Observational constraints on the sub-grid variability of cloud and rain: Implications for microphysical parameterization

The transition of cloud water to rain water through the autoconversion and accretion processes are highly non-linear. Furthermore, accretion is sensitive to the covariance of cloud and rain water. Accurately representing these process in global models therefore requires assumptions regarding the sub-grid variability and co-variability of cloud and rain hydrometeors. In addition, recent results suggest that model estimates of the aerosol indirect effect are largely determined by the balance between accretion and autoconversion. We present a multi-sensor analysis of the covariance parameters that govern these processes derived from Aqua-MODIS and CloudSat observations for marine boundary layer clouds. These observational results provide critical constraints on the sub-grid variability and co-variability that are assumed in microphysical parameterizations. These results reiterate a substantial dependence of the sub-grid cloud water on cloud regime, which scales well with cloud fraction and cloud water path. A power-law dependence of the rain water content on the cloud water content is found that permits straightforward integration into existing cloud microphysics parameterizations. The power-law covariance scaling between cloud and rain shows only minor variation with cloud regime. However, the interaction of this covariance with the sub-grid cloud variability results in significant regime dependence in the derived accretion rates. In particular, a significant enhancement of accretion rates can be inferred in shallow convection regimes relative to stratocumulus cloud regimes. This result is particularly relevant to modelling efforts that seek to unify microphysical representation across cloud regimes that have traditionally been treated by independent parameterization schemes. Initial implementation of the observational constraints in the Community Atmosphere Model (CAM) suggest that obtaining the correct balance between autoconversion and accretion requires careful attention to both sub-grid varibility of cloud water and the covariance of cloud and rain water.