Upper Tropospheric Humidity and Cloud Radiative Forcing: A Tropical Perspective

Earth's energy budget defines the balance between the incoming radiant solar energy reaching Earth and the energy returning to outer space. Clouds play a significant role in Earth's energy budget. Cloud Radiative Forcing (CRF) is the difference between the radiative fluxes at the top of the atmosphere in clear-sky and all-sky conditions. Clouds introduce two contrasting effects on the Earth's energy balance: the albedo effect and the longwave effect. Clouds reflect a large amount of incoming shortwave radiation and cool the Earth, known as the Albedo effect. The energy associated with the albedo effect is known as shortwave cloud radiative forcing (SWCRF). The longwave effect or longwave cloud radiative forcing (LWCRF) denotes the warming of Earth by the cloud-trapped longwave radiation that would otherwise escape to space. Understanding the variability in the amount and distribution of clouds in a warming climate is essential as they modulate the shortwave and longwave cloud radiative feedbacks (Harrison et al., 1990; Bony, S. et al., 2006) and, thereby, the Net CRF. Upper Tropospheric Humidity (UTH) is a vital climate variable that impacts the amount of outgoing longwave radiation. In the tropics, UTH is mainly driven by deep convection. The present study analyzes the influence of UTH on the longwave cloud radiative forcing in the tropics from 2000 to 2021. This study uses the satellite microwave (MW) and infrared (IR) UTH measurements. Clouds affect IR UTH measurements, while MW measurements provide UTH under all sky conditions. Clouds and the Earth's Radiant Energy System (CERES) satellite datasets are used to calculate cloud radiative forcing. This study quantifies the UTH-LWCRF relationship and shows that UTH can explain LWCRF variability in the tropics to a large extent. The joint distribution analysis shows that UTH has a significant impact on the variability of LWCRF over land, whereas over ocean regions, sea surface temperature plays a role in modulating the UTH-LWCRF relationship. Also, the UTH-LWCRF relationship is better represented with MW UTH than IR UTH, which can be attributed to the more comprehensive and accurate MW measurements even in cloudy conditions.