Radiative fluxes, heating rates and cloud radiative forcing at the ARM Eastern North Atlantic observatory

Description

Clouds and water vapor are an integral component of the Earth’s radiation budget as they modulate the shortwave and longwave radiation at variety of spatial and temporal scales. The radiative impacts of clouds and water vapor are poorly represented in Earth System Model (ESM) used for predicting the future climate, thereby leading to model errors.

Characterizing the effect of water vapor and clouds on radiative fluxes is an important step to understand the long-term impact of clouds on climate. Clouds affect incoming and outgoing radiation through their macro-physical and micro-physical properties. They regulate how much radiation reaches the surface by reflecting solar radiation and emitting longwave radiation. Besides the surface effects, there is a complex interaction in the boundary layer that involves the exchange of radiation between the cloud top and the free troposphere and cloud top and the surface layer. For example, low-level marine clouds reflect much greater amount of solar radiation back to space as compared to the ocean surface, thereby cooling the Earth’s surface during their presence. On the other hand, cirrus clouds can have heating or warming effect depending on the season and location.

In this work we calculate longwave and shortwave fluxes from 7 years of data collected at the Eastern North Atlantic (ENA) site. The observed and retrieved profiles of atmospheric vapor, temperature, clouds, and drizzle properties are used as an input to the Rapid Radiative Transfer Model (RRTM). Profiles of radiative fluxes between the surface and the Top of the Atmosphere (TOA) are calculated at a 1-minute temporal and 50 m vertical resolution. The radiative flux profiles are averaged to hourly timescale for further analysis. The RRTM calculated downwelling fluxes at the surface are compared to surface observations made at the site. Uncertainties in the input parameters (observations and retrievals) that affect RRTM flux calculations are examined with particular attention to the impact of cirrus clouds on the TOA fluxes. Uncertainties in the retrieved cloud and drizzle liquid water path also have an effect on the calculated fluxes and are examined.

After quantifying uncertainties, we estimate the annual and diurnal cycles of cloud radiative forcing at the TOA and at the surface and presents a climatology of cloud top radiative cooling at the site. Because of the variety of conditions experienced at the site, the analysis provides a quantification of the regional cloud conditions affecting radiation in the Eastern North Atlantic.