Climate-Scale Oceanic Rainfall Based on Passive Microwave Radiometry - Satellite-based Applications on Climate Change - page 219

or (METH), is based on the use of histogram of brightness temperature over the time
period, providing a characterization of the non-raining portion of the observations
(Wilheit et al. 1991 ;Chiuetal. 2010 ). The technique is robust and is suited for
examining rainfall estimates across different satellite platforms and sensors. Inter-
sensor and inter-satellite calibrations are crucial for establishing multi-platform
multi-sensor rainfall record for climate studies.
In Sect. 15.2 , the model structure and the underlying theory is briefly described.
Section 15.3 examines the product output parameters. Section 15.4 describes
examples of the technique to climate studies, and Section 15.5 discusses future
work and potential improvements to improving this product.

15.2 Background

SSM/I is a seven-channel, four-frequency (19.35, 22.235, 37, and 85.5 GHz) conically
scanning microwave radiometer (Hollinger et al. 1990 ). The Special Sensor Micro-
wave Imager/Sounder (SSMIS) is a 24-channel microwave radiometer and sounder
with frequencies range from19 to 183 GHz (Kunkee et al. 2008 ). It combines an SSM/I
with a microwave sounder that provide temperature and moisture profile information.
They are flown on board DMSP satellites. Description of the SSM/I and SSMIS
sensors and their operations can be obtained from the National Snow and Ice Data
Center (NSIDC)'s web site ( http://nsidc.org/data/docs/daac/f8_platform.gd.html ).
Given an atmospheric profile, the observed microwave radiation from a satellite
can be calculated via radiative transfer. This is the forward problem. The inverse (or
retrieval) problem is to estimate parameters of the atmospheric column from the
observed radiance.

15.2.1 Atmospheric Model

The atmospheric model consists of a cloud layer on top of a rain column over the
ocean surface (see Fig. 15.1 from Wilheit et al. 1977 ). A surface relative humidity
(RH) of 80% is assumed which increases linearly to saturation (100%) at the freezing
level (FL) (Wilheit et al. 1977 , 1991 ). The FL is the height of the zero degree
isotherm. A non-precipitating cloud layer with a density of 0.5 g/m 3 and 0.5 km in
thickness is present near the freezing level. Underneath the FL is a rain column
consisting of rain drops that follow a Marshall Palmer (M-P) distribution (Marshall
and Palmer 1948 ).
With the assumption of the humidity and M-P rain drop distribution, the FL
specifies the moisture condition of the rain column. A brightness temperature T ,
defined as twice the brightness temperature of the vertically polarized 19 GHz
minus the 22 GHz ( T ¼

2 T b 19V T b 22V ), is used. This combination channel