For any meaningful analysis of geostationary satellite data images, it is necessary to have accurate geographic locations
of each pixel which requires accurate information about the satellite altitude, attitude and scanning geometry. Except
above, in general, users are provided with the information about the count values of satellite image, sub-satellite point
geographic location, image size and scanning direction. Sometimes, the geographic location data is available at coarse
interval which is not adequate for the purpose of geophysical parameter retrieval, validation and other applications. The
problem is further compounded by data dropouts and garbling affecting the automated detection of earth disk required
for deriving geographic locations of the pixels. An automated procedure has been developed to find the geographic
locations of the earth view pixels. For the situations with noisy data and attitude errors in roll and pitch but with correct
and stable sub-satellite point geographic location, methods have been developed for space view noise rejection, earth
disk detection, proper overlaying of continental boundaries and finally for determining geographic locations of desired
pixels. These procedures, except the yaw attitude error, automatically correct other attitude errors for each image. The
basic assumption made here is that the sub-satellite point location is correct and stable. The method is useful in the
absence of any information except the image specifications. Examples of INSAT and METEOSAT images will be
presented.
In this study global temperature profiles available from Argo have been assimilated into an Ocean General Circulation
Model (OGCM) to study its impact on ocean temperatures. In the control run the model was forced with daily
QuickSCAT derived scatterometer winds for the period Jan-June 2004 and air temperature, specific humidity, net shortwave
and net long wave radiation from NCEP reanalysis. Two assimilation experiments were performed for Jan-Jun
2004; one in which the monthly averaged Argo profiles were assimilated in the OGCM using nudging technique (Exp-1)
and another experiment (Exp-2) in which daily data from Argo was assimilated into the OGCM using Cressman
technique. Temperature outputs from all the three runs (control and assimilation runs) were first inter-compared and then
compared with independent observations from one of the Indian Ocean TRITON buoys. Errors in surface temperature at
the TRITON buoy location are reduced by 37% and 16% in Exp-1 and Exp-2 respectively. However, the variance
explained in surface temperature with respect to observations is reduced in the assimilated runs as compared to the
control run. Subsurface features like ILD and D20 show significant improvement in terms of error reduction in both the
experiments implying improvement in the mixed layer and the thermocline region. Exp-2 scores over Exp-1 in terms of
the explained variance of ILD and D20. This is so because in exp-1 monthly averaged data is assimilated which
constraints the high frequency variability of the parameter.
The realism of the impact of penetrative solar radiation, an effect we refer to as biological heating, on the upper ocean
thermodynamics has been studied using an Ocean General Circulation Model (OGCM). Daily fields of winds, air
temperature, specific humidity, net long-wave and shortwave radiation from NCEP were used to force the model. In the
control run (cntl-R), diffuse attenuation coefficient (Kd) which signifies the visible radiation penetration is parameterized
for clear water condition. In the experimental run (exp-R), attenuation coefficient for blue-green wavelength (Kd_490)
obtained from SeaWiFS sensor was used to determine the penetrative depth of solar radiation. Use of satellite derived
Kd_490 alters the upper ocean thermodynamics quite significantly. Model simulated parameters sea surface temperature
(SST), current and mixed layer depth (MLD) were found to be sensitive to the choice of diffuse attenuation coefficients
that limit the penetration of solar radiation into the ocean. The SST cools and MLD deepens in clear water regions (large
attenuation depths) due to heat penetration in deeper layers, while the surface gets heated and MLD shoals in regions of
high turbidity (low attenuation depths) due to heat trapping.
A detailed analysis and validation of ocean thermodynamic variables, simulated, using an Ocean general circulation
Model (OGCM) forced by QuikSCAT scatterometer (QS-R) has been carried out in this study. The results are
discussed in terms of comparison of vertical profiles of temperature against TRITON observations for the year 2004.
Root mean square error (RMSE) of Sea Surface Temperature (SST) simulated by QS-R showed an error of the order of
0.6°C. QS-R SST also has a cooler bias. Another OGCM simulation was carried out in which the National Center for
Environmental Prediction (NCEP) reanalysis winds (NCEP-R) were used to force the OGCM. The simulated SST
resulted in lesser RMSE (~0.5°C) and there was no bias. However, variabilities in SST were captured more realistically
in scatterometer forced solution. However the RMSE in SST was higher which could be partly due to physical
inconsistency between scatterometer winds and other air-sea exchange parameters used from NCEP. A detailed statistics
of the model simulated temperatures at different depths show excellent performance of scatterometer forced simulations
as compared to NCEP-R. Maximum error in the temperature profile was in the thermocline region (~1.5°C) in QS-R as
against 3.5°C in NCEP-R. Possible causes of these errors in relation to air-sea exchange parameters used in forcing the
OGCM are described in this work.
Emission based radiative transfer simulations have been carried out to study the impact of atmospheric humidity on
clear-sky microwave emissions at various channels of Megha-Tropiques SAPHIR and NOAA AMSU-B sounders.
Detailed investigations reveal that under cool and drier conditions, water vapour channels in the far wing region like
183.31±7 GHz of AMSU-B and others behave like microwave imagers in contradiction to these being sounding
channels. This feature affects their utility for sounding the lower atmosphere. Simulation study confirms that the layer
average relative humidity is retrieved better as compared to its other forms requiring temperature information and has
logarithmic dependency on radiation. Present study deals with development of retrieval algorithms using multi-channel
sounder data for deriving average relative humidity for different layers of the atmosphere. AMSU-B data during June
and October 2002 over Indian region have been used for testing the algorithms to derive relative humidity in three layers
between 300 to 1000 hpa. The satellite derived humidity fields have been compared and found to be in good agreement
with those from NCEP data.
The forthcoming Indian satellite Oceansat-2 to be launched in 2007 will carry a microwave scatterometer and an
ocean colour monitor onboard. The scatterometer, a Ku-band pencil beam sensor similar to that onboard Quikscat
satellite, will provide surface vector winds over global oceans with a two days repetivity. An algorithm for retrieving
wind vector from scatterometer has been developed with a solution ranking criteria of minimum normalized standard
deviation (NSD) of wind speeds derived using backscatter measurements through a geophysical model function
(GMF). Using Quikscat observational geometry and QSCAT-1 GMF, simulation based evaluation of algorithm
performance under different noise conditions and its comparison with standard algorithm known as Maximum
Likelihood Estimator (MLE) algorithm have been performed. Besides having retrieval performance closely
comparable with MLE, the present algorithm has quality and rain flagging provisions. Moreover, it is
computationally efficient with least subjectivity on various retrieval related parameters. These features are equally
desirable for the operational implementation. Results of simulation studies related to retrieval, quality control and
rain flagging along with its implementation to limited Quikscat data are presented.
Interpretation of microwave radiometric measurements over land for atmospheric studies requires representative
information about surface emissivity. A simulation study has been performed to derive some of the meteorological
parameters like atmospheric water vapour content over land using TRMM TMI data through assumed land surface
emissivity satisfying the radiative transfer model. The process of minimization of simulated and observed radiation at
TMI frequencies using a large number of simulated atmospheric and surface conditions simultaneously yields many
atmospheric and surface parameters over land. Preliminary analysis of TRMM TMI data over India and adjoining land
region for few days during different season has been carried out. The derived surface emissivity difference of vertical and horizontal polarization is found highly resembling with Quikscat radar backscatter of land surface over the same area
corroborating the estimation of land surface emissivity parameters. The study has importance in synergic use of
microwave radiometer and scatterometer for studying the surface features as well as retrieval of geophysical parameters
over land in view of forthcoming Megha-Tropiques and Oceansat-2 satellites. Typical examples of TRMM-TMI and
Quikscat scatterometer are presented here.
Radiative transfer simulation based study was carried for developing sea surface temperature algorithms for
ISRO's next geostationary satellite INSAT-3D that will be similar to GOES-9 configuration. Characterization of Indian
tropical marine atmosphere was done by utilising the surface and atmospheric parameters like temperature, pressure and
humidity observed onboard research vessels, covering entire Indian oceans. These parameters were further perturbed in
order to achieve the full temporal and spatial variability in the Indian region. 1392 such atmospheric profiles were
generated as input to the radiative transfer model. Brightness temperatures for INSAT-3D imager and sounder channels
were simulated for these profiles. Various combinations of the channels suitable for sea surface temperature and total
water vapor estimation were considered and depending on the statistical parameters and retrieval errors, daytime and
nighttime SST retrieval equations were finalised. These equations were applied to GOES-9 data over eastern pacific and
the retrieved SST fields were validated with insitu ship observations. The rms error achieved was ~ 0.68 K. Finally SST
retrieval equations were suggested for INSAT-3D. The advantage of frequent sampling by geostationary satellites was
also demonstrated by studying the diurnal variability of SST and improving the cloud free SST fields using INSAT-3A
data. It was found that cloud free fields can be increased to ~ 25% in a day by compositing eight images for that day.
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