We examine integrated water vapor fields and rain intensity patterns derived from the Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) for several rapidly deepening and non-rapidly deepening midlatitude cyclones in the North Atlantic. Our goal is to identify features in the satellite data unique to the rapidly deepening cases, and to explore how these data can potentially be used in the analysis and forecasting of these events

Katsaros, Kristina

Mcmurdie, Lynn

English

NASA Technical Reports Server

J1.4 N 9 3 - 1 0 3 4 9
SATELLITE DERIVED INTEGRATED WATER VAPOR AND RAIN
INTENSITY PATTERNS: INDICATORS OF RAPID CYCLOGENESIS
Lynn McMurdic and Kzistina Katsaros
Department of Atmospheric Sciences
University of Washington, Seattle WA 98195
1. INTRODUCTION
Rapidly intensifying extratropical cyclones are
accompanied by severe weather conditions and frequently
develop over the data sparse ocean areas. Although
operational numerical model performance has improved in
recent years, the strength can be poorly predicted, due in
pan to an inadequate initial analysis (Sanders, 1987).
Recent research has demonstrated the importance of moist
processes and latent heat release in the development of these
cyclones (e.g. Kuo et al., 1991). Hence, an adequate
representation of the moisture fields appears to be important
for accurate prediction of these storms (Reed et al., 1988).
Atmospheric water vapor and precipitation intensity
can be retrieved over the ocean from satellite microwave
radiometers. These fields have been used to delineate
frontal zones (McMurdie and Katsaros, 1985; Katsaros et
al., 1989), to characterize typical water vapor distributions
and content in storms occurring in various oceanic basins
(McMurdie and Katsaros, 1991), and to study moisture
pattern evolution in the Gulf of Mexico following the
passage of a cold front (Rabin et al., 1991).
In this paper, we examine integrated water vapor
fields and rain intensity patterns derived from the Scanning
Multichannel Microwave Radiometer (SMMR) and Special
Sensor Microwave/Imager (SSM/I) for several rapidly
deepening and non-rapidly deepening midladtude cyclones
in the North Atlantic. Our goal is to identify features in the
satellite data unique to the rapidly deepening cases, and to
explore how these data can potentially be used in the
analysis and forecasting of these events.
2. DATA ANALYSIS
The SMMR and the SSM/I are passive microwave
instruments on the Nimbus 7 and the Defense
Meteorological Satellite Program (DMSP) polar orbiting
satellites, respectively. Information on each instrument is
presented in table 1. The algorithms used to calculate
SMMR integrated water vapor (IWV) are given in Chang et
al. (1984) and Petty and Katsaros (1990) for the SSM/I.
They are valid over ocean areas and are as accurate as
radiosondes within a rms error of 2.1 and 2.0 kg nr2,
respectively. Since accurate measures of rainrate are
difficult to obtain from satellite, we use a normalized
polarization difference ^37) in the 37 GHz channel to
indicate rain intensity for the SSM/I cases. It is defined as
the measured 37 GHz vertically polarized brightness
temperature (Tb) minus the 37 GHz horizontally polarized
Th divided by the clear sky Tb polarization difference. Low
values (0.1 or 0.2) indicate high rain intensity and high
values (greater than 0.8) indicate no rain. For the SMMR
cases, a simple polarization difference in the 37 GHz
channel is used to indicate rain.
Thirty-four North Atlantic storms that occurred
during the winters of 1979 - 1981 (SMMR) and 1988
(SSM/I) are divided into two groups: 1) Rapidly deepening
Table 1 - Comparison of SMMR and SSMI instruments
SMMR SSMI
Dales
Swach width
Channels
Polarization
Resolution
(IWV.Pj,)
1979 -1985
780km
6.6. 10.7,18 GHz
21,37 GHz
Vertical and Horizontal
all channels
SO km x SO km
1987 -present
1394km
19.22 .^ 37. 85 GHz
Vertical and Horizontal
all channels. 2L2 in
vertical only
SOkmxSOkm
53
storms, where the storm experienced a deepening rate of
1.4 or greater for 24 hours (a deepening rate of one is
defined in Sanders and Gyakum (1980) as a drop of 24 mb
in 24 hours multiplied by (sin <t> / sin 60°) where o is the
latitude of the storm); and 2) Standard storms, where the
storm experienced a deepening rate less than 1.4. The
NMC hemispheric surface analysis is used to determine the
central pressure and the position of the surface fronts.
Water vapor fields and rainfall intensity are compared
during the incipient (more than twelve hours prior to the
period of most rapid deepening) and deepening stages.
IWV and P;7 fields from individual passes over a
storm are analyzed as follows. Only the passes that
sampled the majority of the storm near the low center are
included in the study. Maximum water vapor content in the
warm sector, or along the cold and warm fronts, is
estimated from the IWV patterns by picking the highest
contour (contour interval is every 2 kg nr2) in the analyzed
field along, or within 200 km of the frontal zones. In
addition, the area! extent of the region of high water vapor
content is estimated by counting the number of pixels
greater than 30 kg nr2, multiplying that number by the pixel
resolution and rounding to the nearest 103 km2. The region
considered does not extend south of 25°N or 5° of latitude
south of the southern portion of the cold front in order to
include just the warm sector and frontal zones sampled by
the satellite. The value 30 kg rrr2 was choosen arbitrarily
since it is approximately the average ma-iimum IWV content
for North Atlantic storms found by McMurdie and Katsaros
(1991). For the rain intensity fields, the areal extent of the
region of rain (quantified as fyj less than 0.8 for the SSM/I
and 37 GHz polarization difference of less than 30°K for
the SMMR cases) is calculated in the same manor.
Maximum water vapor, areal extend of high vapor content
and areal extent of precipitation from each pass over a storm
are averaged together for rapidly deepening storms and
standard storms. The averages in similar categories (e.g.
maximum water vapor content during the incipient stage in
rapid vs. standard cases) are tested to be significantly
different using the two sample t-test.
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Figure 1 - IWV from SSM/I for orbit 2970 at 10 UTC 16 January
1988. The field is contoured every 4 kg m'2 and the region greater
than 28 kg m'2 is shaded. Frontal analysis is estimated from the
f^fMC hemispheric surface charts at 06 and 12 UTC 16 January.
Figure 3 - IWV from SSM/I for orbit 3274 at 23 UTC 6 February
1988. The field is contoured every 4 kg m'2 and the region greater
than 28 kg m'2 is shaded. Frontal analysis is estimated from the
NMC hemispheric surface charts at 00 UTC 7 February.
3. WATER VAPOR
3.1 Rapidly peepening Caser 15 - 17 January 198R
The surface low of this system developed from a
weak trough of 1018 mb situated at 32°N 72°W at 00 UTC
16 January. At this time a strong shortwave trough was
situated along the east coast from Maryland southward. In
six hours the surface low deepened to 1008 mb and a
surface cold front is analyzed. By 12 UTC 16 January, an
upper-level cut-off low formed with a height of 558
decameters situated just west of the now rapidly deepening
surface low. The storm subsequently deepened to 992 mb
and moved zonally to 32°N 62°W by 00 UTC 17 January.
The storm achieved a maximum deepening rate of 1.5
The IWV field from the SSMI at 10 UTC 16
January, prior to deepening, is shown in figure 1. In the
region east of the cold front, along the prefront trough
(dashed line in figure) and south of the warm front, the
moisture content is high. Maximum values of 44 kg m*2
are evident along the trough. The region where the water
vapor content is greater than 30 kg rrr2 is 85.0 x 10* km2.
Figure 2 - IWV from SSM/I for orbit 2977 at 22 UTC 16 January
1988. The field is contoured every 4 kg m'2 and the region greater
than 28 kg m"2 is shaded. Frontal analysis is estimated from the
NMC hemispheric surface charts at 18 UTC 16 Jan. and 00 17 Jan.
SON I •'ON
Vapor (kgm-2)/
7"SCW ...__
.'ON
Figure 4 - IWV from SSM/I for orbit 3287 at 21 UTC 7 February
1988. The field is contoured every 4 kg m'2 and the region greater
than 24 kg m"2 is shaded. Frontal analysis is estimated from the
NMC hemispheric surface charts at 18 UTC 7 February and 00 UTC 8
February.
In figure 2, the IWV field at 22 UTC 16 January
after rapid deepening is shown. The water vapor content is
still high from the warm front southwards. Maximum
water vapor content of 40 kg nr2 is evident at the trough
and cold front, and the area! extent of water vapor content
greater than 30 kg m'2 is still large, approximately 95.0 x
104 km2. The water vapor pattern has evolved into a
comma shape, and the strength of the water vapor gradient
behind the cold front has increased.
3.2 Standard Cyclone: 6 - 8 February 1988
This storm developed from a wave on a pre-existing
surface cold front. At 12 UTC, a closed low of 1012 mb is
analyzed at 30°N 73°W at the cold front. The surface low
did not begin to deepen until 12 UTC 7 February when a
rapidly moving upper-level shortwave trough moved just
upstream of the surface low. The surface low progressed
northeastward to 46°N 54°W and deepened to 992 mb by 00
UTC 8 February. The storm had a maximum deepening
rate of 1.0.
54
J12
The IWV field at 23 UTC 6 February, prior to
deepening, is shown in figure 3. In contrast to figure 1,
high water vapor content is confined to a narrow band
along the front and in a narrow region south of the front.
The maximum water vapor content in this region is only 32
kg nr2, and the region where the IWV content is greater
than 30 kg nr2 is only 30.0 x 10* km2.
In figure 4, the IWV pattern at 21 UTC 7 February
is shown. At this stage, the low has moved substantially
northward and the water vapor content is significantly
lower than in figure 3. The maximum water vapor content
is 28 kg nr2 along the warm front. The next orbit over this
storm (Orbit 3288, not shown) samples the southern end of
the cold front. Eventhough the front is south of 30°N, the
maximum IWV content is also low, 36 kg nv2, and
confined to a narrow region along the front
3.3 Summary of all cases
It was shown above that the rapidly deepening case
exhibited substantially more water vapor content throughout
its development than the standard case. In order to examine
if this relationship holds for all cases, average maximum
water vapor content and average area of IWV content
greater than 30 kg nr2 were calculated for all the storms
examined to date and are displayed in tables 2 and 3
respectively. In table 2, the average maximum IWV content
is higher in rapidly deepening storms than in standard
storms during both stages. However, since the number of
samples is small and the standard deviations large, the
averages are not significantly different at a confidence level
of 90% or greater.
In table 3, average area of IWV content greater than
30 kg nr2 for storms is shown. Since the swath width of
the SMMR is half that of the SSM/I, it is only possible to
compare results from each instrument separately. In the
SMMR standard cases, during the inicipient stage, only
three of the eight storms had any IWV values greater than
30 kg nr2, and of those three, the area! extent was quite
small In contrast, the areal extent of high vapor content for
the SMMR rapidly deepening cases during their incipient
stage was on average a factor of ten times larger.
Therefore, despite the very large standard deviations and
small number of cases, the average area of IWV content
greater than 30 kg nr2 is significantly larger for SMMR
rapidly deepening cases at the 95% confidence level.
During the deepening stage, and for the SSM/I cases, the
differences between rapidly deepening cyclones and
standard cyclones are not significant due to large standard
deviations and small number of cases.
Table 2 - Average maximum water vapor content for
rapidly deepening and standard cyclones during the
incipient and deepening stages. Ave denotes the average in
kg nr2 , s.d. denotes the standard deviation and n the
number of cases used.
A) RAPIDLY DEEPENING
Incipient
ave. s.d. n
SMMR 27.6 7.3 6
SSM/I 36.7 7.0 3
together 30.4 8.1 9
B) STANDARD CYCLONES
Incipient
ave. s.d. n
SMMR 26.4 5.5 9
SSM/I 34.3 2.3 2
together 27.9 5.9 11
Deepening
ave, s.d. n
32.5 6.5 12
36.2 4.7 4
34.2 6.1 16
Deepening
ave. s.d.
31.3 5.0
35.0 1.4
31.7 4.8
Table 3 • Average area in warm sector where the IWV is
greater than 30 kg or2 for rapidly deepening and standard
cylones. Ave denotes the average area in 104 km2, s.d is
the standard deviation and n is the number of samples used.
A) RAPIDLY DEEPENING
Incipient
ave. s.d. n
SMMR 53.6 78.5 5
SSM/I 43.6 13.1 3
B) STANDARD CYCLONES
Incipient
ave. s.d. n
SMMR 5.5 9.1 8
SSM/I 34.3 21.2 2
4. RAIN
Deepening
ave. s.d. n
35.6 20.1 8
56.2 32.2 4
Deepening
ave. s.d. n
27 J 23.6 13
30.1 1.2 2
The rain distribution given by the P^-j fields from
the SSM/I for the two storms discussed in the previous
section are given in figures 5 and 6. The area covered by
precipitation is quite extensive in the rapidly deepening
case, figure 5. In addition, there are several areas of
intense rainfall, evident as ¥yi values of 0.2 or less located
along the trough, cold front and north and west of the
developing surface low. In order for the latent heat of
condensation (by precipitation) to influence the
development of the surface low, the laplacian of the
heating, not just the heating itself, is important (e.g., see
eq. 3 in Kuo et al, 1991). Near the low center and cold
front, non-raining areas are in close proximity to raining
areas. Although it is not possible to calculate the magnitude
of the latent heating, the spanal pattern of precipitation alone
indicates that the contribution to the development of the low
by diabatic heating may be important in this case.
In figure 6, the precipitation area is smaller than in
the rapidly deepening case. In addition, the regions of
intense rainfall are much fewer and smaller in this case.
Furthermore, the raining areas are more continuous in the
vicinity of the developing low center and the spatial pattern
of rain is less complex. Again, it is not possible to calculate
the magnitude of the latent heating; however, the rain
distribution indicates that the contribution by diabatic heat to
the low development may be less in this case than in the
rapidly deepening case.
In table 4, the average area covered by precipitation
in rapidly deepening and standard cyclones are presented.
Due to the different swath widths and the different ways of
indicating rain, the SMMR and SSM/I cases must be
considered separately. During the incipient stage, the
SMMR standard cases had very light rainfall, and three of
the eight cases exhibited no rain. The average rain area for
incipient rapidly deepening cyclones is significantly larger
than the average rain area for incipient standard cyclones at
the 95% confidence level. In addition, the average rain
areas are significantly different during the deepening stages
at the 90% confidence level. The differences between
rapidly deepening and standard cyclones for the SSM/I
cases are not significant due to the small number of cases.
5. SUMMARY
In this paper we present IWV and Pyj -analyses for
thirty-four rapidly deepening and standard cyclones that
occurred in the North Atlantic during the winters of 1979 -
1982 and 1988. We found that the maximum water vapor
content was not significantly different in these two types of
storms. Yet, during the incipient stage in the SMMR cases
analyzed, the area of IWV amounts greater than 30 kg nr2
in the warm sector and the area of precipitation were
significantly larger in rapidly deepening cyclones than in
standard cyclones. The examples presented in section 4
II
J13
\ 7QW Precipitation \
Normalized Polarization Difference '
SOW
70W
Raoidly \
Deepening \
Storm
Figure 5 - P37 from SSM/1 for orbit 2970 at 10 UTC 16 January
1988. The field is contoured every 0.2 and the region less than 0.8
indicates rain and is shaded. Frontal analysis is estimated from the
NMC hemispheric surface charts at 06 and 12 UTC 16 January.
Table 4 - Average rain area where P37 is less than 0.8 for
rapidly deepening and standard cyiones. Ave denotes the
average area in 10* km2, s.d is the standard deviation and n
is the number of samples used.
A) RAPIDLY DEEPENING
Incipient
ave. s.d. n
SMMR 12.8 8.7 5
SSM/I 94.6 203 3
B) STANDARD CYCLONES
Incipient
ave. s.d. n
SMMR 3.5 3.7 8
SSM/I 128.1 74.9 2
ave. s.d. n
17.8 10.0 11
72.2 32.1 4
ave. s.d. n
10.4 11.5 10
99.8 67.9 2
demonstrated that differences in rainfall intensity and
rainfall pattern variability may exist between rapidly
deepening and standard storms. However, more cases
need to be analyzed in order to quantify these differences.
The results obtained from the cases studied to date are
consistent with the hypothesis that moisture availability and
latent heating are important in the development of rapidly
deepening cyclones.
Acknowledgments - We are grateful to Erika Francis and
Frank Wentz of Remote Sensing Systems, Inc. who
generously provided the SSM/I data tapes. The SMMR
CELL-ALL data tapes were provided by the National
Space Science Data Center at Goddard Space Flight Center.
Special thanks are extended to Iftekar Bhatti for assistance
in the SMMR analysis and Zhao Hongwei for assistance in
the SSM/I analysis. We thank Ms. Kay Dewar for expertly
drafting the figures. This work was supported by NASA
Grants NAS8-36473 and NAGW-1688.
sow60WPrrcipimtoo
I Po4»mmtx» DifTcraKc
_ jow—-—
60W
Standard Stonn
Figure < - P37 from SSM/I for orbit 3274 at 23 UTC 6 February
1988. The field is contoured every 0.2 and the region less than 0.8
indicates rain and is shaded. Frontal analysis is estimated from the
NMC hemispheric surface charts at 00 UTC 7 February.
6. References
Chang, H.D.. P.M. Hwang, T.T. Wilheit. A.T.C. Chang. D.H.
Staelin and P.W. Rosenkranz. 1984: Monthly Distributions of
Precipitable Water from the Nimbus 7 SMMR Data. J Geophys
Res., 89, 5328-5334.
Katsaros. K.B.. I. Bhatti. L. McMurdie and G. Petty, 1989:
Identification of atmospheric fronts over the ocean with
microwave measurements of water vapor and rain. Weather and
Forecasting. 4, 449-460.
Kuo. Y.-H., M. A. Shapiro and Evelyn G. Donall. 1991: The
interaction between baroclinic and diabatic processes in a
numerical simulation of a rapidly intensifying extratropical
marine cyclone. Man. Wea. Rev..119, 368-383.
McMurdie, L.A. and K.B. Katsaros, 1985: Atmospheric water
distribution in a mid-latitude cyclone observed by the Seasat
Scanning Multichannel Microwave Radiometer. Man Wea.
Rev.. 113. 584-598.
McMurdie, LJV. and K.B. Katsaros. 1991: Satellite derived integrated
water vapor distribution in oceanic midlatitude storms: variation
with region and season. Mon.WeaJlev.. 119,589-605.
Petty, G.W. and K.B. Katsaros, 1990: New geophysical algorithms
for the Special Sensor Microwave/Imager. Preprint Volume,
Fifth International Conference on Satellite Meteorology and
Oceanography, September 3-7.1990. London. England.
Rabin, R. M., L. A. McMurdie. C. M Hayden and G. S. Wade,
1991: Monitoring precipitable water and surface wind over the
Gulf of Mexico from microwave and VAS satellite imagery.
Wea. and Forecasting. 6.227-243.
Reed, R J.. AJ. Simmons. M.D. Albright and P. Unden, 1988: The
role of latent heat release in explosive cyclogenesis: three
examples based on ECMWF operational forecasts. Wea. &
Forecasting. 3,217-229.
Sanders. F.. 1987: Skill of NMC operational dynamical models in
prediction of explosive cyclogenesis. Wea. and Forecasting, 2,
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Satellite derived integrated water vapor and rain intensity patterns: Indicators of rapid cyclogenesis

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