Arctic Clouds and Precipitation in the Community Earth System Model Version 2.

Area of Science:

• Climate Science
• Atmospheric Science
• Arctic Research

Background:

• The Arctic is warming twice as fast as the global average.
• Accurate climate models are crucial for predicting Arctic climate evolution.
• Representing small-scale cloud processes in global climate models (GCMs) remains a challenge.

Purpose of the Study:

• To evaluate Arctic cloud and precipitation representation in the latest Community Earth System Model (CESM2 with CAM6).
• To compare CESM2 with its predecessor (CESM1 with CAM5) using preindustrial control runs.
• To understand how model updates affect Arctic climate simulations.

Main Methods:

• Comparison of preindustrial control runs between CESM1 (CAM5) and CESM2 (CAM6).
• Analysis of changes in Arctic cloud properties (ice and water content).
• Assessment of liquid-containing cloud (LCC) frequency and its impact on surface radiation.

Main Results:

• CESM2 shows a decrease in Arctic cloud ice and a dramatic increase in cloud water.
• Liquid-containing cloud frequency increased from 19% in CESM1 to 51% in CESM2.
• Increased LCCs led to higher downwelling longwave radiation (+22 W m⁻²) and lower shortwave radiation (-23 W m⁻²).
• Mean Arctic surface temperature increased by 3 K in CESM2 (257 K to 260 K), with a notable winter increase of 6 K.
• Annual snowfall decreased slightly, while rainfall increased.

Conclusions:

• CESM2 exhibits significant changes in Arctic cloud characteristics compared to CESM1.
• These cloud changes influence Arctic radiative balance and surface temperature.
• The updated model indicates shifts in Arctic precipitation patterns, with more rainfall and less snowfall.