The role of anomalous soil moisture on the inland

Nat Hazards
DOI 10.1007/s11069-011-9966-6
The role of anomalous soil moisture on the inland
reintensification of Tropical Storm Erin (2007)
Olivia Kellner • Dev Niyogi • Ming Lei • Anil Kumar
Received: 15 January 2011 / Accepted: 22 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Prior research on tropical storm systems that have made landfall and undergone
a period of sustainability or reintensification has been linked to the synoptic environment at
the time the storm restrengthened. Tropical Storm (TS) Erin is an interesting case study in
that it did not take on hurricane-like structure nor reach hurricane intensity until it moved
through west-central Oklahoma on August 19, 2007. This study seeks to examine the
possible impact of anomalously wet soils across much of Oklahoma on the reintensification
of TS Erin during the early morning hours of August 19, 2007. To determine the degree to
which the antecedent soil state impacted TS Erin’s inland evolution and reintensification,
analyses of the synoptic environment and the mesoscale environment/boundary layer
environment are undertaken using operational and research datasets such as upper air
soundings, surface soil moisture and temperature data, and multiple products from the
Storm Prediction Center (SPC) mesoanalysis archive. This observational assessment is
complemented with numerical experiments using the Weather Research and Forecast
Model, Advanced Research Version 3.2 (WRF-ARW) to further study the role of soil
moisture availability and surface fluxes that may have led to the boundary layer feedback
and inland reintensification. Observational analysis and model results indicate that
anomalously wet conditions over the central Oklahoma region may have helped develop a
regional boundary layer feedback that appears to have contributed to the inland reintensification of TS Erin. Thus, the anomalously wet land surface had a positive role in TS Erin
reintensifying over Oklahoma during the early morning hours of August 19, 2007.
O. Kellner (&) D. Niyogi (&) M. Lei
Indiana State Climate Office, Purdue University, LILY 2-420, 915 W State Street,
West Lafayette, IN 47907-2054, USA
e-mail: [email protected]
D. Niyogi
e-mail: [email protected]
A. Kumar
NASA/GSFC, Hydrological Sciences Branch and ESSIC, University of Maryland,
College Park, MD, USA
Nat Hazards
Keywords Tropical Storm Erin Storm reintensification Storm intensity Land surface processes Soil moisture Landfalling cyclones
1 Introduction
While there is progress being made in hurricane track prediction, forecasting storm
intensity change continues to be a challenge (NOAA-HFIP 2010). Schade and Emanuel
(1999) identified three variables that can be potentially useful in predicting storm
intensity: the storm’s initial intensity, the thermodynamic state of the atmosphere the
storm resides in, and the heat exchange between the surface boundary layer and the
storm core. While the synoptic environment is one of the dominant mechanisms
affecting the storm structure, feedback from the surface heat source and boundary layer
has also been shown to be a contributing factor to storm system intensity (Bister and
Emanuel 1998; Hong et al. 2000; Emanuel et al. 2004; Persing et al. 2002). Numerous
studies have contributed to the understanding of the boundary layer’s role in storm
intensity over the ocean waters and the land surface near the ocean (Anthes and Chang
1978; Emanuel et al. 2004; Smith and Thomsen 2010; Schade and Emanuel 1999;
Wang and Wu 2004).
Tropical systems in coupled dynamic models continue to be a target of coordinated
research and operational initiatives such as the NOAA Hurricane Forecasting Improvement Project (HFIP). A majority, if not all of the intensity change studies deal with the
issue of offshore intensity changes and assume a decay in the tropical cyclone intensity
after landfall, not continued longevity of the storm far inland (e.g., DeMaria et al. 2005).
This paper investigates onshore intensity changes to tropical storm systems with a case
study on the reintensification of Tropical Storm (TS) Erin over west-central Oklahoma
on August 19, 2007. The term ‘‘reintensification’’ applies to post-landfall systems that
experience an observed lowering of the central pressure after an increased observation of
central pressure far inland away from a warm water source. This is noted in the HPC
Tropical Cyclone Report on TS Erin (Knabb 2008). This study seeks to assess the
reintensification of TS Erin relative to land surface interactions, specifically due to
anomalous soil moisture values.
2 Background
After landfall, most tropical systems are deprived of their surface heat source that fuels
the storm or the storm system encounters a high shear environment that leads to the
eventual decay of the warm core system (Rhome and Raman 2006). However, some
tropical storm systems have reintensified inland, away from any warm water source. A
recent storm of notable reintensification is TS Erin in 2007. The storm saw a brief period
of reintensification over west-central Oklahoma where winds speed increased, copious
amounts of rainfall fell, pressure dropped, and lives were lost due to flash flooding
(Arndt et al. 2009). Thus, knowing why this tropical storm system reintensified over land
is important for the forecasting and emergency response community. This study will
review the inland reintensification of TS Erin emphasizing the role of anomalously wet
soil conditions as the possible contributor for the tropical storm system sustainment and
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2.1 Impacts of varying soil moisture
Soil moisture anomalies or soil moisture gradients present on or within the land surface
alter surface heat fluxes that can act to enhance mesoscale convection (Holt et al. 2006;
LeMone et al. 2007; Niyogi et al. 2006; Pielke Sr. 2001). This study applies the soil
moisture anomaly concept at a larger scale, hypothesizing that when a soil moisture
anomaly is present across a large enough area and at a large enough amount, it may act to
enhance a tropical storm system (Dastoor and Krishnamurti 1991). Numerous studies
support the idea that the antecedent soil moisture state can affect mesoscale thunderstorms
and the mean storm track over the United States, showing a possible direct relationship to
higher antecedent soil moisture values and the subsequent storm systems that develop
(Findell and Eltahir 1997; Lanicci et al. 1987; Pal and Eltahir 2003; Lintner and Neelin
2.2 Hypothesis
The working hypothesis of this study is: Anomalously wet soil conditions aided in the
reintensification of TS Erin by helping to generate a thermodynamically-rich storm environment suitable for the sustainment and reintensification of a warm core system. An
alternate hypothesis that will be examined is that exceptionally dry soils could have acted
to increase the rate at which the tropical cyclone would have decayed over land.
These hypotheses build on earlier work by Dastoor and Krishnamurti (1991), Raman
and Niyogi (personal communication, 1998; unpublished manuscript related to Hurricane
Fran), Kehoe et al. (2000), Emanuel et al. (2008), Chang et al. (2009), and Kishtawal et al.
(2010) who study the effects of pre-storm soil moisture on storm intensification and inland
duration. For example, analysis by Chang et al. (2009) shows that higher antecedent soil
moisture conditions can lead to lower surface pressures in monsoon depressions postlandfall. The experiments also isolated the effects of antecedent soil conditions and the
storm’s rainfall and latent heat release to provide evidence that warm and wetter antecedent soil conditions can lead to more intense and sustained post-landfall monsoon systems. The validity of these results and findings is further strengthened by the investigation
of 183 landfalling monsoon depressions by Kishtawal et al. (2010). This study shows that
there may be a number of antecedent soil states that can allow for the inland sustainment of
tropical storm systems (depressions or cyclones). The potential broader applicability of the
antecedent soil moisture hypothesis seems likely because of similar results found in a
global model used by Dastoor and Krishnamurti (1991) over the Indian Monsoon region
and by Emanuel et al. (2008) over Western Australia. While the Indian Monsoon region
and associated monsoon depressions are different atmospheric phenomena as compared to
the mid-latitudinal tropical systems, the convection and processes affecting the land
atmosphere interactions can be argued to be similar in nature originating with tropical
characteristics and moving inland.
While the impact of land surface heterogeneity on convection has been the topic of
research for many years (e.g., Pielke Sr. 2001), whether land surface heterogeneity and
antecedent soil moisture conditions can also affect synoptic scales down to mesoalphascale systems such as tropical cyclones needs to be intensively investigated. The aforementioned studies hint at the possibility that antecedent soil moisture may have played an
active role in the post-landfall evolution of TS Erin and acts as the basis for this research
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Over the last several years, TS Erin has become a case study in several other meteorological research projects. These studies, however, are widespread in focus. They range
from observational summaries regarding the roles of the synoptic and mesoscale environments utilizing observational datasets such as the Oklahoma Mesonet and radar and
satellite data archives (Arndt et al. 2009; Monteverdi and Edwards 2010) to modeling
studies on the synoptic and mesoscale environment (Evans et al. 2010; Galarneau et al.
2008, 2009). Additional studies include the assimilation of satellite and radar data into the
WRF-ARW model for a better forecast of TS Erin upon making landfall (Xue et al. 2008)
and the type of convection and tornadoes associated with TS Erin over Oklahoma (Holt
and Kloesel 2011). Only one other known study similar in nature to that discussed in this
paper (soil moisture sensitivity analysis using WRF) is underway (Evans et al. 2011). The
findings of Evans et al. (2011) also suggest that although synoptic-scale dynamic forcing
(or lack thereof) appears to have contributed to the inland reintensification of TS Erin to a
certain extent, soil moisture appears to have played an important role as well to the inland
reintensification of TS Erin. To reach a better understanding of the role of synoptic and
mesoscale dynamics and the role of the anomalous soil moisture observed in Oklahoma
during the spring and summer of 2007 on TS Erin’s inland reintensification, an operational
analysis and model verification study will be undertaken in the following sections.
3 Operational analysis of TS Erin
3.1 Synoptic-scale environment
The synoptic-scale atmospheric environment is the main driver of weather systems in the
mid-latitudes. Baroclinicity that exists from uneven surface heating and air mass discontinuities is responsible for the majority of storm systems that traverse over North America
(Holton 2004). In most cases, any tropical system that makes landfall decays or undergoes
extratropical transition (DeMaria et al. 2005). Extratropical transition (ET) occurs when
the tropical storm system becomes embedded in mid-latitude, westerly flow, and takes on
baroclinic characteristics (Hart and Evans 2001). The tropical storm system then transitions
to a cold core system instead of maintaining its warm core self-sustaining structure (Jones
et al. 2003). To determine whether TS Erin became extratropical, the 300, 500, and 700 mb
winds, along with surface observations (not shown) and 500 mb vorticity maps (not
shown) are reviewed at 0000Z and 1200Z August 18, 2007 (Fig. 1) and 0000Z and 1200Z
August 19, 2007 (Fig. 2). Upper air analyses are retrieved from UCAR’s Locust Image
Archive website (
The 300 mb maps are reviewed to determine the placement and orientation of the jet
stream at the time of TS Erin’s inland propagation and reintensification. Previous studies
on tropical cyclones that have restrengthened have found a common occurrence with
tropical storm systems being influenced by an exit region of a jet streak, the presence of a
shortwave trough to aid in upper-level divergence, or a vorticity maximum embedded in
westerly flow (Hanley et al. 2001; Wu and Cheng 1999). The 300 mb maps (Figs. 1d, f, 2c, f)
from 0000Z August 18 to 1200Z August 19 (12 h plots) indicate that TS Erin is not
influenced by a jet streak, the jet stream wave pattern, or a well-pronounced vorticity
maximum originating in westerly flow. However, a weak shortwave trough was present at
the time of reintensification. This trough does appear to have contributed to upper-level
divergence, similar to how a tropical upper tropospheric trough (TUTT) acts to help initial
tropical disturbances organize into a tropical system and/or helps in the sustainment of a
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Fig. 1 700, 500, and 300 mb upper air charts for 0000Z (a–c) and 1200Z August 18, 2007 (d–f),
respectively. TS Erin moved northward into the southern Texas panhandle by 1200Z August 18, 2007 and
weakened by this time, with no clear circulation visible in the 0000Z charts and only in the 1200Z. 300 mb
streamlines show a shortwave embedded in the streamlines with diffluent flow over the storm circulation in
both the 0000Z and 1200Z August 18, 2007 maps. The box indicates the region of Tropical Storm Erin’s
warm core system by transporting latent heat release downstream of the storm (Rhome and
Raman 2006).
The 500 mb maps are often used to determine the mean storm track for weather systems
across North America (Holton 2004). Upper-level flow at 500 mb during TS Erin’s life
cycle shows relatively zonal flow with the jet stream located over the north-central plains.
Reviewing 12 h maps from 0000Z August 18 to 1200Z August 19 (Figs. 1b, e, 2b, e), the
500 mb maps also reveal nothing of significance regarding TS Erin except when the storm
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Fig. 2 Same as Fig. 1 but for August 19, 2007. TS Erin has tracked northeast from the Texas Panhandle
into west-central Oklahoma by 1200Z August 19, 2007. At 0000Z, the 500 mb chart shows a closed isobar
over TS Erin’s circulation center and the 300 mb streamlines show a shortwave over the circulation center.
Upper-level shear is still weak, not exceeding 30 knots. By 1200Z, TS Erin is clearly visible at its peak
intensity with a closed isobar and nearly closed second isobar. The 1200Z 500 mb chart shows an open wave
over TS Erin, increasing storm top divergence at the time of reintensification. The 1200Z 300 mb chart
shows weak upper-level flow
structure reaches the 500 mb level in the form of a closed isobar. Closed isobars for TS
Erin are observed on the 1200Z August 17, 2007 map (figure not included), the 1200Z
August 18, 2007 map, and the 0000Z August 19 map (Figs. 1e, 2e). By 1200Z, August 19,
2007 (time of reintensification), TS Erin becomes embedded in the 500 mb flow as a
shortwave feature on the map (Fig. 2e) showing that the vertical structure as a warm core
system is confined to the lower levels of the atmosphere. 500 mb vorticity plots show that a
vorticity maximum exists from 0000Z August 18, 2007 to 0000Z August 20, 2007 that
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resulted from TS Erin’s reintensification and was not originally embedded in westerly flow
(not shown).
In a developed tropical system, warm core structures are predominantly present below
700 mb; therefore, the 700 mb maps during TS Erin’s life span over land were reviewed.
These maps provide convincing evidence that TS Erin becomes hurricane-like over land
through the duration of the storm over Oklahoma. A closed isobar at 700 mb is present
over TS Erin’s center from 1200Z August 17, 2007 (not shown) to 1200Z August 19, 2007
(Fig. 2e). The 1200Z map for August 19, 2007 shows a small closed isobar over westcentral Oklahoma with a nearly closed second isobar around the storm center. At the time
of peak intensity, TS Erin was confined to the lower levels of the atmosphere and did not
reach beyond the normal depth of a warm core structure (roughly 700 mb). The 500 mb
maps and 850 mb maps only show a shortwave feature at this time and location, not a
closed structure. Arndt et al. (2009) discuss the presence of a *30 knot (15 m s-1) lowlevel jet (LLJ) feeding into TS Erin from the Gulf of Mexico at the time of storm reintensification. This feature is visible on 850 mb maps (figure not included) and is indeed
considered as one of the ingredients in the inland sustainment of TS Erin in this study. The
analysis undertaken thus does not diminish the role of the LLJ, but hypothesizes that the
anomalously wet soil moisture state in Oklahoma (discussed in the follow paragraphs)
enhanced the pre-storm environment with a more unstable air mass that provided a thermodynamically favorable region in which TS Erin could reintensify. We are hypothesizing
that without the soil moisture anomaly, TS Erin may have followed a different path and the
convective life cycle(s) may have taken on a different nature other than remaining a
tropical storm and appearing as in inland hurricane over west-central Oklahoma around
1100Z August 19, 2007.
3.2 Mesoscale and boundary layer environment
Tropical or extratropical storm systems are classified as synoptic-scale phenomena (Stull
1988) or mesoscale phenomena (Orlanski 1975; Thunis and Bornstein 1996). With the
synoptic environment from 0000Z August 18, to 0000Z August 20, 2007 showing no
indications of extratropical or obvious synoptic-scale processes responsible for the reintensification of TS Erin, we look to the mesoscale for additional answers (Monteverdi and
Edwards 2010; Evans et al. 2011). We suggest that reintensification of TS Erin resulted from
mesoscale and boundary layer interactions that evolved in the anomalously moist environment. To investigate the possible feedbacks of the moist mesoscale and boundary layer
environment through which TS Erin migrated, data are collected and analyzed from the
NOAA Storm Prediction Center mesoanalysis archive website (
exper/ma_archive/) and Plymouth State’s Weather Center (
u-make.html). The same time frame (0000Z August 18, 2007 to 0000Z August 20, 2007) as
the synoptic-scale analysis is applied to review parameters. Wind-induced surface heat
exchange, or WISHE, processes require a moist boundary layer for tropical storm sustainment and intensification (Laing and Evans 2010). This mesoscale and boundary layer
analysis will seek to show how the moist and unstable boundary layer present at the time of
reintensification appears to have acted favorably to the WISHE process and the life cycle of
TS Erin.
A number of mesoscale parameters provide a good indication of atmospheric stability.
When considering the possibility of mesoscale phenomena impacting a synoptic-scale
system and helping it reintensify, parameters selected for review are chosen to provide a
representation of an antecedent environment conducive to supporting a tropical storm
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system. These parameters are indicators of moisture availability, wind fields, instability,
and rotation (vorticity) within the boundary layer.
The following parameters are reviewed and discussed in the subsequent paragraphs with
references to figures for possible contribution to the reintensification of TS Erin between
0600Z August 19, 2007 and 1130Z August 19, 2007: surface streamlines, surface plots,
upper air soundings, equivalent potential temperature, dew point temperatures, 0–3 km
MLCAPE (CAPE values from the surface to 3 km tend to favor low-level stretching and
instability) and surface vorticity, surface-based Lifted Index and convective inhibition
(CIN), 0–1 and 0–3 km storm relative helicity, and the 850–300 mb mean wind. Surface
plots are examined to assess the location of frontal boundaries (extratropical cyclone
features), and surface streamline analysis shows regions of surface convergence and
enhanced lift. Upper air soundings are reviewed as well to assess the thermodynamic state
of the atmosphere leading up to and during the time of reintensification. he is reviewed
because it is a strong indicator of instability. Dew points and surface-based Lifted Index
are also used to assess the instability of the air mass. Convective inhibition is reviewed to
determine the presence of a cap at the time of reintensification. Vorticity and helicity are
reviewed to assess rotation in the storm environment. The 850–300 mb mean wind is
reviewed because mesoscale convective systems tend to traverse along the height fields
between 850 and 500 mb and is used to provide estimates of possible mean storm motion.
The 850–300 mb layer is selected because the 850–500 mb mean wind was not available
for review.
Combined with the review of the above mesoscale parameters, other products including
the Oklahoma Mesonet surface and subsurface datasets, Atmospheric Radiation Measurement (ARM) observations, and North American Regional Reanalysis (NARR) data are
used for analysis. Fractional Water Index (FWI) soil moisture maps at 5, 10, and 25 cm
depths from the Oklahoma Mesonet are examined to determine the possible role of soil
moisture boundaries influencing TS Erin’s reintensification and path. ARM datasets provide observations of sensible and latent heat fluxes from the Cordell, Oklahoma site near
the region where TS Erin reintensified. NARR data plots for surface soil moisture, surfacebased CAPE, and the surface latent heat flux are reviewed for comparison with observations recorded at Oklahoma Mesonet sites and to parameters retrieved from the SPC
mesoanalysis archive.
Surface streamline analysis indicates a southerly LLJ over Texas and Oklahoma at the
time of TS Erin’s reintensification. Surface streamlines were examined to identify whether
any surface convergence boundaries were acting to enhance lift and help deepen the central
pressure of the storm. By 1800Z August 18, 2007, a convergent flow pattern into the storm
center is noted, followed by prominent southerly and southeasterly flow over Texas and
Oklahoma from 0000Z August 19, 2007 until 0000Z August 20, 2007 (Figure not shown).
While it is known that low-level flow inland from the Gulf of Mexico is a moisture and
instability transport mechanism, the soil moisture anomaly still appears to have a role in
the storm evolution. The degree to which the low-level inflow impacts the storm’s intensity
and the soil moisture anomaly impacts the storm’s intensity is reviewed in the modeling
section of this paper.
Surface analysis charts provide a quick and simple summary of the air mass affecting a
given region including but not limited to surface temperatures, surface dew point temperatures, surface wind speed and direction, surface pressure, and cloud cover. Surface
weather charts were mainly used in this study to determine whether the extratropical
process occurred leading up to or during the time of TS Erin’s reintensification. Surface
analysis charts show no generation of frontal boundaries, which is an indication of the
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extratropical process. Neither surface temperature gradients/frontal locations, nor dew
point temperature gradients (another means of frontal boundary determination) developed
around TS Erin’s circulation center (Figure not shown).
Thermodynamic diagrams provide valuable information on air mass characteristics
through the depth of the atmosphere, providing details on atmospheric instability and
atmospheric inhibition to convection. To assess the air mass in Oklahoma that predominantly influenced TS Erin as it reintensified and decayed, a cross-section analysis of three
upper air sounding sites is generated from 0000Z August 19 to 0000Z August 20, 2007 and
reviewed. Data from sounding sites around Oklahoma are analyzed to complete a general
north to south transect through the area where TS Erin underwent reintensification. Sites in
the analysis include Dodge City, Kansas; Norman, Oklahoma; and Dallas/Fort Worth,
Texas. The sounding data are retrieved from the University of Wyoming website
( The soundings, for 0000Z August 19,
2007 to 0000Z August 20, 2007 Norman, Oklahoma (as shown in Fig. 3), show marginal
instability, weaker wind shear that varies little in speed through the upper levels of the
atmosphere (typical flow over North America sees increasing wind speeds with height),
and predominant southerly to easterly low-level inflow into the storm system over
Oklahoma. These profiles show a thermodynamic environment suitable for the sustainment
Fig. 3 Skew-T diagrams to determine air mass characteristics in a cross-section across Oklahoma at the
time of Tropical Storm Erin’s reintensification. A1 (0000Z August 19), A2 (1200Z August 19, 2007), and
A3 (0000Z August 20, 2007): Dodge City, Kansas. B1 (0000Z August 19), B2 (1200Z August 19, 2007),
and B3 (0000Z August 20, 2007): Norman, Oklahoma. C1 (0000Z August 19, 2007), C2 (1200Z August 19,
2007), and C3 (0000Z August 20, 2007): Dallas-Fort Worth, Texas
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of a warm core system as winds are greater near the surface and have weak shear a loft and
agree with the findings of Monteverdi and Edwards (2010).
Of all the mesoscale parameters reviewed for this operational analysis, plots of surface
equivalent potential temperature (he), which is analogous to moist static energy, result in
the most convincing feature that TS Erin’s storm motion and reintensification may be
related to the moisture available to the storm near the ground surface. Equivalent potential
temperature is often used as an instability parameter because ridges of he (‘‘theta-e ridges’’)
often act as an initiation point for thermodynamically induced thunderstorms and mesoscale convective systems. These ‘‘theta-e ridges’’ are most commonly found in areas
undergoing the greatest warm air and moisture advection (Bister and Emanuel 1998).
Reviewing the surface he maps for TS Erin, the building of a theta-e ridge from southcentral Texas north into Oklahoma is apparent at 0600Z August 18, 2007. At 0000Z
August 19, 2007, the he contours align with the storm track over the next 12 h. The 1200Z
he map shows a ridge centered directly over central Oklahoma, coinciding with the time
frame of peak convection and reintensification (Fig. 4a).
High dew point temperatures can be indicative of instability, with a common threshold
value in severe weather forecasting around 70°F (294 K). Dew point maps from 0000Z
August 18, 2007 to 0000Z August 20, 2007 reveal a tongue of moisture extending through
central Texas and north into central Oklahoma. Maps show that the western edge of the
gradient is relatively strong over a short distance. At 0000Z August 19, 2007, the gradient
takes a southwest to northeast orientation similar to the path of TS Erin through Oklahoma
at this time (Fig. 4b). Dew point temperatures are supportive of a favorable thermodynamic environment and appear to serve as a possible vorticity boundary for the reintensification and steering of TS Erin through Oklahoma.
The 0–3 km mid-level CAPE and surface vorticity plots reveal that at 1500Z August 18,
2007, surface vorticity contours concentrated around TS Erin’s circulation center begin to
tighten as the cyclone aligns along a CAPE boundary oriented parallel to the storm path
motion that it follows into Oklahoma. TS Erin follows the CAPE gradient through the
duration of the storm movement through Oklahoma. At 0700Z August 19, 2007, the
surface vorticity increases as TS Erin undergoes reintensification (Fig. 4c).
Mesoanalysis of the surface-based Lifted Index and CIN shows that from 0000Z August
18, 2007 to 1000Z August 19, 2007, there is a tongue of marginal instability through
central Oklahoma. At 0100Z August 19, 2007, a pocket of uncapped air is present over
western Oklahoma. This air mass is located along the backside of a Lifted Index gradient
and over TS Erin’s circulation center. The uncapped air is visible on maps through the rest
of TS Erin’s life cycle allowing for the development of deep convection to help sustain and
feed the warm core system. Figure 4d shows the 0600Z August 19, 2007 plot of surfacebased Lifted Index and CIN.
Storm relative helicity (SRH) is monitored at two levels in the troposphere: 0–1 and
0–3 km. Higher values in the 0–1 km layer contribute more to low-level rotation in storms
than the 0–3 km layer. Low-level rotation is generated by directional and speed shear
present in the lower levels of the atmosphere. The wind shear contributes to a higher rate of
evaporation from the land surface, which is required for WISHE processes (Laing and
Evans 2010; Rauber et al. 2004), on top of low-level vorticity from which TS Erin could
draw momentum. The highest amount of 0–3 km SRH seen with TS Erin is 500 m-2 s-2.
This value occurs when TS Erin is undergoing reintensification over west-central Oklahoma. From 0000Z August 18, 2007 till 0400Z August 19, 2007, values of 0–3 km SRH
vary every hour between 0100Z and 0400Z. At 0400Z August 19, 2007, the 0–3 km SRH
climbs to 500 m-2 s-2, remaining at that amount for 2 h, and then decreases back to
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Fig. 4 Mesoscale analysis for 0600Z August 19, 2007 just prior to TS Erin’s organization. An eye-like
feature formed between the hours of 0900Z and 113Z. a shows a large theta-e ridge over Oklahoma with
contours similar in orientation to the mean path of TS Erin into and through Oklahoma. b shows the dew
point gradient through western Oklahoma, oriented similar to the mean track path of TS Erin. c shows a
couplet of 0–3 km CAPE and surface vorticity over TS Erin in west-central Oklahoma. d shows the region
of uncapped air and the surface-based Lifted Index gradient in the vicinity of where reintensification begins.
e shows a strong bulls-eye of 0–1 km SRH and f shows the mean wind (which was weak allowing for warm
core structure sustainment) oriented from the southwest to northeast across Oklahoma
200–300 m-2 s-2 by 1600Z August 19, 2007. Upon reviewing the 0–3 km SRH and then
0–1 km SRH, it is seen that a majority of the SRH was confined below 1 km in the
atmosphere, with values varying between 150 and 300 m-2 s-2 during the time of reintensification. 300 m-2 s-2 is the highest value noted for the SRH when Erin reintensified
from 0300Z August 19, 2007 to 0800Z August 19, 2007 (Fig. 4e), after which the SRH
value began to decrease.
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Reviewing the 850–300 mb mean wind charts shows a 20 knot pocket of shear that
migrates along with TS Erin as the storm progresses through Texas and into Oklahoma.
There is rotation visible in the wind fields showing the presence of TS Erin moving with
the mean flow till 0800Z August 19, 2007. The rotation then transitions into an open-wave
pattern by 0900Z August 19, 2007. After this hour, the 20 knot shear becomes embedded in
the westerly flow located over the northern central plains and the circulation disappears
(Fig. 4f). The 20 knot pocket of shear may have acted to sustain TS Erin through Oklahoma by providing needed storm top divergence to carry heat downstream of the storm top.
The 20 knot (*10 m s-1) shear appears to also have helped steer TS Erin through
Oklahoma providing the vortex with continuous access to a warm and moist land surface.
The process is similar in fashion to shear needed to advect a tropical storm system across
the ocean preventing cold water upwelling generated by the storm itself from weakening
the warm core system. Some upper-level shear is crucial for the self-sustainment of the
warm core system (Laing and Evans 2010).
3.3 Discussion of synoptic and mesoscale features
Upper air plots of the synoptic-scale environment during TS Erin’s life cycle show conclusive evidence that the storm maintains warm core structure far inland. Despite being
visible on some 500 mb maps (Figs. 1b, e, 2b, e), TS Erin is mostly confined below
700 mb during its peak intensity hours, a key characteristic of warm core structures
(Figs. 1c, f, 2c, f). 850 mb maps (figure not included) show the presence of a LLJ from the
Gulf that helps to feed TS Erin warm, moist air, and sustain associated convection, but
does not appear to be the sole moisture source for the storm, as we discuss in the following
paragraphs. The 300 mb charts (Figs. 1a, d, 2a, d) show no signature of reintensification
from a jet streak or a vorticity maximum.
Vertical analysis of the atmosphere reveals the closed-off nature of a warm core system
and the surrounding environment across Oklahoma. It is apparent in the 0000Z and 1200Z
soundings on August 19, 2007 that TS Erin is situated near and over Norman, Oklahoma.
These profiles show nearly saturated air and stronger wind shear (up to 35 knots or
18 m s-1) (Fig. 3) aloft, supportive, and common with warm core systems (Jones et al.
2003; Monteverdi and Edwards 2010). It appears that TS Erin remained a warm core
Mesoanalysis products show an uncapped atmosphere over TS Erin as it reintensified
and progressed through Oklahoma. In addition to this uncapped region of air over the
circulation center, the environment was highly moist and unstable leading up to and during
the time of TS Erin’s reintensification, providing an ample environment for convection
near the circulation center to strengthen and briefly organize into a hurricane-like structure.
3.4 Additional operational analysis products
The Oklahoma Mesonet soil moisture maps (Fig. 5) for the Fractional Water Index (FWI)
at 5, 25 and 60 cm depths are reviewed to find any possible antecedent soil moisture
boundaries that may have influenced TS Erin. The data are analyzed from July 2007 to
August 20, 2007 and reviewed to determine the temporal scope of the anomalous soil
moisture boundary present in the area where TS Erin reintensified. A prominent gradient of
soil moisture coincides with TS Erin’s track across Oklahoma. This, paired with the
notable h-e ridge, leads us to believe that the enhanced soil moisture zone may have acted
as a gradient of surface vorticity that may have acted favorably to the reintensification and
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steering of TS Erin. To complement the point information from the Oklahoma Mesonet,
NARR surface soil moisture plots are used. Data plots for 0000Z August 19, 2007 from the
Oklahoma Mesonet at 5, 10, and 25 cm along with a NARR surface plot of soil moisture
are reviewed closely because 0000Z provides a picture of the environment prior to reintensification. The NARR plots show a similar soil moisture gradient present in the area
where TS Erin restrengthened despite NARR plots displacing a majority of the projected
soil moisture in south-central Oklahoma.
Several observational sites supported by the Department of Energy Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) site region are within the
region of TS Erin’s reintensification. Data from the Cordell, Oklahoma site, were obtained
and reviewed (Fig. 6) to assess the changes in surface energy flux corresponding to the
passage of TS Erin over Oklahoma on August 19, 2007. Plots of sensible heat flux, latent
heat flux, net radiation, and average soil heat flux were reviewed. As expected, the sensible
and latent heat surface fluxes reveal a change corresponding to the time of TS Erin’s passage
over the Cordell, OK site (time window being *1600Z August 18, 2007 to 2300Z August
18, 2007). For each day during peak heating hours, there is a greater latent heat flux from the
surface than a sensible heat flux, indicative of a moisture-rich surface and a flux pattern
more suitable for warm core sustainment and possible reintensification.
NARR plots of surface soil moisture, surface-based CAPE, and the surface latent heat
flux are analyzed to delineate the mesoscale boundaries where TS Erin reintensified. All
three variables are reviewed from 1800Z August 18, 2007 to 1200Z August 19, 2007
(Fig. 7). A discernable boundary in the region of TS Erin’s reintensification is present in all
plots for the different variables. Interestingly, the stated hypothesis that higher soil
Fig. 5 a NARR surface soil moisture for 0000Z August 19, 2007. b Fractional Water Index maps from the
Oklahoma Mesonet for (b) 5 cm, c 25 cm, and d 60 cm depths. The maps show a strong soil moisture
gradient in the vicinity of reintensification
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Fig. 6 Surface flux data measurements from the Atmospheric Radiation Measurement Program (ARM) site
at Cordell, Oklahoma, at 0000Z August 17, 2007—0000Z August 19, 2007. Cordell, Oklahoma, was
selected due to its proximity to TS Erin’s path through Oklahoma. TS Erin passed over Cordell, OK from
1600Z August 18, 2007 to 2300Z August 18, 2007. During this time frame (circled for clarity), the latent
heat flux (blue line) exceeds that of the sensible heat flux (orange line)
Fig. 7 A1–A4 NARR surface plots of surface latent heat flux, B1–B4 soil moisture, and C1–C4 CAPE.
Column 1 contains respective plots for 1800Z August 18, 2007. Column 2 contains respective plots for
0000Z August 19, 2007. Column 3 contains respective plots for 0600Z August 19, 2007. Column 4 contains
respective plots for 1200Z August 19, 2007. The region of reintensification is circled for clarity
moisture values act to generate a favorable thermodynamic environment for TS Erin
appears to be possible when reviewing the NARR latent heat flux gradients at the time of
reintensification. The distinct boundaries resulting from the spatial distribution of the soil
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Fig. 8 Proposed schematic of the life cycle and primary mesoscale features that seem to have favored TS
Erin’s inland reintensification
moisture anomaly in Oklahoma serve as a visible surface boundary of vorticity that may
have influenced the storm’s track and evolution.
Figure 8 summarizes the life cycle and primary mesoscale influences that appear to
have aided in TS Erin’s reintensification. The two subset maps show findings that illustrate
the reintensification of TS Erin as a warm core structure over Oklahoma. The tornado track
map shows that when TS Erin entered Oklahoma and began to restrengthen, five tornadoes
touched down in the right front quadrant of the storm (seven symbols are plotted, as
tornadoes that cross county lines are reported separately. Most tropical storm systems have
a higher frequency of tornadogenesis in the right front quadrant of the storm. The most
recent tropical cyclone-induced tornado climatology is documented by Schultz and Cecil
(2009), in which TS Erin’s tornadoes are included, further supporting the fact that during
the reintensification stage, TS Erin was still tropical in nature. The mesoscale boundary
map is a schematic showing the location of numerous overlapping mesoscale boundaries at
0600Z August 19, 2007 and is plotted as a vertical cross-section in Fig. 9. Boundaries
present on the map include 0–3 km CAPE and surface vorticity, Lifted Index and CIN,
dew point contours, surface EPT contours, mean wind (850–300 mb flow), latent heat flux,
CAPE, and soil moisture. Knowing that low-level moisture is a contributing factor in the
value of most of these parameters, it again suggests that a wet boundary layer resulting
from moist, hot soils clearly could have helped in the generation of the inland hurricanelike features during the storm reintensification.
4 Modeling analysis
The observational analysis compares well with findings from prior studies that suggest
land surface feedback in the form of soil moisture can enhance convection (Pielke 2001)
and possibly tropical storm systems (Chang et al. 2009; Emanuel et al. 2008; Kishtawal
et al. 2010). The observational analysis provides evidence that the anomalously wet soils
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Fig. 9 Vertical cross-section summarizing the surface and boundary layer processes that may have aided in
TS Erin’s reintensification. Of interest is the multiple overlapping mesoscale boundaries in the vicinity of TS
Erin’s reintensification
may have impacted the evolution of TS Erin from 0600Z to 1130Z over central Oklahoma on August 19, 2007. The synoptic environment before and during TS Erin’s
reintensification shows no sign of reintensification resulting from a jet streak or vorticity
maximum originating in westerly flow. Surface plots show no frontogenesis occurring,
and the 300, 500, and 700 mb maps show TS Erin followed normal warm core characteristics at the time it reintensified over west-central Oklahoma. Review of mesoscale
parameters influenced by values of low-level heat and moisture reveal that most of the
instability within the planetary boundary layer at the time of TS Erin’s reintensification
was located over one of the wettest regions of Oklahoma leading up to the display of
hurricane-like gusto during the morning hours of August 19, 2007. Thus, TS Erin may
well be another example of an inland tropical storm system sustained from antecedent
soil moisture conditions, similar to the inland intensification of Typhoon Abigail over
Australia (Emanuel et al. 2008).
To further test the hypothesis regarding the role of antecedent soil moisture, numerical
experiments with WRF-ARW Ver. 3.2 are undertaken with different anomalous soil
moisture conditions prior to TS Erin’s landfall. This model analysis aims to test that
although most inland tropical storm systems that have reintensified were a result of ET,
inland tropical systems may be impacted by the land surface they pass over. Results will
also look to test the alternative hypothesis that the storm may decay more rapidly with
antecedent drought-like conditions since there is no surface latent heat flux to sustain the
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4.1 Model setup
We used the WRF-ARW 3.2 (April 2010 release) with the Slab Land Surface Model
(LSM) as the land surface scheme. The slab model was chosen because it maintains
constant soil moisture values and prognosticates soil temperatures (Dudhia 1996). The
LSM was chosen to determine the initial impact of soil moisture anomalies on the storm,
with results serving as a preliminary guide to future research experiments. North
American Regional Reanalysis (NARR) data are selected for model initial conditions.
The NARR-based WRF-ARW simulations were conducted at three different spatial
resolutions: 9, 3, and 1 km resolutions, with higher resolution over Oklahoma. The
model was run from 0000Z August 18, 2007 to 0000Z August 20, 2007. The 1-km
resolution is selected for analysis to focus on the feedback of soil moisture on storm
evolution since the region of antecedent soil moisture anomalies appears to be localized.
Other model specifications include USGS 24 class landuse, WRF Single Moment (WSM)
5-class microphysics, RRTM longwave radiation, Dudhia MM5 shortwave radiation, 5
layer thermal diffusion slab soil model, YSU boundary layer scheme, and for the outermost domain new-Eta Kain–Fritsch (KF2) cumulus convection scheme. Model
experiments were conducted to account for impact of soil moisture changes with dry,
default (control), and wet conditions over Oklahoma. The Control run used the default
soil moisture values of the model, the ‘‘Dry’’ run reduced the soil moisture by half
(0.5 9 SM), while the ‘‘Wet’’ run increased the soil moisture by 50% (1.5 9 SM). The
‘‘Dry,’’ ‘‘Control,’’ and ‘‘Wet’’ runs were analyzed to study varying soil moisture impacts
on boundary layer heat, mass, and moment fluxes on the evolution of TS Erin. Soil
moisture modification was undertaken only in the innermost domain, which roughly
coincides with the observed region of the soil moisture anomaly.
Variables analyzed include surface pressure tendency, surface and 850 mb winds,
reflectivity, surface fluxes (sensible and latent heat), parcel trajectories at several layers
(surface, 850 and 700 mb), a vertical cross-section of he and vertical velocities, sounding
surface wind speed and direction, along with sounding temperature and dew point temperature (which are compared to Oklahoma Mesonet observations), and dew point temperature plots near the surface. Surface pressure is analyzed because it is the main variable
used in this study to classify a storm as ‘‘reintensifying.’’ Surface and 850 mb winds are
reviewed to determine the degree to which differing soil moisture values affect boundary
layer convergence and flow patterns that could act to enhance TS Erin. Model reflectivity
plots are generated for comparison with the radar imagery to determine which model run
resulted in the most realistic storm structure. Also generated are parcel trajectories maps to
show moisture transport from regions other than the LLJ, and dew point plots to show
moisture availability.
4.2 Discussion of model results
The pressure tendency for each model run was plotted as a cumulative sum (i.e., for the
duration of the run) and reviewed. Results (Fig. 10) indicate that the Wet soil run generated
the greatest pressure decreases over time, which were also concentrated over a central
region in south-central Oklahoma (the forecasts by the WRF displaced the storm south of
observed mean path in all runs). On the contrary, the Dry run saw the least difference in
surface pressures and had the largest amount of spatial variation as compared to the
Control and Wet runs.
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Fig. 10 Pressure (mb) tendency plots from 1800Z August 18, 2007 to 1200Z August 19, 2007 for a Dry
run, b Control run, and c Wet run as described in the text. The Wet run results in the most concentrated and
intense region of lower pressure
Tropical storm system dynamics generate the strongest surface winds to the right of the
storm track in the Northern Hemisphere because of westerly flow and the Coriolis
parameter (Jones et al. 2003). Model results from this experiment also suggest that TS Erin
maintains warm core characteristics with its strongest surface winds found to the right of
the storm track through the duration of the model run through Oklahoma. Plots reveal that
the Wet run clearly produces stronger surface winds to the right of the storm track, with the
Dry run soils resulting in the weakest surface winds right of the storm track. Overall,
surface winds increase with wetter soils and appear to generate greater convergent flow
into the storm at the surface (Fig. 11). The 850 mb wind plots generated by the Dry,
Control, and Wet runs reveal an impact of the antecedent soil moisture on the boundary
layer (850 mb) winds. Higher values of dissipative wind shear are generated in the 850 mb
Dry run, followed by the Control run, and the weakest shear is present in the Wet run
during the time of reintensification on August 19, 2007. The lower shear in the Wet run
provides for a more tropical-like environment to evolve and favor sustainment of the warm
core system over Oklahoma (Fig. 12).
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Fig. 11 Surface 10 m wind plots for (column 1) Dry, (column 2) Standard, and (column 3) Wet run during
the period of reintensification. The top, middle, and bottom row show the forecast for hours 1100Z, 1200Z,
and 1300Z, respectively. As expected with tropical storm systems, winds are strongest to the right of the
Reflectivity plots of each run provide insight into the type of convective environment
generated from the differing values of soil moisture and their impact on TS Erin’s structure
and evolution in simulated environments. Reflectivity plots are also compared to the
observed reflectivity radar fields to determine which storm environment best simulates TS
Erin at the time of reintensification. There are differences in the amount of convection
represented in each run. The Wet run shows more widespread rainfall over Oklahoma with
the least amount of concentrated/intense convection. The Dry run shows the opposite
pattern with the most spatially concentrated convection (Fig. 13). Note that because this is
an ‘‘idealized’’ scenario with the use of the Slab LSM, distinct tropical storm features
typically visible in real-time radar data are not simulated by the model.
The working hypothesis of this paper suggests that wetter antecedent soils result in more
thermodynamically favorable environment with larger heat and moisture fluxes available
to sustain and possibly reinvigorate a tropical storm system over land. This is based on
literature discussing the role heat fluxes from the ocean surface, with more specific focus
on the amount of moist static energy (analogous to he, discussed in the previous section of
this paper) available to the hurricane via latent heat flux (Laing and Evans 2010). We are
proposing that the heat and moisture flux from the anomalously wet soils acted similar in
nature to latent heat flux from the ocean surface (though not to the same degree) providing
increased moist static energy that has also been attributed to rainfall development over land
(Eltahir 1998). The operational analysis of the antecedent convective environment over
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Fig. 12 Same as Fig. 11 but for 850 mb
west-central Oklahoma prior to TS Erin’s arrival shows an environment heavily favored by
a warm and moist boundary layer that may have evolved from the higher surface latent
heat fluxes resulting from the abnormally moist soils present at the time (cf. Emanuel et al.
2008). The WRF runs also provide some support to this hypothesis. Because TS Erin
reintensified over night, model runs during the daytime afternoon hours for August 18,
2007 are also analyzed to determine the state of the boundary layer environment before TS
Erin moved into the region. Results show once again that the gradients caused by the
anomalously wet soils (in the Wet run) adjacent to the much drier soils of western
Oklahoma resulted in the most favorable surface flux plots showing distinctive regions of
heat flux where TS Erin reintensifies several hours later during the early morning of August
19, 2007 (Figs. 14, 15). The least favorable flux plots for storm reintensification were
generated by the Dry run.
A LLJ was readily observed in observational maps at 850 mb during the time of TS
Erin’s reintensification. This LLJ fed into the storm from the southeast adding warm, moist
air into the storm, and may have also played a role in sustaining the storm inland. However,
results already discussed also indicate that the soil moisture may have further impacted the
evolution of the storm, sustaining and leading to the evolution as an inland tropical system.
To further delineate this, parcel trajectory maps were generated for the surface, 850 and
700 mb level to assess the moisture transport originating outside of the LLJ. Recall that the
mesoscale environment discussed earlier in Sect. 4 shows that TS Erin was uncapped at the
time of reintensification (Fig. 4d). Parcel trajectory plots for the surface show most parcels
originating from the south and southeast (in all runs), away from the wettest soils observed
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Fig. 13 Model-generated reflectivity dBZ for a dry, b standard, and c wet runs. d Observed NEXRAD
mosaic included for comparison. Model runs failed to capture the eye-like feature as observed by NEXRAD
in Oklahoma at the time; however, originating in a maritime tropical air mass, similar to
the findings of Monteverdi and Edwards (2010). However, trajectory plots at 850 and
700 mb shift parcel trajectories to regions southwest and west of the main reflectivity core.
Warmer and dry air entrainment such as this typically weakens tropical storms and most
likely acted as a limiting factor into the time frame of TS Erin’s reintensification (Fig. 16).
A cross-sectional analysis of he and vertical velocities along a north/south axis through
Oklahoma at 1100Z was also generated. This was done to obtain a better understanding of
how the wet surface may have impacted the vertical structure of the atmosphere during TS
Erin’s passage through Oklahoma (Fig. 17). It is clear in the images that the Wet run had
the largest he values (a more unstable environment) and the greatest vertical velocities
through the depth of the cross-sections. Vertical velocity and he values were the weakest in
the Dry run.
Past studies examining the meteorological impacts of excess soil moisture such as from
irrigation show a feedback of soil moisture values into the boundary layer mainly affecting
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Fig. 14 Surface latent flux W m-2 (Row 1), sensible heat flux W m-2 (Row 2), and surface temperature
plots Kelvin (Row 3) for 2200Z August 18, 2007. (Column 1) dry, (Column 2) standard, and (Column 3) wet
runs. Region of reintensification is circled for clarity
average maximum and minimum temperatures and convergence above the inundated
region (Mahmood et al. 2004). To determine the soil moisture feedback into the overlying
boundary layer, its effect on temperature and dew point temperature in each of the three
model runs, Skew-T plots were generated at latitude and longitude locations at the different
Oklahoma Mesonet sites. These sites include locales where direct observations at the time
of TS Erin’s passage were recorded to compare to forecast model environments. Oklahoma
Mesonet sites selected include Bessie (35.24°N, 99.03°W), Ft. Cobb (35.08°N, 98.27°W),
El Reno (35.32°N, 98.02°W), Hinton (35.29°N, 98.28°W), Watonga (35.50°N, 98.31°W),
Weatherford (35.30°N, 98.46°W), Kingfisher (35.52°N, 97.54°W), Fairview (36.15°N,
98.29°W), Putnam (35.53°N, 98.57°W), Erick (35.12°N, 99.48°W), Lahoma (36.23°N,
98.06°W), Butler (35.35°N, 99.16°W), and Camargo (36.01°N, 99.20°W). Parameters
analyzed and compared to actual observations for 1100Z are surface temperature and
surface wind direction (Fig. 18). Surface wind speeds were difficult to accurately discern
from the generated Skew-T diagrams, as winds plotted are not necessarily at the surface
level. Findings show additional evidence that the Wet run generated the closest to observed
forecasts at 1100Z and generated the most similar surface wind direction forecasts compared to the Control and Dry runs. The average surface temperature for the Wet run was
overestimated by the WRF model by 3.8°F, and by 4.7°F in the Control run, and 4.5° F in
the Dry run. Wind was best forecast in the Wet run as well, with four of thirteen sites
matching observed surface wind directions, with the Control and Dry runs matching two of
thirteen sites for surface wind direction (Fig. 18).
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Fig. 15 Plots of surface latent heat flux A1–A3 W m-2, sensible heat flux B1–B3 W m-2, and ground
surface temperature C1–C3 Kelvin for (Column 1) Dry run, (Column 2) Standard run, and (Column 3) Wet
run for August 19, 2007 just prior to organization and reintensification at 0700 UTC (0200 LST). The region
of reintensification in the ‘‘wet’’ run has the largest latent heat flux. The region of reintensification and eye
generation is circled for clarity. The Wet run provides the strongest latent heat flux gradient
Model plots of dew point temperature through the domain show the highest dew points
present over Oklahoma in the Wet run, with the lowest dew point temperature forecasts in
the Dry run (Fig. 18). Oklahoma Mesonet observations are not available for dew point
temperatures; therefore, only surface temperature and relative humidity were compared to
model output. Since relative humidity is most commonly calculated from a known temperature and dew point temperature, relative humidity was computed for each Oklahoma
Mesonet site listed above and averaged for all three runs and compared to the average
observed relative humidity observed at the same Oklahoma Mesonet stations. Results
indicated that the Wet run most accurately forecasts surface dew point temperatures with
the average relative humidity differing only by 1.35% below the average observed relative
humidity (91.5%). The Control run differs by 7.2% (below observed), and the dry differs
by 8.5% (below observed).
5 Conclusions
Analyzing different operational products such as surface maps, soil temperature and soil
moisture data from the Oklahoma Mesonet, Storm Prediction Center mesoanalysis products, radar data, ARM data, and synoptic-scale maps suggests a possible link between
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Fig. 16 Reflectivity and parcel trajectory lines (red lines) at the surface for the Dry (a), Standard (b), and
Wet (c) run, at 850 mb for the Dry (d), Standard (e), and Wet run (f), and 700 mb for the Dry (g), Standard
(h), and Wet (i) run. Although parcels at the surface originate to the south of the storm, at 850 and 700 mb
parcel trajectories originate over regions with wetter soils with an uncapped boundary layer at the time of
reintensification. This suggests possible moisture transport from the warm and wet surface layer into upper
anomalous soil moisture conditions over Oklahoma and the reintensification of TS Erin
over west-central Oklahoma in the early morning hours on August 19, 2007. A large
positive soil moisture anomaly present from a spring and early summer that was the second
wettest on record for Oklahoma (Yueh et al. 2008; Arndt et al. 2009) appears to have
generated an environment thermodynamically favorable for warm core reintensification in
the vicinity where TS Erin reintensified. The different soil moisture parameters, mesoscale
parameters, and boundary layer parameters analyzed support the possibility that an antecedent moist environment allowed for the reintensification of TS Erin.
Verification of the impact of anomalously wet soils through the application of synthetic
case studies using high-resolution WRF-ARW 3.2 simulations reveal that a more conducive storm environment for tropical storm sustainment and reintensification was generated
by the Wet run rather than the Control and Dry runs. Higher surface winds were observed
to the right of the storm center in the model, weaker winds at 850 mb, highest latent heat
flux values, greatest he values, and vertical velocities were simulated most realistically by
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Fig. 17 North/south cross-sectional plots through Oklahoma showing contours of equivalent potential
temperature and colored regions up upward vertical velocities. a is the dry run, b is the standard run, and c is
the wet run. Vertical velocities are colored red for upward vertical motion and blue for downward vertical
motion. The wet run forecasts the strongest vertical velocities that happen to exist where two theta-ridges
meet between 100 and 200 km (x-axis)
the Wet run, along with the most realistic surface temperatures, dew point/relative
humidity, and surface wind direction forecasts. These findings all support the possible
surface feedback from the anomalous soil moisture in creating a favorable environment for
the intensification and the track of TS Erin through Oklahoma.
Combining the observational support with numerical model results indicates that
anomalously wet soils may have played an active role in the inland evolution of TS Erin
over Oklahoma. However, uncertainty still remains regarding the temporal feedback of the
soil moisture anomaly on the storm reintensification. Model results show that wetter soils
simulate the most realistic life cycle of TS Erin indicating that the antecedent amount of
soil moisture does plays role in TS Erin’s evolution; however, the limitations because of
the simple Slab model used in this study need to be highlighted. While this study should
not be taken as the conclusive proof that the observed soil moisture anomaly in Oklahoma
at the time of TS Erin’s reintensification did cause the storm to restrengthen, it is readily
apparent that synoptic-scale features played a limited role in the reintensification. This
elevates the role of mesoscale features such as soil moisture and how it feeds back into the
storm environment as a possible cause for the inland reintensification of TS Erin. Our
results support the perspective that antecedent soil moisture conditions may influence the
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Fig. 18 Surface dew point plots (K) over Oklahoma for the dry (a), standard (b), and wet (c) runs at 1,100
UTC. The Wet run clearly has the highest dew point plots. Because Mesonet stations do not record
observations for dew point temperatures, but do record relative humidity data, model relative humidity
forecasts were generated from model Skew-T diagrams for each respective Mesonet latitude and longitude.
The wet model run generated the closest forecast to observed values in all fields (relative humidity, air
temperature, and surface wind direction). Surface wind speeds were not included, as the generated Skew-T
has too many overlapping wind barbs near the surface to determine the forecast speed accurately
post-landfall evolution of a tropical storm system as the storm moves inland through the
subsequent generation of enhanced surface moisture and heat fluxes. These findings are
consistent with those of Chang et al. (2009), Emanuel et al. (2008), and Kishtawal et al.
(2010). Anomalous soil moisture appears to play a role in TS enhancement and should be
reviewed and studied further for consideration in forecast models and operational synthesis
dealing with tropical storm system track and intensity forecasts further inland.
Acknowledgements Study benefited in part from NASA Earth and Space Science Fellowship (Dr. W. W.
Wang), and the DOE—ARM 08ER64674 (Dr. Rick Petty), and discussions with Prof. Marshall Shepherd at
the University of Georgia, and Dr. Jeff Basara at the University of Oklahoma. Special thanks are given to
Dr. Michael Baldwin and Dr. Jonathon Harbor of Purdue University and to Daniel McCarthy of the
Indianapolis National Weather Service for continued feedback and support of the research project.
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