Regime dependence of ice cloud heterogeneity – a convective life‐cycle effect?

Abstract

Cloud condensate varies on scales smaller than those typically resolved by global weather and climate models. In order to accurately predict the radiative and microphysical process rates representative of the entire model grid box, the effect of the subgrid‐scale heterogeneity of cloud must be taken into account. In this study, observed ice water content retrieved from A‐Train satellite observations is used to explore how spatial ice condensate variability, characterized by the fractional standard deviation (FSD, the standard deviation divided by the mean), varies with cloud regime. FSD is generally lower for overcast cloud scenes, but an additional predictor based on convective activity is needed to capture the high FSD associated with more turbulent clouds and reproduce the observed latitudinal and height variations of FSD. Convective clouds that extend only a few kilometres above the freezing level are likely to be smaller at an earlier stage in their life cycle, and actively growing with a higher FSD. In contrast, more mature clouds reaching the tropopause are larger and generally have a lower variability and smaller FSD. To capture this life‐cycle effect, a new parametrization is tested which uses the ratio of a model's convectively detrained condensate to the existing cloud condensate mass as a proxy for the cloud's convective life stage to highlight areas with enhanced condensate variability. The parametrization is scale‐adaptive, situation‐dependent and captures seasonally varying global patterns and the zonal mean vertical structure of observed ice condensate variability well. Ground‐based observations obtained from five Atmospheric Radiation Measurement sites provide independent confirmation that the parametrization satisfactorily captures condensate variability in high‐altitude ice clouds.

A Flexible and Efficient Radiation Scheme for the ECMWF Model

“…Another challenge is to refine many of the assumptions made in the radiation scheme that determine the radiative effect of clouds, such that they agree as well as possible with the latest observations, and are consistent with the IFS cloud scheme and the observational forward operators in the data assimilation system. This could include switching from EXP‐EXP to EXP‐RAN overlap, changing the fractional standard deviation of in‐cloud water content from 1 to a value closer to observations (e.g., Ahlgrimm & Forbes, ; Shonk et al, ), representing convective clouds in the radiation scheme, and making use of an ice optics parameterization that represents more recent findings that ice particles are optically rough (Baran et al, ; Yi et al, ). Although not affordable operationally, ecRad also has the option to represent 3‐D effects for the first time in a global model.…”

Cirrus Horizontal Heterogeneity and 3‐D Radiative Effects on Cloud Optical Property Retrievals From MODIS Near to Thermal Infrared Channels as a Function of Spatial Resolution

“…From Atmospheric Radiation Measurement ground‐based observations, Ahlgrimm and Forbes () confirm that the FSD of isolated cirrus clouds, at 80‐km scale, falls largely within the range of parameterized values from Hill et al (). Ahlgrimm and Forbes () refined the formulation by Hill et al (), wherein FSD is dependent on total water and is enhanced for convective situations based on the detrainment ratio.…”

Enhanced Cloud Top Longwave Radiative Cooling Due To the Effect of Horizontal Radiative Transfer in the Stratocumulus to Trade Cumulus Transition Regime

“…To quantify f σ , the ratio of the standard deviation of in‐cloud water content to the mean in‐cloud water content, the cloud water content sub‐grid variability needs to be provided. The sub‐grid variability of cloud water content may be assumed or diagnosed (e.g., Ahlgrimm & Forbes, 2017). Some GCMs (e.g., Bogenschutz et al., 2012) have adopted the probability density function (PDF)‐based boundary layer cloud parameterization scheme (Golaz et al., 2002) to treat shallow convection.…”