IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India,2002 
INTERCOMPARISON OF OCEAN SURFACE WIND SPEED FROM TOPEX RADAR 
ALTIMETER AND TRMM MICROWAVE IMAGER 
A.K. Varma, R.M. Gairola and Vijay K. Agarwal 
Oceanic Sciences Division 
Meteorology and Oceanography Group 
Space Applications Centre, ISRO 
Ahmedabad 380015, India 
KEY WORDS: TOPEX Radar Altimeter, TRMM Microwave Imager, Radiometer, Wind Speed, Significant Wave Height 
ABSTRACT: 
An Intercomparison of concurrent measurements of wind speed for TOPEX Radar Altimeter (RA) is made with that from TRMM 
Microwave Imager (TMI). A large number of observations (40770) during non-raining situations from TOPEX and TMI are 
colocated within temporal difference of 30 minutes and spatial difference of 0.05°. It is found that wind measurements by RA and 
TMI have good comparison with correlation of about 0.91, rms difference of 1.61 ms’, bias of 0.72 ms''. The standard deviation of 
the difference of two winds is 1.44 ms. 
The effect of Cloud Liquid Water (CLW), Integrated Water Vapour (IWV), and Ocean Surface Waves on wind comparison statistics 
is studied. Results are discussed in terms of possible effect of these parameters on retrieval accuracy of wind measurements from RA 
and TMI. 
1. INTRODUCTION 
For more than last two decades, advances in the areas of 
satellite borne oceanographic instrumentation and retrieval 
techniques have lead to the possibility of measurement of single 
oceanographic parameter with different instruments and 
retrieval techniques. An example of this is the measurement of 
wind speed over oceans with various space borne active 
(altimeter and scatterometer) and passive (radiometer) sensors. 
In such cases, there is a possibility of some inconsistency in the 
measurements of the same parameter by two or more such 
sensors due to difference in the physics involved in the retrieval 
algorithms, sensors characteristics, and their differential 
dependence on atmospheric and ocean surface conditions. 
Varma et al. (1998) have made a comparative study of near 
concurrent DMSP-SSM/I and Geosat-altimeter measurements 
of surface wind speeds over the Indian oceanic region. They 
have found that the winds retrieved by these two sensors differ 
within 3.0 ms! for about 84% cases. Varma et al. (2002) 
compared MSMR, SSM/I and TMI derived geophysical 
parameters that included winds over the global oceans. They 
found a reasonably good agreement between MSMR and SSM/I 
/ TMI derived wind speeds. They, however, found a much 
superior agreement between SSM/I and TMI. 
One sensor involved in the present study is the state-of-art 
Radar altimeter onboard TOPEX/Poseidon satellite, which has 
made a history by providing continuous observations over the 
global oceans for last one decade. The TOPEX/Poseidon 
satellite carries a dual frequency TOPEX-altimeter, an 
experimental solid state Poseidon altimeter operating at single 
frequency, and a three-frequency nadir looking microwave 
radiometer. The other instrument involved in the study is the 
Tropical Rainfall Measuring Mission (TRMM) Microwave 
Imager (TMI). TRMM was launched on November 27, 1997, 
by United States and Japan, to carry out a systematic study of 
tropical rainfall required for major strides in weather and 
climate research. The TRMM includes three principal types of 
instruments. The first and most innovative of the three is the 
first quantitative precipitation radar (PR) in space. The second 
type of instrument is a combination of cross track scanning 
multichannel dual polarization passive TRMM Microwave 
Imaging radiometer (TMI). A complementary VIS and IR 
sensor for rain estimation called VIRS and Cloud and Earth's 
Radiant Energy Systems (CERES) and the Lightening Imaging 
System (LIS) are also included in the TRMM. The space 
segment of TRMM was a satellite in 350 km circular orbit with 
35? inclination angle till 2001. The complete description of 
sensor package of TRMM is given in Kummerow et al. (1998). 
In the present study TMI derived ocean geophysical parameters 
are used. The characteristics of TMI are given in table 1 below: 
Table 1: TMI Characteristics 
  
  
TMI 
  
Freq./Wave Length | Dual Pol. 10.65, 19.35, 37, 85.5 GHz 
& Vertical Pol. 21 GHz 
  
  
  
  
Scanning Mode Conical 
Ground Resolution | 5 km at 85 GHz to 45 km at 10 GHz 
Swath Width 760 Km 
Science Appl. Surface rainfall rate, rain type, 
  
  
distribution and structure 
  
  
This study is aimed to bring out discrepancies in the 
measurements of wind speed by TOPEX/Poseidon-altimeter 
and TML This study is done with ‘collocated’ 
TOPEX/Poseidon (T/P) -altimeter and TMI wind speed 
estimates. By ‘collocated’, we mean here the estimates which 
are having temporal difference of less than 30 min. and spatial 
difference of less than 0.05°. 
   
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