Comparison study on rainrall retrieval
Algorithm: model simulation and armar data
Jinli Liu, Zhehu Cui, Ling
Zhang, Xiankang Dou, Daren Lu
LAGEO, Institute of Atmospheric Physics, Chinese Academy of Sciences
Beijing 100029, China
Keyword: Rainfall remote sensing, SSM/I,
AMSR, TOGA COARE, Microwave
radiative transfer model
Abstract
Although space-borne Microwave (MW)
radiometers have been actively used for
global rainfall estimation, to date the retrieval
accuracy of rainfall rate still can't meet the
requirement of operational use and research,
one of the main reasons is the highly
complexity and rapid variation of the vertical
structure of precipitating cloud. For
improving the accuracy of retrieval rainfall
rate, a proper cloud structure should be
chosen.
In this paper, By using ARMAR
airborne radar-radiometer data, which was
obtained in TOGA COARE IOP, and the
dynamic cloud model we established two
kinds of precipitating cloud models:
convective and stratified precipitating cloud
with vertical structure. Based on these cloud
models the simulation relationship of Tb-R
(brightness temperature versus rainfall rate)
were compared with the real observation of
ARMAR’s data. Also, several retrieval
algorithms of rainfall rate are compared based
on these cloud models.
Introduction
Rainfall distribution over global and
regional scales plays important role in
research and application for climate change
and global water cycle within the ocean-land-atmosphere
system. Satellite-borne
microwave (MW) radiometry, such as
DMSP’s SSM/I has been proved as a
powerful tool for global rainfall estimation.
Although it is successful in revealing global
pattern and annual variation of global rainfall
distribution, there is still big discrepancy in
the accuracy and correlation of retrieval
rainfall to real rainfall.
For improving the retrieval accuracy of
rainfall rate, establishment of precipitating
cloud model is also a key step. Tropical
region covers big part of global oceans,
remote sensing of rainfall in this area is of
significance. There are mainly two different
kinds of rainfall in this area, i.e., convective
and rather homogeneous stratified rainfall.
We analyzed the ARMAR[ Durden et al
1994] airborne radar/radiometer observation
data which was measured over equatorial
Pacific Ocean during TOGA /COARE
Intensified Observation Period ( IOP ) and
established stratified and convective cloud
models. With these two cloud models, MW
radiative transfer (R-T) simulations were
made by a vector R-T code. All channels of
SSM/I and AMSR were simulated with
various rainfall rates. In the following
sessions, two precipitating cloud models are
introduced and validated; For extending the
algorithm derived from SSM/I data to AMSR
data, the R-T simulations have been made
with two cloud models by changing the
corresponding MW frequencies.
Models
Vertical structure of precipitating clouds
There are basically two kinds of
precipitating clouds in tropical area:
convective cloud and stratified cloud. Here
we used the ARMAR radar/radiometer data
observed during TOGA COARE IOP as the
basis of cloud model. Airborne radar
observed vertical structure of radar
reflectivity Z
e and the MW radiometer
observed the brightness temperature of
underneath cloud-ocean surface system. The
observation revealed very clear stratified
characteristics with brightband as well as
relatively horizontal homogeneity of
precipitating cloud. With these data we made
statistics of Z
e profiles and produced an
average (typical) profile, which may
represent as basis for R-T simulation. Since
the data set did not contain the phase
information except the brightband showing
the height of 0°c level, we used the simulation
output of a cloud dynamical model developed
by Hu & Yan (1986), and used radio
sounding data and ocean surface condition in
TOGA COARE period as the input, the
output contained vertical structure of
raindrops, cloud droplets as well as various
kind of ice particles. Based on composition of
radar observation and cloud model output of
microphysical structure, we established
stratified and convective precipitating cloud
models respectively (see Fig.1 and Fig.2).
Fig.1 Stratified cloud model
Fig2. Convective cloud model
Radiative Transfer Model
In present simulation vector R-T model
developed by Evans [Evans & Stephens
1995] was used. Simulations were made for
the following situations: Range of rainfall
rate: 0 – 50 mm/hr, MW channel: 13.8 GHz,
all 7 channels of SSM/I, 10 channels of
ADEOSII AMSR, ground surface: calm
ocean, and Marshall-Palmer spectra were
used for both liquid and ice particles. In
simulation, both the relation between Tb
(brightness temperature) and R (rainfall rate)
and between Tb and TWC (total column
water content) were concerned.
