Search for a Cloud Phase Feedback in the Arctic Climate System

Abstract

Recent research on climate system feedbacks in global climate model simulations indicates that cloud phase transitions have greater implications than previously realized. This applies not just to the Arctic but to most extratropical regions, and also in the evaluation of equilibrium climate sensitivity increases in the latest versions of the major global climate models. At the same time, this consideration is particularly important to the Arctic, as much of its current amplified climate warming involves surface and lower-atmosphere transitions from just below to just above the freezing point of water.

When a cloud transitions from the mixed-phase to entirely liquid water, one should expect concomitant changes in optical properties that impact shortwave radiation backscattered to space and transmitted to the surface, longwave radiation from the cloud to the surface, and additional potential changes involving cloud amount and lifetime. There has been recent innovative research using satellite imager retrievals of cloud properties, clearly showing the greater importance of phase transitions as compared with single-phase cloud property changes. However, use of in situ data has so far been limited. The recent successful completion of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) and Cold-air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) ARM Mobile Facility campaigns, combined with the North Slope of Alaska (NSA) facility, now provide advanced sensor data from three contrasting Arctic regions that collectively offer a wide variety of lower troposphere scenarios in which temperatures cross the freezing point and cloud properties change accordingly. Compared with satellite data analysis, in situ ARM Program data have the advantages of thorough vertical profiling in the lower troposphere, and high time resolution in atmospheric thermodynamics and cloud properties as temperatures vary.

This project will address a basic question: What happens with real high latitude clouds — in both microphysics and radiation — when lower troposphere temperatures in a given region increase from just below to just above freezing? Existing empirical literature on Arctic mixed-phase clouds is extensive, but most previous efforts comprise either exploratory work on basic cloud properties, climatological assessments, or model applications to individual cloud systems well-studied by advanced sensor suites. In this work we will provide a new approach – we will identify the episodes in the most advanced high latitude ARM data during which lower troposphere temperatures transition from a range encompassing approximately -10°C to -3°C, to the freezing point or just above, in either direction. We will create case studies based on this criterion, typically spanning several days to a few weeks, in which cloud properties will be derived using the most current techniques. There are several ARM value-added products (VAPs) we can compile for initial evaluation of cloud properties. However, we find that the case studies will be more valuable if we perform additional retrievals of cloud effective particle size and optical depth using Atmospheric Emitted Radiance Interferometer (AERI) and shortwave multispectral radiometer data. We also have a new technique using Ka-band zenith radar data that reveals secondary ice production processes, which are significant in the temperature range just below freezing that we are considering in this work. We will perform k-means clustering on meteorological reanalysis data to determine synoptic influences on the cases, and we will characterize the cases in terms of lower troposphere stability (LTS). This comprehensive characterization of each case, combined with ARM measurements of surface radiative fluxes and satellite observations of top-of-atmosphere radiative fluxes, will provide a complete picture of the phase transition as it manifests in the climate system.

We hypothesize that a transition from slightly supercooled to the freezing point or slightly warmer will have three potential manifestations: (1) larger shortwave flux transmitted to the surface as supplemental ice absorption by the cloud disappears; (2) or conversely, larger optical depths in liquid-only clouds that decrease shortwave and increase longwave flux at the surface; and/or (3) higher level mixed-phase or ice clouds might offset the other two effects. We will test these hypotheses as we develop the cases and make them available to climate modelers. MOSAiC provides robust sampling of the high Arctic north of 85°N; COMBLE samples a classical maritime Arctic region, and NSA samples an Arctic region subject to frequently alternating influences of high pressure in the Arctic Ocean and low pressure in the North Pacific.