Comparison With In-Situ Observations and Model Improvements of Ice Cloud Properties Simulated by the Community Atmosphere Model (CAM5)

Abstract:

Detailed measurements of ice crystals in cirrus clouds were used to compare with results from the Community Atmospheric Model Version 5 (CAM5) global climate model. The observations are from two different field campaigns; Atmospheric Radiation Measurements Spring Cloud Intensive Operational Period in 2000 (ARM-IOP), which was characterized primarily by midlatitude frontal clouds and cirrus, and Tropical Composition, Cloud and Climate Coupling (TC4), which was dominated by anvil cirrus. Results show that the model typically overestimates the slope parameter of the exponential size distributions of cloud ice and snow, while the variation with temperature (height) is comparable. The model also overestimates the ice/snow number concentration (0^th moment of the size distribution) and underestimates higher moments (2^nd through 5^th), but compares well with observations for the 1^st moment. The mass-weighted terminal fallspeed is lower in the model compared to observations, which is partly due to the overestimation of the size distribution slope parameter. Sensitivity tests with modification of the threshold size for cloud ice to snow autoconversion (D_cs) do not show noticeable improvement in modeled moments, slope parameter and mass weighed fallspeed compared to observations. Further, there is considerable sensitivity of the cloud radiative forcing to D_cs, consistent with previous studies, but no value of D­_cs improves modeled cloud radiative forcing compared to measurements. Since the autoconversion of cloud ice to snow using the threshold size D_cs has little physical basis, we suggest improvements by removing the artificial distinction of cloud ice and snow, combining them into a single ice-phase category, and improving the representation and self-consistency of ice particle properties in the model using empirical particle mass-dimensional and projected area-dimensional relationships. Preliminary results with the improved parameterization will be shown.