Algorithms for retrieving geophysical parameters from the MADRAS and SAPHIR sensors of the Megha‐Tropiques satellite: Indian scenario

Abstract

The major scientific objective of the Megha-Tropiques (MT) satellite, an Indian Space Research Organisation (ISRO)-Centre National d'Études Spatiales (CNES) collaborative project, is to understand the energy and water cycles in the global tropical region. With its 20 • inclined orbit, it will frequently measure radiation emitted by the Earth-Atmosphere System in the visible, infrared and microwave spectrum through its four sensors on board. Various geophysical parameters, namely water vapour, cloud liquid water and surface winds over oceanic regions, and the rainfall, humidity profile and top-of-atmosphere radiative fluxes over land as well as over oceanic regions will be derived from the measurements made by these instruments. This article deals with the efforts made by ISRO to develop algorithms for deriving these geophysical parameters from the microwave imager and sounder, mentioning the pre-launch specifications with prelude examples from existing space-borne sensors of similar types. The sensor-specific algorithms are presented in different sections.

Estimating Confidence Intervals around Relative Humidity Profiles from Satellite Observations: Application to the SAPHIR Sounder

“…The best retrievals are obtained between 250 and 600 hPa, where the highest correlation coefficients (above 0.93) are associated with the lowest values of errors (means , 1.8%) and standard deviations (,5.2%). This is consistent with the findings of Kumar et al (2014) for the retrieval scheme layeraveraged relative humidity (LARH) detailed by Gohil et al (2013) (the Indian scenario for SAPHIR) when compared to NWP model profiles.…”

“…Globally, the method produces a slightly biased midtroposphere (9.19% RH in the 400-600-hPa layer and 9.79% RH in the 650-700-hPa layer) with RMS below 12.12% RH, comparable to other techniques reaching an RMS of ;10% RH in the midtroposphere Gohil et al 2013). One can refer to Sivira et al (2015) for a discussion on the improvements brought by the microwave imager Microwave Analysis and Detection of Rain and Atmospheric Systems (MADRAS), which is also part of the MeghaTropiques payload and was declared nonoperational after almost 15 months of operations (a serious mechanical anomaly affecting the scan mechanism).…”

“…For instance, for C6 (183.31 6 11 GHz) the warmest BTs (i.e., the driest profiles) are associated with low peaking J RH . Despite this feature, common to all microwave humidity sensors, a few papers have already demonstrated the usefulness of SAPHIR measurements to estimate layeraveraged RH profiles Brogniez et al 2013;Gohil et al 2013;Sivira et al 2015), with an increased improvement of the retrievals with respect to the three-channel operational sensors toward the edges of the troposphere.…”

Validation of INSAT-3D sounder data with in situ measurements and other similar satellite observations over India

“…It is a passive remote-sensing microwave sounder which operates at six channels close to 183.31 GHz (±11.0, ±6.60, ±4.30, ±2.8, ±1.2), retrieving layer-averaged RH values in six pressure layers (1000-850, 850-700, 700-550, 550-400, 400-250 and 250-100 hPa) within ±30 • latitudinal belt. The algorithms related to retrieval for the sounders of the MT satellite are discussed by Gohil et al (2012). These data have been validated against similar satellites and reanalysis data sets and found to be good except in level 1 (1000-850 hPa) (Venkat .…”

“…The satellite crosses the equator twice a day, once during the ascending node at ∼ 13:30 LT and once during the descending node at ∼ 01:30 LT. It is a high spectral sounder with 2378 channels measuring the IR radiances at wavelengths in the range of 3.7-15.4 µm with a swath of 1650 km and horizontal spatial resolution of 13.5 km at nadir (Aumann et al, 2003). We used the level 3 version 5 daily gridded data products (Susskind et al, 2006) obtained from the IR radiances of the AIRS sounder during 2014 and 2015.…”