Abstract
AbstractThe cloud radiative forcing (CRF) quantifies the warming or cooling effects of clouds. To derive the CRF, reference values of net (downward minus upward) irradiances in cloud‐free conditions are required. There are two groups of techniques to estimate these reference values; one is based on radiative transfer modeling, and a second group uses measurements in cloud‐free situations. To compare both approaches, we first look at a case study from the airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX) campaign, where a moving cloud field with a sharp edge separating a cloudy boundary layer from an adjacent evolving cloud‐free area was probed. These data enabled the quantification of the impact of changing atmospheric and surface properties relevant for the reference net irradiances in cloud‐free conditions. The systematically higher surface albedo below clouds compared to cloud‐free conditions, results in a 11 W·m−2 smaller shortwave cooling effect by clouds estimated from the radiative transfer approach compared to the measurement‐based one. Due to the transition of thermodynamic parameters between the cloudy and cloud‐free atmospheric states, a 20 W·m−2 stronger warming effect is estimated by the radiative transfer approach. In a second step, radiative transfer simulations based on radiosoundings from the Surface Heat Budget of the Arctic Ocean campaign are used to quantify the impact of the vertical profiles of thermodynamic properties on the CRF. The largest difference between the longwave CRF estimated by the two methods is found in autumn with up to 25 W·m−2.
Highlights
Clouds significantly influence the local surface energy budget in the Arctic
Radiative transfer simulations based on radiosoundings from the Surface Heat Budget of the Arctic Ocean campaign are used to quantify the impact of the vertical profiles of thermodynamic properties on the cloud radiative forcing (CRF)
Relevant parameters for the CRF and the radiative energy budget (REB) derived from instruments on the aircraft, like surface brightness temperature (BT) (measured by a Kelvin infrared radiation thermometer (KT-19)), sea ice concentration, and in situ observations of the thermodynamic state are introduced in Stapf et al (2020a)
Summary
Clouds significantly influence the local surface energy budget in the Arctic. Long-term observations in the Arctic clearly demonstrated the close link between synoptically driven atmospheric and radiative states (Graham et al, 2017; Miller et al, 2017; Stramler et al, 2011; Walden et al, 2017). A truly observation-based comparison between the CRF values derived from both approaches covering shortwave and longwave radiative effects, as well as a measurement-based quantification of the influence of the assumed thermodynamic profile properties has not been reported so far To close this gap, we present and analyze observations from two campaigns: the airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX) campaign, and the Surface Heat Budget of the Arctic Ocean (SHEBA) expedition (Uttal et al, 2002). AFLUX we were fortunate enough to observe an atmospheric situation over sea ice with a sharp cloud edge separating a cloudy, potentially coupled boundary layer from an evolving cloud-free atmosphere with a surface-based inversion (Section 2) This specific constellation enabled the direct measurement of the radiative impact of clouds on the surface REB with closely adjacent measurements in cloudy and cloud-free conditions over the same type of surface. To study the related differences in longwave CRF estimates on a longerterm basis and investigate seasonal changes of CRF estimates based on both methods, SHEBA observations are analyzed (Section 4)
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