Temporal Characteristics of Cloud Radiative Effects on the Greenland Ice Sheet: Discoveries From Multiyear Automatic Weather Station Measurements

Abstract

The impact of clouds on Greenland's surface melt is difficult to quantify due to the limited amount of in situ observations. To better quantify cloud radiative effects (CRE), we utilize 29 automatic weather stations and provide the first analysis on seasonal and hourly timescales in both accumulation and ablation zones. Seasonal CRE shows opposing cycles across geographical regions. CRE generally increases during melt season in the north of Greenland, mainly due to longwave CRE enhancement by cloud fraction and liquid water. CRE decreases from May to July and increases afterwards in the middle and south of Greenland, mainly due to strengthened shortwave CRE caused by surface albedo reduction. This finding resolves the contrasting seasonal distributions in previous studies at different Arctic locations. Longwave seasonal cycle also shifts from increasing in the north to decreasing in the south, implying spatial variations in cloud and atmospheric conditions. On hourly timescales, longwave downwelling radiation exhibits a bimodal distribution peaking at ∼ −70 W/m 2 (i.e., clear state) and ∼ 0 W/m 2 (i.e., cloudy state), suggesting that Greenland alternates between fairly clear and overcast. In the cloudy state, CRE strongly correlates with the combined influence of solar zenith angle and albedo (r = 0.85, p < 0.01) probably because clouds are thick enough for CRE to become saturated. The close relationship between CRE and albedo over dark surfaces suggests a stabilizing feedback: Net CRE is negative at low albedo caused by snow melt and snow metamorphism and thus tends to increase albedo and decelerate surface melt.

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

“…Clear‐sky radiation is simulated using the Column Radiation Model (Zender, ) driven by the Level‐3 Atmospheric Infrared Sounder, AIRS (AIRS Science Team/Joao Texeira, ). The uncertainty of clear‐sky simulations is less than 10 W/m 2 (Wang et al, ).…”

“…Clouds warm surfaces through increased longwave radiation (i.e., positive CRE) and cool surfaces through the shortwave shading effect (i.e., negative CRE). The net effect, warming or cooling, strongly depends on location (Wang et al, ). In the high elevation accumulation zone of Greenland, clouds can augment surface heating caused by warm southerly advection, trigger massive surface melt (Bennartz et al, ), and enhance meltwater runoff (Van Tricht et al, ).…”

“…Although the active sensors onboard CALIPSO provide more accurate instantaneous observations of clouds, especially the prevalent low‐level clouds in the Arctic, compared to passive sensors (Cesana & Chepfer, ; Chan & Comiso, ; Henderson et al, ; Kay & L'Ecuyer, ), they have difficulty capturing the climatological cloud effects due to their limited spatial‐temporal sampling (Kay & L'Ecuyer, ; Liu, ). At the southern and western coasts where surface melts frequently in summer (Hall et al, ), surface albedo rather than cloud properties might play a more important role in determining CRE (Wang et al, ).…”

Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo–cloud interactions

“…Besides temperature and humidity changes, clouds modify the illumination and reflection of the surface. For highly reflecting snow surfaces, radiative transfer simulations show that two processes are crucial: (i) a cloud-induced weighting of the transmitted downward irradiance to smaller wavelengths, causing an increase in shortwave surface albedo, and (ii) a shift from mainly direct to rather diffuse irradiance in cloudy conditions, which decreases the shortwave albedo (Warren, 1982). Observations have shown that there is a tendency for which the surface albedo is larger in cloudy compared to cloud-free conditions (e.g., Grenfell and Perovich, 2008), which was demonstrated for a seasonal cycle by Walsh and Chapman (1998) for highly reflective surface types.…”

Strong Summer Atmospheric Rivers Trigger Greenland Ice Sheet Melt through Spatially Varying Surface Energy Balance and Cloud Regimes

“…2017; Cullather and Nowicki 2018;Wang et al 2018), and future GrIS melt projections are highly sensitive to modeled cloud properties (Hofer et al 2019). Given the large fluxes of water vapor delivered by ARs, it is likely that some parts of the GrIS experience SMB losses under cloudy conditions during AR events.…”