Cloud Microphysical and Radiative Properties in the Warmer Arctic

Description

As the high Arctic continues to be affected by polar amplification, we can expect an increasing frequency and duration in episodes when the lower troposphere is at temperatures above freezing. In these episodes clouds will be deglaciated, implying significantly different radiative properties from mixed-phase clouds at just slightly lower temperatures (down to -10°C) where secondary ice production (SIP) influences the microphysics. We surveyed North Slope of Alaska (NSA) data from 2001-2024 and evaluated the temperature dependence in cloud liquid water path (LWP) over the cloud base temperature range -25°C to +10°C.  We find that the increase in LWP at above-freezing temperatures depends strongly on the prevailing meteorological regime. The prevailing meteorological regimes are identified using k-means clustering on NSA surface meteorological data, and comprise four distinct clusters:

(1) High pressure North of NSA, with Northeasterly onshore flow;

(2) Low pressure North of Utqiaġvik, with Northwesterly onshore flow;

(3) High pressure North of NSA with concomitant low pressure in the Gulf of Alaska, and strong Northeasterly to Easterly flow at NSA;

(4) Strong Southerly flow through the Bering Strait, with weak Southerly to Southeasterly surface flow and stronger Southerly flow in the middle and upper levels at NSA.

Cluster 1 comprises colder than average conditions at NSA. Clusters 2 comprises moderate conditions. Clusters 3 and 4 entail the warmest conditions at NSA, influenced by the strong Southerly flow. The coldest cluster (1) conditions do not produce a significant number of instances where cloud temperatures are above freezing, in any season. In clusters (2) and (3), LWP at above-freezing temperatures is not significantly different than in the slightly colder SIP regime. In cluster (4) LWP is approximately twice that observed in the SIP regime. We also used ERA5 reanlysis data to investigate multi-decadal trends in relative frequencies of the four clusters. No trends emerge in the summer season (JJA), but during spring (MAM) and autumn (SON), the coldest cluster frequency decreases while the two warmer clusters' frequencies increase. This suggests a cloud phase radiative feedback during seasons of sea-ice melt and re-freeze in the Beaufort and Chukchi Seas, associated with gradual Arctic warming.