Upper-tropospheric Moisture Distribution in the Vicinity of Convection
Abstract
The water vapor feedback on climate change depends in part on the mechanisms
that transport moisture from tropical convection into the subtropics. Observational evidence that
moist air is mixed into the environment from all vertical levels within convectively active
regions is presented in this paper. Atmospheric soundings in the western tropical Pacific during
TOGA-COARE show the frequent occurrence of thin moist layers in an otherwise dry
environment. These layers are likely the result of the vertical interleaving of moist and dry air
masses in the vicinity of tropical convection. This interleaving would be in contrast to
detrainment near the tropopause providing the sole source of subtropical air. This process,
however, is consistent with recent high-resolution tropical convection models that maintain
constant relative humidity in climate warming scenarios.
1. Introduction
The mechanisms for moisture transport from tropical convection to the dry subtropical
environment are critical to the water vapor feedback in climate change (Held and Soden, 2000).
Several studies have shown how the horizontal distribution of moisture in the tropics can be
explained simply by advection and subsidence of air exiting regions of active convection
(Pierrehumbert, 1998; Salathé and Hartmann, 1997; Sherwood, 1996) and that this process may
be captured by a global model (Salathé and Hartmann, 2000). However, there remains some
uncertainty regarding the way air exits the convective regions – either primarily through
detrainment just below the tropical tropopause (Hartmann and Larson, 2002; Lindzen, 1990) or
through a more complex set of parcel trajectories that mix air at multiple levels in the vertical
(Held and Soden, 2000).
A picture of these complex trajectories emerges from a recent study with an idealized
model. Iwasa et al. (2002) performed model simulations that show moist air entering the
subtropics at all levels below the tropopause. They find that vertical motions in the free
troposphere are purely radiatively driven, while convectively driven circulations are short-lived
and confined to a narrow convectively active region. As a result, relative humidity simulated in
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the free troposphere simply depends on the vertical distance air has subsided since exiting the
convectively active region.
In global warming experiments with the same models (Iwasa et al., 2004), as the
atmosphere warms, the radiative cooling rate does not change. Consequently, the circulation
remains geometrically similar under warming such that air parcels at a given location experience
similar subsidence since exiting convection in the control and warm scenarios. Since the
subsidence remains similar, and the relative humidity depends on the subsidence, relative
humidity is constant under global warming. Thus, if near-saturated air enters the large-scale
subsiding environment from all vertical levels, relative humidity tends to be preserved as the
atmosphere warms. This result is in accordance with the simple model of the water vapor
feedback originally formulated in radiative convective models in the 1960s (Manabe and
Wetherald, 1967) where relative humidity was proscribed as constant.
Iwasa et al. (2002) contrast their depiction of the tropical water vapor transport with a
“chimney” model, attributed to Lindzen (1990). In the chimney model, air parcels exit
convection primarily just below the tropopause, which creates a cold trap that controls
subtropical relative humidity. If the tropical tropopause rises and cools in response to global
warming, the cold trap would cause specific humidity to decrease yielding a negative feedback.
However, as described in Hartmann and Larson (Hartmann and Larson, 2002), detrainment near
the tropopause out of convective areas is driven by the gradient in radiative cooling, and the
temperature is relatively constant at 200 K independent of surface warming. Thus, while upperlevel detrainment is clearly a factor in tropical water vapor transport, the evidence suggests that it
does not lead to a negative feedback. Below the detrainment level transport is characterized by
horizontal mixing, which dilutes this detrained air with moister air originating lower within the
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convective region. Mixing may also bring in drier air of extra-tropical origin (Pierrehumbert,
1998)
In this paper, we examine the vertical moisture distribution observed at subtropical
sounding stations during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere
Response Experiment (TOGA-COARE) (Webster and Lukas, 1992) and present evidence that
subtropical air is detrained from convectively active regions at all levels. These soundings show
vertical moisture features that are consistent with mechanisms simulated by Iwasa et al. (2004;
2002) that provide moisture transport at all vertical levels. In particular, these features are not
consistent with the idea that all or even most air in the subtropics subsides from convective
outflow near the tropopause.
Analyses of atmospheric soundings from TOGA-COARE (Brown and Zhang, 1997; Mapes
and Zuidema, 1996; Zhang and Chou, 1999; Zhang et al., 2003) reveal important variability in
the moisture field in the areas surrounding tropical convection. Brown and Zhang (1997)
illustrate a wide, bimodal distribution of free-tropospheric relative humidity, with probability
peaks corresponding to periods of relative drought and of enhanced precipitation. Mapes and
Zuidema (1996) describe dry tongues of air that intrude into otherwise moist profiles near
regions of active convection. This moisture structure suggests that active convection and dry
subsiding regions are distinct air masses. Transport of moisture between them is highly variable
and depends upon complex mesoscale circulation patters surrounding tropical convection.
Analogous to dry tongues, away from active convection, rawinsondes frequently show
moist tongues or layers embedded in the dry environment. In tropical soundings taken during
TOGA-COARE, there frequently occur layers of anomalously high relative humidity of a few
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Figure 1. Sounding of Relative Humidity (left, solid), potential temperature (left, dashed),
wind speed (right, solid), and wind direction (right, dashed) observed over Guam (13.6 N,
144.800 E) during TOGA-COARE at 11 UTC 28 February 1993. Three moist layers are
centered at 460, 400, and 350 hPa
tens of millibars vertical extent within an otherwise dry, cloud-free environment. Dry tongues
suggest an interleaving of moist and dry air masses.
Below we shall discuss the structure of these layers, their frequency, and their vertical and
horizontal distribution. We shall also explore how these layers could be maintained while
advected over significant distances. Finally, we discuss how these observations relate to the
various theories of water vapor transport in the tropics and the consequences for the water vapor
feedback.
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Figure 2. Cloud top pressure (shading; dark indicates high clouds) derived from
Geostationary Meteorological Satellite (GMS) on 27 (top) and 28 (bottom) Feb 1993.
Streamlines are for concurrent 300 hPa winds from the NCEP/NCAR reanalysis.
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2. Structure of moist layers
An example of moist layers is shown in the sounding plotted in Figure 1. This sounding
was observed over Guam (13.6 N, 144.800 E) during TOGA-COARE at 11UTC 28 February
1993. Three moist layers are centered at 460, 400, and 350 hPa. As indicated by the potential
temperature profile, the moist layers tend to have lower static stability than their environment
indicating they are actively mixed. Furthermore, each moist layer is associated with a shift in the
horizontal wind speed and direction, which suggests that the layers originate in a different
airmass from the dry background environment.
The large-scale structure associated with this sounding is shown in Fig 2. Shading shows
the cloud top pressure derived from Geostationary Meteorological Satellite (GMS)
observations as part of the International Satellite Cloud Climatology Project (ISCCP) (Rossow
and Schiffer, 1999; Schiffer and Rossow, 1983); dark shades indicate high clouds. Streamlines
are for the 300-hPa winds from the NCEP/NCAR reanalysis (Kalnay et al., 1996). The top panel
is 24 hours before and the lower panel coincident with the sounding. Guam is located at the point
marked “G” (13.6 N, 144.800 E). In the period prior to the sounding, the station was in the
outflow of a deep convective system to the southeast. This system dissipates just before the
sounding was launched as winds shift to the northeast and air is supplied from an anticyclone in
the arid subtropics. The wind profile in Fig 1 is remarkably consistent with this large-scale
transition. The southerly winds in the moist layers suggest moist air is supplied by the convective
region to the south. The more easterly winds in the dry environment are consistent with dry air
flowing from the subtropical anticyclone.
Thus, this profile shows the vertical interleaving air masses. Such a dry air mass, as
described in Salathé and Hartmann (1997), would have spent several days subsiding after exiting
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a convectively active region. The satellite image suggests the moist air mass is the product of
active tropical convection that persisted until a few hours before the sounding was taken. The
convectively-conditioned moist air is evidently drawn into the environment at multiple levels in
the vertical. Furthermore, the relative humidity of each moist layer is comparable, with the
lowest layer having the highest relative humidity. This observation is inconsistent with the moist
air masses subsiding from the same original altitude, close to the tropopause. Rather, it appears
that each parcel subsided a similar amount since exiting the convective region, and thus
originated from three different levels.
In the following sections, we present the results of analysis of a large collection of such
layers to examine how frequent this phenomenon is and thus whether it is a robust feature of the
tropical moisture transport processes.
3. Composite structure of layers
In Mapes and Zuidema (1996), dry tongues were identified by a sharp relative humidity
gradient at the bottom of the tongue. While Mapes and Zuidema (1996) considered dry tongues
intruding into the moist convective environment, we consider the layers as moist tongues
intruding into the dry subsiding environment. By analogy, we identify moist layers as having a
sharp RH gradient at their top. We further define the bottom of the layer as the level where RH
returns to the value at the top. Layers are required to be between 50 and 200 hPa in thickness and
are only selected from cloud-free soundings. Cloud-free soundings are identified by RH less than
80% at all levels above 800 hPa.
Using this definition, we searched 8,729 soundings from 35 stations archived from TOGACOARE for the period 1 November 1992 to 28 February 1993. The search revealed 5,108 cloud-
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Figure 3. Composite of 2,871 moist layers from TOGA-COARE soundings. Pressure is
normalized to the layer depth. Solid line is relative humidity in percent; dashed is potential
temperature perturbation from value at layer bottom.
free soundings and 2,871 moist layers. Clearly, over the whole tropical western Pacific during
the northern winter, moist layers are a common feature of the free troposphere
A composite profile can be made by averaging together many layers. The average is done
in a scaled vertical pressure coordinate so that the top and bottom of layers are at 0 and 1 in the
scaled pressure coordinate; that is, the vertical coordinate is scaled to match the top and bottoms
of all layers as they are composited. To show the background, the composite is extended above
and below the layer by 0.2 normalized pressure units. The resulting RH composite of all layers is
shown in Fig 3. Potential temperature is composited in a similar manner and is plotted in Fig 3.
The potential temperature at a normalized pressure of 1.2 is subtracted from each layer profile so
that potential temperature is in units of K relative to the bottom of the composite. As is seen in
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this figure, the composite shows the structure we attempt to search soundings for, a sharp RH
gradient at the top of the layer. On average, layer relative humidity peaks at 53% in a 30%
environment. Although the layers are selected according to the humidity feature, the composite
potential temperature shows increased stability near the top indicating that the layers tend to have
lower stability than the environment and suggesting they tend to be actively mixed.
4. Horizontal and vertical distribution of layers
Layered structures are distributed geographically as shown in Fig 4a. The average number
of layers per cloud-free sounding at a given site is printed at the site location and the distribution
is indicted by hand-drawn contour lines. Likewise, the mean relative humidity from 500 to 200
hPa (UTH) for these soundings is shown in Fig 4b. UTH is high in a band along the equator
while the layers are most frequent in the northwest and southeast corners of the region, the driest
portions of the domain. The asymmetric distribution of layers can be related to the UTH
distribution and the mean meridional winds. Gray contour lines indicate the meridional wind
from the NCEP-NCAR reanalyses (Kalnay et al., 1996) averaged over the period 1 November
1992 to 28 February 1993 (solid contour lines are at 0.5, 1, 1.5, and 2.0 m/s; dashed contours are
corresponding negative values; there is no zero contour). The southward and northward arrows
indicate meridional wind maxima that would tend to zonal asymmetry and supply dry air. Thus,
layers are not confined to the regions around convection, where UTH is high, but extend well
into the subtropics and appear to be associated with the movement of dry air into the tropics.
Fig 5 shows the vertical distribution of moist layers from all layers used in the composite.
This distribution is similar to that found for dry tongues in Mapes and Zuidema (1996), and, as
discussed there, the drop off at upper levels may be due to the limitations of rawinsonde moisture
measurements rather than to a physical mechanism. Layers appear at all levels between 700 and
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Figure 4 a) Number of layers per cloud-free sounding at each station, with hand-drawn
contour lines (black). Mean meridional winds (solid gray contour lines at 0.5, 1, 1.5, and
2.0 m/s; dashed contours are corresponding negative values; no zero contour) from NCEPNCAR reanalysis for period 1 November 1992 to 28 February 1993. b) Average uppertropospheric humidity from TOGA-COARE soundings used in layer analysis, with handdrawn contour lines.
250 hPa, peaking at 400 hPa. This vertical distribution suggests that moisture is transported from
convectively active regions into the large-scale dry environment by layers at all levels, not
simply at the top of convective towers or at the freezing level.
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Figure 5. Vertical distribution of moist layers during TOGA-COARE. Percent occuence is the
percent of total layers that occur in a given pressure interval.
5. Radiatively driven mixing in layers
The potential temperature profile in moist layers, as indicated in the example from Guam
(Fig 1) or the layer composite (Fig 2), suggest that the layer is undergoing vertical mixing, which
is suppressed at the top of the layer. The concept of “cooling to space”, as described by Mapes
and Zuidema (1996), suggests how radiation affects the temperature of the moist layer. At the top
of the layer, there is considerable amount of water vapor to emit or absorb radiation, yet there is
relatively little above to block its exposure to space. At night, the layer will cool rapidly at its top
as infrared radiation is emitted to space, and near noon, solar absorption and infrared emission
will tend to balance. Over time, the net effect is to cool the layer at its top, which helps produce
the stable lid at the top of the layer. This stability would tend to allow the layer to persist as it is
drawn well away from the convective source region.
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6. Discussion
In this paper, we have shown that interleaving of moist and dry layers is a frequent feature
in soundings observed in the eastern tropical Pacific during TOGA-COARE (November 1992 to
February 1993). Moist layers are found at all levels in the free troposphere below 250 hPa. These
layers are broadly distributed geographically, but appear to be most frequent in regions of low
upper tropospheric humidity. The layers are likely robust features as they are reinforced by
radiative cooling and mixing. From examining a single sounding in detail, it appears that the
layers result from the interleaving of convective and subtropical air masses.
The structure and distribution of these layers suggest a picture of tropical moisture
transport highly consistent with the modeling results of Iwasa et al. (2002) and with ideas
discussed in Held and Soden (2000). Convective motions – updrafts and downdrafts – are
constrained to the immediate vicinity of the cumulus tower while descending motion in the free
troposphere is driven by radiative cooling. Cumulus convection exists as full, deep towers only
for brief periods, 10% of the time in simulations by Iwasa et al. (2002).
Convective activity is transient in time and space, producing moist air masses that become
drawn into the large-scale environment. In the horizontal, the trajectory simulations performed
by Pierrehumbert (1998) show how air masses are extruded into filaments. The layers presented
here are the signature of this extrusion in the vertical, and suggest the following scenario for
moisture transport: Air is detrained from convective systems at various points in their life cycle
at various levels in the vertical. Decayed convection also may leave behind a column of moist
air, preconditioned to a constant relative humidity profile by convection. Shearing motions
interleave this moist air with dry air, forming the layers presented here. Dry air originates either
from subtropical air that has subsided a considerable distance or from the cold extra tropics. The
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stability of the layers, due to radiative cooling and overturning within the layer, allows the moist
air to be drawn deep into dry air masses. Over time, dynamic instability and cascading to smaller
scales will homogenize the vertical profile.
As shown by Iwasa et al. (2004), this sort of moistening process, combined with
radiatively-driven subsidence in the clear regions, would tend to maintain constant relative
humidity in a climate warming scenario. If the vertical motions remain geometrically similar in
the warming scenario, then the vertical humidity structure in the convective regions is simply
transferred to the subtropics. Assuming relative humidity within deep convection remains
constant, or nearly constant if precipitation efficiency increases, then a strong water vapor
feedback controlled by the exponential Clausius-Clapeyron relationship will result. Other
mechanisms that tend to decrease humidity must work against this strong positive feedback.
7. Figure list
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