Spatial Distribution of Melt Season Cloud Radiative Effects Over Greenland: Evaluating Satellite Observations, Reanalyses, and Model Simulations Against In Situ Measurements

Abstract

Arctic clouds can profoundly influence surface radiation and thus surface melt. Over Greenland, these cloud radiative effects (CRE) vary greatly with the diverse topography. To investigate the ability of assorted platforms to reproduce heterogeneous CRE, we evaluate CRE spatial distributions from a satellite product, reanalyses, and a global climate model against estimates from 21 automatic weather stations (AWS). Net CRE estimated from AWS generally decreases with elevation, forming a “warm center” distribution. CRE areal averages from the five large‐scale data sets we analyze are all around 10 W/m2. Modern‐Era Retrospective Analysis for Research and Applications version 2 (MERRA‐2), ERA‐Interim, and Clouds and the Earth's Radiant Energy System (CERES) CRE estimates agree with AWS and reproduce the warm center distribution. However, the National Center for Atmospheric Research Arctic System Reanalysis (ASR) and the Community Earth System Model Large ENSemble Community Project (LENS) show strong warming in the south and northwest, forming a warm L‐shape distribution. Discrepancies are mainly caused by longwave CRE in the accumulation zone. MERRA‐2, ERA‐Interim, and CERES successfully reproduce cloud fraction and its dominant positive influence on longwave CRE in this region. On the other hand, longwave CRE from ASR and LENS correlates strongly with ice water path instead of with cloud fraction or liquid water path. Moreover, ASR overestimates cloud fraction and LENS underestimates liquid water path substantially, both with limited spatial variability. MERRA‐2 best captures the observed interstation changes, captures most of the observed cloud‐radiation physics, and largely reproduces both albedo and cloud properties. The warm center CRE spatial distribution indicates that clouds enhance surface melt in the higher accumulation zone and reduce surface melt in the lower ablation zone.

Importance of Orography for Greenland Cloud and Melt Response to Atmospheric Blocking

“…In this vein, high pressure systems may warm Greenland by supporting northward moisture transport (Neff et al 2014) and formation of clouds with a net warming impact. More recent analysis using automatic weather station data during the melt season corroborates the regional dependence of cloud impacts: clouds in the accumulation zone promote a dominant longwave warming, while clouds in the ablation zone cause dominant shortwave cooling (Wang et al 2018(Wang et al , 2019. The regionally dependent influence of Greenland clouds on longwave and shortwave surface radiation motivates analysis of the spatial distribution of CRE under high pressure conditions.…”

“…Hofer et al (2017) find declining cloud cover since the mid-1990s to be strongly correlated with increasing surface shortwave downwelling radiation and surface melt. Strong correlations also exist between cloud fraction and net CRE over Greenland (Wang et al 2019). Furthermore, Wang et al (2018) suggest that Greenland surface albedo may play a more important role than cloud properties in determining CRE.…”

“…Strong correlations also exist between cloud fraction and net CRE over Greenland (Wang et al 2019). Furthermore, Wang et al (2018) suggest that Greenland surface albedo may play a more important role than cloud properties in determining CRE. Regional variations in surface CRE are closely linked to variations in surface albedo, which strongly modulates the strength of the shortwave CRE.…”

“…More specifically, clouds over dark surfaces in the ablation zone cause dominant shortwave cooling, while clouds over bright surfaces in the accumulation zone promote dominant longwave warming during melt season. The contribution of additional cloud properties such as liquid and ice water path to Greenland CRE are further discussed in Bennartz et al (2013), Miller et al (2015), Van Tricht et al (2016, and Wang et al (2019).…”

“…CERES EBAF Ed4.0 also contains biases in polar cloud fraction and surface radiative fluxes, particularly in overestimating cloud fraction and longwave downwelling radiation at Summit during polar night (Kato et al 2018). Wang et al (2019) compare an earlier version of the product used to produce EBAF data, CERES Synoptic Radiative Fluxes and Clouds Edition 3A, as well as ERA-Interim and other reanalyses, with in situ cloud observations from the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS). Of these datasets for summers from 2011 to 2013, CERES most closely resembles Summit observations in terms of magnitude and spatial variability of both cloud fraction and liquid water path.…”