Direct observations of the multi-year seasonal mean diurnal variations of TOA cloud radiative forcing over tropics using
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Direct observations of the multi‑year seasonal mean diurnal variations of TOA cloud radiative forcing over tropics using Megha‑Tropiques‑ScaRaB/3 Ashok Kumar Gupta1,2 · K. Rajeev1 · Edwin V. Davis1,4 · Manoj Kumar Mishra1 · Anish Kumar M. Nair3 Received: 17 May 2020 / Accepted: 23 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Diurnal variation of cloud radiative forcing (CRF) is a major factor that controls the global radiation balance. This study presents multi-year seasonal mean diurnal variations of longwave cloud radiative forcing (LWCRF) and daytime shortwave cloud radiative forcing (SWCRF) at the top of atmosphere over tropics, derived from the broadband radiation measurements made by ScaRaB/3 onboard the low-inclination Megha-Tropiques satellite. The largest LWCRF (60–80 Wm−2) occurs over the oceanic regions of the east equatorial Indian Ocean and the western Pacific during all seasons, as well as the South Pacific Convergence Zone, the northeast Bay of Bengal, Amazon region, central and southern Africa and north Indian landmass (monsoon trough) during the local summer. Diurnal variations of 15–25 Wm−2 in LWCRF (20–35% of the mean) are observed with peak values occurring at 18–21 local time (LT) over continents and 00–06 LT over oceans. The minimum LWCRF occurs at 09–12 LT throughout the tropics. Over convective regions, SWCRF maximizes at 12–15 LT (− 220 to − 300 W m−2) and has a higher magnitude over continents due to early convection occurrence, indicating the importance of diurnal phase. Certain specific features including the CRF associated with the double inter-tropical convergence zone, day-night changes ∼ in net CRF, and the effect of El Nin o on CRF are also presented. The net CRF and its zonal variations are strikingly similar ∼ during the normal and El Nin o periods because the changes in LWCRF and SWCRF are mutually compensated. Keywords Cloud radiative forcing · Diurnal variation · Tropics · El Nino · Deep convection · Double ITCZ ∼
1 Introduction Clouds are among the largest modulators of the radiation and energy balance of the earth-atmosphere system and have a pivotal role in regulating atmospheric thermodynamics, circulation, hydrological cycle, weather and climate (e.g., Ramanathan et al. 1989; Stephens and L’Ecuyer 2015; Trenberth * K. Rajeev [email protected]; [email protected] 1
Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India
2
Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37240, USA
3
Meteorological Facility, Thumba Equatorial Rocket Launching Station, Vikram Sarabhai Space Centre, Thiruvananthapuram 695 022, India
4
Research Fellow in Physics, University of Kerala, Thiruvananthapuram, India
et al. 2016). The net radiative impact of individual cloud types is distinct: most of them produce a net cooling of the earth’s surface, while thin cirrus clouds generally have a warming effect (e.g., Ramanathan et al. 1989; Hartmann and Berry 201
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