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Megha-Tropiques-ScaRaB/3 makes multi-year direct observations of the diurnal variation of cloud radiative forcing over tropics

Clouds are the largest modulators of the radiation budget of the earth-atmosphere system and have a pivotal role in regulating the weather and climate. Radiative impact of clouds and the associated feedback are among the largest uncertainties in numerical atmospheric models and quantification of earth’s energy budget. Shortwave cloud radiative forcing (SWCRF) refers to the shortwave (solar) flux reflected back to space by clouds. The longwave cloud radiative forcing (LWCRF) is the amount of terrestrial longwave radiation trapped within the earth-atmosphere system by clouds. Net cloud radiative forcing (NCRF) is the sum of SWCRF and LWCRF.

The ScaRaB/3 payload on-board the low-inclination (19.98°) orbit Indo-French Megha-Tropiques satellite measures the broadband reflected and emitted radiances at different local times (LT) over the tropics during its 51-dayprecession cycle. This unique nature of MT-ScaRaB/3 has been used to make the first direct observations of the multi-year seasonal mean diurnal variations of LWCRF and SWCRF at the to-of-atmosphere (TOA) over the entire tropics. The MT-ScaRaB/3 data during 2012-2016 was obtained from the Meteorological & Oceanographic Satellite Data Archival Centre (MOSDAC). In addition to quantifying the seasonal mean CRF and its diurnal variations, this study reveals several specific features hitherto unexplored.

For the first time, this study brought out substantial diurnal variation in the spatial extent of the “pool of inhibited cloudiness” over the southwest Bay of Bengal during the Asian summer monsoon season: it maximizes during 09-15 LT by a factor of 4 compared to the minimum at night. Diurnal variation of CRF associated with the double inter-tropical convergence zone, which is persistent over the western Pacific throughout the year, has been brought out for the first time. For the first time, this study reveals that the magnitudes of NCRF and its zonal variations at TOA are remarkably similar during the normal and ElNi o periods, because the changes in LWCRF are compensated by the corresponding changes in SWCRF over the above oceanic regions.Diurnal variations of LWCRF over convective regions maximize during 18-21 LT over continents and 00-06 LT over oceans. On average, the NCRF produces considerable daytime cooling and night-time warming over the tropics. This decreases the diurnal variation of radiative fluxes at the surface and hence diurnal variation of surface temperature which, in turn, would feedback to modify the circulation.

This study was carried out at Space Physics Laboratory (SPL), Vikram Sarabhai Space Centre (VSSC) and the results are recently published in Climate Dynamics (https://doi.org/10.1007/s00382-020-05441-w). In an earlier study carried out at SPL/VSSC, MT-ScaRAB observations were used to make the first direct observations of the diurnal variation of aerosol radiative forcing at TOA over the highly turbid mineral dust-dominated Arabian Sea (dust transported from West Asia) and the Atlantic Ocean (dust transported from Sahara) during the summer season (https://ieeexplore.ieee.org/document/8004524).

Seasonal mean net cloud radiative forcing (NCRF; unit W m-2) averaged during the daytime (left panel) and nighttime (rightpanel) during the four seasons (averaged for 2012-2016).DJF=Northern Winter (December-February), MAM=Northern Spring (March-May), JJA=Northern Summer (June-August), SON=Northern Autumn (September-November)

Fig.1: Seasonal mean net cloud radiative forcing (NCRF; unit W m-2) averaged during the daytime (left panel) and nighttime (rightpanel) during the four seasons (averaged for 2012-2016).DJF=Northern Winter (December-February), MAM=Northern Spring (March-May), JJA=Northern Summer (June-August), SON=Northern Autumn (September-November).