COMPARISON OF Ka-BAND AND POLARIZED S-BAND MOMENT PROFILES FOR INFERENCE ON
MIXED-PHASE CONDITIONS
Christopher J. Johnston a*, David J. Serke a, Andrew L. Reehorst b, Marcia K. Politovich a
a
NCAR- National Center for Atmospheric Research
Research Applications Laboratory
Boulder, Colorado
b
1.
NASA Glenn Research Center
Cleveland, Ohio
INTRODUCTION
In-flight icing is a contributing factor to many
aviation accidents, and the reliable detection of this
hazard is a major concern to aviation safety. As an
aircraft flies through supercooled liquid water (SLW),
the droplets freeze onto the leading edges of the
airframe and consequently alter the aerodynamic
characteristics of the aircraft. In the past 30 years,
several advances in the research and detection of SLW
have been made, including the development of the
National Center for Atmospheric Research’s (NCAR)
Current/Forecast Icing Product’s (CIP/FIP), the
Aviation Digital Data Service (ADDS) and the NASA
Icing Remote Sensing System (NIRSS).
Figure 1. NIRSS hardware located at Platteville,
Colorado.
NIRSS (fig. 1) is a prototype system that uses three
vertically pointing sensors; a vertically pointing 9 mm
*Corresponding author address: Christopher J.
Johnston, National Center for Atmospheric Research,
PO Box 3000, Boulder, CO 80307, chrisj@ucar.edu
wavelength Doppler Metek Ka-band radar, a Vaisala
model CL-51 laser ceilometer and a Radiometrics
Corporation model 3000 multi-channel microwave
radiometer (Reehorst et al., 2006). When combined, all
three of these sensors assist in warning for potential
areas of SLW related to in-flight icing.
An upcoming improvement in the polarization of
the network of countrywide Next-Generation Radars
(NEXRADs) has led to research investigating the
ability of polarized radar in detecting SLW. From
November 2010 to May 2011, NIRSS was relocated
from Cleveland, Ohio to Platteville, Colorado (Serke et
al., 2011). The goal of bringing NIRSS to the Front
Range of Colorado was to determine the relationship
Figure 2. Colorado State University’s CHILL radar
located near Greeley, Colorado.
between reflectivity in NIRSS’ vertically pointing 9
mm wavelength Metek Ka-band pulsed Doppler radar
and Colorado State University’s CHicago, ILL (CSUCHILL) scanning 10 cm wavelength pulsed S-band
polarized radar located near Greeley, CO (fig. 2) in
scenarios of known in-flight icing conditions. NIRSS
was located south of CSU-CHILL at an azimuth of
~195° at a range of 30 km, in an unblocked section of
the radar scan range. The purpose of this work was to
determine the practicality of using only S and/or K a
reflectivity profiles in icing and non-icing cases.
atmospheric water vapor and temperature profiles
(Solheim et al., 1998). This information is ingested into
NIRSS’ fusion algorithm which then allows the system
to detect the location of subfreezing layers and the
existence of SLW. For this study, the absorption of
liquid water found from the DWR was compared to the
ILW output of the radiometer.
2. BACKGROUND
2.2
2.1
Review of Error Sources
Radar Methodology
Previous research (Ellis and Vivekanandan, 2011)
shows that the difference in reflectivity (dB) of
collocated, matched beamed radars, one sensitive (Kaband) and the other non-sensitive (S-band), is known as
the Dual Wavelength Ratio (DWR), which is
proportional to liquid water content. In our study the
radars were not collocated, although they had identical
beamwidths of approximately one degree. In essence,
the authors’ approximated the DWR with vertical Kaband profiles and S-band range height indicators (RHIs)
over NIRSS from the CSU-CHILL radar. Although this
method is unorthodox compared to other radar
techniques, the authors’ sought to investigate if there
was any correlation with two non-collocated radars of
differing wavelengths.
One of the fundamental properties of the Ka-band
radar is that it emits higher frequencies than most other
radars (with the exception of the V-band and W-band
radars). This property makes the Ka-band subject to
higher atmospheric absorption and attenuation.
However, this provides very high resolution of
reflectivity and velocity data, a high data renewing rate
and does not affect the ability of the radar to measure up
to heights of 15 km. The advantage of the high
resolution measurements is the Ka-band’s ability to
detect small cloud droplets. This is unlike lower
frequency/larger wavelength radars (such as S-band),
which detect atmospheric particles of much larger sizes.
This in turn makes the use of DWR a very reliable
source of information for liquid water content,
especially in detecting SLW droplets within an
atmospheric column.
In winter clouds, SLW can be suspended in the
atmosphere (at temperatures below 0°C) until nearby ice
crystals remove the liquid water from the air through the
process of rapid evaporation and deposition onto the
growing ice crystals (Rogers and Floyd, 1989).
The Radiometrics model 3000 multi-channel
radiometer passively collects incoming microwave
radiation using a number of channels in the K and Vbands of the electromagnetic spectrum. This allows the
radiometer to derive integrated liquid water (ILW),
Rayleigh scattering describes the elastic scattering of
light by spheres which are much smaller than the
wavelength of light. Errors in radar analysis can occur
due to the violation of the Rayleigh scattering
approximation which is known as Mie scattering. This
occurs when radar backscattering by targets have
dimensions somewhat greater than 1/10 the wavelength
of the radar but less than several radar wavelengths
(Rinehart, 2010).
Figure 3. Ka-band Mie scattering transition (green
text, bottom right).
The Ka-band radar operates in the Mie scattering
regime
when
measuring
backscatter
above
approximately 20 dBZ (Fig. 3, green). For this analysis,
the gate-to-gate differences in the S-band and Ka-band
profiles were computed above the Ka-band’s Mie
scattering layer (see case studies section revealed in
section 3.1).
2.3 Pilot Reports
Pilot Reports (PIREPs) are voluntary reports made
by pilots to report on the presence or absence of inflight icing conditions and other weather-related
conditions. Both a subjective icing severity (trace, light,
moderate, heavy or severe) and icing type (rime, clear
or mixed) are included in each report. PIREPs are the
only means of in-situ diagnoses of actual atmospheric
conditions encountered by pilots and their aircraft in the
absence of expensive icing research flights or specially
instrumented fleet aircraft (Johnston et al., 2011).
3.
3.1
ANALYSIS AND DISCUSSION
Dual Wavelength Ratio Case Studies
Four example DWR case studies are presented here
to illustrate the significance of the DWR approximation
with coincident ILW from the radiometer. Each case
represents either low or high ILW and is compared to
the absorption of reflectivity (dB) over the depth of the
layer above the Mie scattering regime for the Metek Kaband radar. In each case CSU-CHILL was operated in
the RHI scanning mode.
3.1.1 DWR Case Study December 15, 2010
Late on 15 December 2010, a strong arctic cold front
crossed from north to south across the urban corridor of
Colorado's Front Range. At 22:52 UTC GOES West
infrared satellite imagery displayed a shallow layer of
stratiform clouds with cloud top temperatures near -26
°C over Platteville. This temperature was within the
range that could support SLW droplets. Only four hours
prior to this cold frontal passage a severe PIREP was
reported near Cheyenne, Wyoming (108 km north of
Platteville). Seen in figure 4 left column (blue shaded
area) the Metek Ka-band was in the Mie regime from
the surface to ~ 3 km.
Above the Mie regime to the minimum reflectivity
at the top of the layer, DWR was calculated by
differencing the reflectivities of the Metek Ka-band
from CSU-CHILL S-band (fig. 4). At this time, the
radiometer calculated an ILW value of 0.05 g m-2 which
compared very well with the shallow DWR value of 6
dB km-1.
Figure 4. 22:52 UTC CSU-CHILL S-band and Metek
Ka-band reflectivity [dBZ] versus height [km] plot (top)
and calculated DWR reflectivity [dB] versus height
[km] plot (bottom) for December 15, 2010.
3.1.2 DWR Case Study December 30, 2010
Late in the day on 30 December 2010, an
intensifying synoptic low created a modest upslope
flow along the eastern plains of Colorado’s Front
Range. At 20:52 UTC GOES West infrared satellite
imagery revealed fairly deep stratiform clouds with
cloud top temperatures close to -38 °C over Platteville.
The Mie regime of the Metek Ka-band radar was from
the surface to ~ 1.4 km (shown in fig. 5, blue shaded
area).
DWR again was calculated above the Mie regime to
the minimum reflectivity at the top of the layer (fig. 5).
The radiometer measured an ILW of 0.21 g m-2. The
DWR calculation was very high with a value of 30 dB
km-1. This value corresponded positively with the
radiometers high ILW measurement.
regime of the Metek Ka-band was from the surface to ~
3.4 km (shown in fig. 6, blue shaded area).
DWR was computed from the top of the Mie regime
to the minimum reflectivity at the top of the layer (fig.
6). The radiometer measured a very low ILW value of
< 0.01 g m-2 which compared well to a low DWR of 2.7
dB km-1.
Figure 5. 20:52 UTC CSU-CHILL S-band and Metek
Ka-band reflectivity [dBZ] versus height [km] plot (top)
and calculated DWR reflectivity [dB] versus height
[km] plot (bottom) for December 30, 2010.
3.1.3 DWR Case Study December 31, 2010
Early in the day on 31 December 2010, the synoptic
area of low pressure that affected the Front Range of
Colorado the previous day was continuing to intensify.
At 00:22 UTC GOES West infrared satellite imagery
exposed a large band of deep stratiform clouds that
extended across much of northeastern Colorado,
including the Platteville area. The cloud top
temperature was near -52°C over Platteville, which
supported the range of temperatures for SLW,
especially in the shallow parts of the cloud. The Mie
Figure 6. 00:22 UTC CSU-CHILL S-band and Metek
Ka-band reflectivity [dBZ] versus height [km] plot (top)
and calculated DWR reflectivity [dB] versus height
[km] plot (bottom) for December 31, 2010.
3.1.4 DWR Case Study May 18, 2011
On 18 May, 2011 a synoptic low developed south of
Denver, Colorado which was influenced largely from a
strong jet aloft. In addition, weak convection developed
throughout the day. At 19:51 UTC GOES West Infrared
satellite imagery showed a very shallow layer of
cumulus clouds over Platteville. The cloud top
temperatures above Platteville were approximately -28
°C which were supportive for droplets of SLW. The
Mie regime of the Metek Ka-band was from the surface
to ~ 6.5 km (fig. 7, blue shaded area).
As before, DWR was measured above the Mie
scattering regime to the top of the layer (fig. 7, bottom).
The radiometer approximated a moderate to high ILW
value of 0.18 g m-2 while the DWR computation came
out to be 12 dB km-1. Again a high correlation was seen
between the radiometer ILW and the DWR
approximation.
3.1.5 Dual Wavelength Ratio Analysis Summary
Overall, the DWR calculations compared strongly
with the NIRSS radiometer values of low ILW cases (<
0.10 g m-2) and moderate to high ILW cases (≥ 10 g m2
). As further research is conducted in the use of
polarized radar to detect in-flight icing conditions, this
simple radar comparison method may be implemented
as a warning tool to inform pilots, air traffic control and
other aviation personnel on the presence of in-flight
icing conditions. Further DWR case studies will be
analyzed in upcoming years after NIRSS is relocated to
Cleveland, Ohio.
3.2 Ka-band Profile Matching to Pilot Reports
In addition to comparing the DWR calculations to
ILW values, an analysis was also completed which
evaluated Ka-band profiles against moderate or greater
PIREPs within 25 km of the Cleveland-Hopkins
International Airport in Cleveland, Ohio between the
years of 2007 to 2009.
Figure 8. Profile depicting mean single and multiplelayer icing profile characteristics: 1. Slope above Mie
region (-51 dB km-1), 2. Second layer separation (1.2
km), 3. Second layer max reflectivity (-8.5 dBZ).
Figure 7. 19:51 UTC CSU-CHILL S-band and Metek
Ka-band reflectivity [dBZ] versus height [km] plot
(bottom left column) and calculated DWR reflectivity
[dB] versus height [km] plot (top right column) for May
18, 2011.
Ka-band profiles during in-flight icing conditions
were categorized into single layer and multiple-layer
scenarios, where single layer scenarios had reflectivity
to the noise threshold of the Ka-band radar (~40 dBZ)
and multiple-layer scenarios were separated a minimum
distance of 0.1 km from each layer. Figure 8 depicts the
mean single and multiple-layer icing profile
characteristics. In total there were 56 icing cases studied
34 % of those were multiple layer cases and the
remaining 66% were single layer cases.
Besides matching profiles to times of moderate or
greater PIREPs, there were 21 non-icing PIREPs
examined as well from 2007 to 2009. These cases
were sorted into only single layer scenarios. The 55.8
dB km-1 slope of the Ka-band reflectivity above the Mieinfluenced cloud layer was slightly lower for icing
versus non-icing cases, due to the presence of SLW.
Moreover, a synoptic weather breakdown of single
and multiple-layer cases (fig. 9) was created to show
which synoptic event was the likely cause of each radar
profile type (Bernstein et al., 1997). These results
indicate that for most synoptic weather events, multiplelayer cases are most likely to occur behind occluded
fronts, cold fronts, ahead of stationary fronts and are
influenced by shortwaves. Conversely, single layer
cases are most likely to occur ahead of cold fronts,
warm fronts, behind cold fronts and are influenced by
shortwaves.
Figure 9. Single-layer (blue) and multiple-layer (red) synoptic weather breakdown.
4. SUMMARY
In-flight icing is known to be one of the most
hazardous weather situations contributing to a great deal
of aviation accidents (NTSB, 2005). Thus, it is
extremely important that in-flight icing conditions are
detected reliably and accurately and that a warning
system is developed to warn the aviation community.
This study was undertaken to determine the usefulness
of the DWR for detecting cases of low, moderate and
high ILW values. Several case studies were touched on
to demonstrate how the DWR may be utilized in winter
conditions to show where possible SLW exists. In
addition Ka-band profiles were matched to times of
icing and non-icing PIREPs in Cleveland, Ohio.
Overall, there were several key points to this study. The
DWR approximation value was consistent for both low
and high ILW value cases. The slope of the Ka-band
reflectivity above the Mie-influenced cloud layer was
slightly lower for icing versus non-icing cases.
Multiple-layer clouds made up 34% of examined icing
cases. Most importantly, the main reflectivity value of
the upper cloud was frequently below the minimum
detectable by research or operational S-band radars.
Future work with NIRSS will include exploring the
usefulness of Doppler radar spectra on detecting SLW
and testing the abilities of a new volumetric scanning
method with the addition of scanning radiometers to the
NIRSS prototype.
5. ACKNOWLEDGEMENTS
The National Center for Atmospheric Research is
sponsored by the National Science Foundation. Any
opinions,
findings,
and
conclusions
or
recommendations expressed in this publication are
those of the authors and do not necessarily reflect the
views of the National Science Foundation. The authors
would like to thank Colorado State University for
providing the CHILL radar data.
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