ScaRaB onboard Megha Tropiques (MT) observations of seasonal mean diurnal variations of cloud radiative forcing (CRF) at top-of-atmosphere over tropics and the impact of El Nio periods (November 2014-February 2016) on CRF

The ScaRaB payload onboard Megha-Tropiques (MT) satellite has been making observations of radiative fluxes at the top of the atmosphere (TOA) for different local time (LT) of the day. This provides a unique opportunity to investigate the diurnal variation of the regional instantaneous cloud radiative Forcing (CRF) at TOA. Using direct observations of radiative fluxes from ScaRaB, seasonal mean diurnal variations of CRF at TOA are investigated during July 2012-Decemeber 2016. Such observations are essential to investigate and quantify the regional differences in the longwave CRF (LWCRF) diurnal cycle and daytime variations of shortwave CRF (SWCRF) over deep convective regions and subsidence zones of the Hadley and Walker circulation cells. One of the most remarkable features observed is the significantly large diurnal variation of LWCRF over deep convective regions, with distinctly different phases over the continents and open oceans. Results show that the magnitude of the diurnal variation of LWCRF is largest over the continental deep convective regions of Brazil and Africa, where the peak-to-trough amplitude of the diurnal cycle of LWCRF is in the range of 20 to 30 Wm $^{-2}$, which is about 30–45% of their diurnal mean values. Peak-to-trough amplitude of the diurnal cycle of LWCRF over the oceanic regions are mostly in the range of $\sim 15$ to 25Wm $^{-2}$ which are about 20 to 35% of the diurnal mean values of LWCRF over these regions. The least diurnal variation of LWCRF over the ITCZ generally occurs over the eastern Pacific (peakto-trough amplitude $< 15$ Wm $^{-2}\,)$. The diurnal maxima of LWCRF over the continental regions occur during 1521 LT while the minima occur at $\sim 03 -09$ LT. Over the deep convective regions of open oceans, the diurnal cycle of LWCRF attains broad maximum during 21–06 LT while the minimum occurs at $\sim 09 -12$ LT. Further, this study indicates that the magnitudes of shortwave CRF (SWCRF) are largest around $\sim 12 -15$ LT, mainly due to the largest incoming solar flux and phase of the diurnal variation of cloud development. Though significant shift in the location of deep convection occurs during El Nino periods (which also introduces corresponding shift in the spatial variations of LWCRF and SWCRF), zonal variations of the average net CRF (NCRF) during the El Nino and normal periods are remarkably similar during all seasons.