Current Science

Nizy Mathew ¹, Viju Oommen John ², C. Suresh Raju ¹, K. Krishna Moorthy ³

Affiliations
1 Space Physics Laboratory, Vikram Sarabhai Space Centre, Indian Space Research Organization, Thiruvananthapuram 695 022, IN
2 Hadley Centre, Met Office, Exeter, GB
3 ISRO Headquarters, Antariksh Bhavan, New BEL Road, Bengaluru 560 231, IN

Abstract

Upper tropospheric humidity (UTH) has been derived using a 'brightness temperature (T_b) transformation' method from the humidity sounder channels of SAPHIR payload on-board Megha-Tropiques (MT). These channels are very close to the water vapour absorption peak at 183.31 GHz. The channel at 183.31 ± 0.2 GHz enables retrieval of humidity up to the highest altitude possible with the present nadir-looking microwave humidity sounders. Megha-Tropi-ques satellite has an equatorially inclined orbit, which ensures frequent spatial and temporal coverage of the global tropical belt. Transformation coefficients for the first three channels for all the incidence angles have been derived and are used to convert brightness temperatures to weighted average upper tropospheric humidity having weighting function peaks at different pressure levels. The methodology has been validated by comparing the SAPHIR-derived UTH with that de-rived from radiosonde observations. Inter-comparison of the derived UTH has been done with layer averaged humidity product from SAPHIR measurements and with UTH product using infrared measurements from Kalpana satellite (MOSDAC). UTH over the tropical belt for six months has been studied taking the ad-vantage of the humidity product with high spatial and temporal resolution. The transformation coefficients and methodology to identify the cloud-free pixels to derive UTH from the three channels for all the possi-ble incidence angles are presented here, so that the users can directly derive UTH from the brightness temperature data.

Keywords

Brightness Temperature, Radiosonde Observations, Sounder Channels, Upper Tropospheric Humidity.

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Yan Feng ¹, M. Cadeddu ¹, V. R. Kotamarthi ¹, R. Renju ², C. Suresh Raju ²

Affiliations
1 Environmental Science Division, Argonne National Laboratory, Argonne, IL 60439, US
2 Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, IN

Abstract

The radiosonde humidity profiles available during the Ganges Valley Experiment were compared to those simulated from the regional Weather Research and Forecasting (WRF) model coupled with a chemistry module (WRF-Chem) and the global reanalysis datasets. Large biases were revealed. On a monthly mean basis at Nainital, located in northern India, the WRFChem model simulates a large moist bias in the free troposphere (up to +20%) as well as a large dry bias in the boundary layer (up to -30%). While the overall pattern of the biases is similar, the magnitude of the biases varies from time to time and from one location to another. At Thiruvananthapuram, the magnitude of the dry bias is smaller, and in contrast to Nainital, the higher-resolution regional WRF-Chem model generates larger moist biases in the upper troposphere than the global reanalysis data. Furthermore, the humidity biases in the upper troposphere, while significant, have little impact on the model estimation of column aerosol optical depth (AOD). The frequent occurrences of the dry boundary-layer bias simulated by the large-scale models tend to lead to the underestimation of AOD. It is thus important to quantify the humidity vertical profiles for aerosol simulations over South Asia.

Keywords

Aerosol Optical Depth and Extinction, Relative Humidity, Regional Climate Model.

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C. Suresh Raju ¹, Tinu Antony ¹, Nizy Mathew ¹, K. N. Uma ¹, K. Krishna Moorthy ¹

Affiliations
1 Space Physics Laboratory, Vikram Sarabhai Space Centre, ISRO, Thiruvananthapuram 695 022, IN

Abstract

This article reports the potential of the 'MADRAS' payload on-board the Megha-Tropiques satellite for land surface studies. The analysis has been divided into two parts as application of MADRAS data for studying the land surface properties and estimation of microwave emissivity directly from MADRAS brightness temperature (TB) data by applying an in-house developed Microwave Radiative Transfer Computation Code. The derived emissivity is further used to characterize the microwave emissivity of different land surface classes. The polarization difference (PD) parameters, the difference between horizontal (H-) and vertical (V-) polarization of TBs at 18 and 36 GHz clearly discern surface features of different surface classes such as deserts, arid/semi-arid and vegetated regions. Land surface microwave emissivity for MADRAS channels is derived on a global basis. These are inter-compared with the emissivity derived from the operational TRMM Microwave Imager and are in reasonably good agreement. The analysis based on emissivity shows spectral variation for different surface classes.

Keywords

Land-Surface Microwave Emissivity, MADRAS Payload, Megha-Tropiques Mission, Microwave Radiometry.

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