Impact of Convective Detrainment on Cloud Optical Properties

In the tropics, convective parameterizations (CPs) contribute a major part to cloud formation through the detrainment of cloud liquid water and ice at the top of convective updrafts. In mass flux-based CPs, the partitioning between cloud liquid water and ice depends on the treatment of condensation/deposition and precipitation in the entraining convective plume model. In most CPs developed for low-spatial resolution numerical weather prediction and climate models, cloud microphysics processes lack the details of those generically parameterized in grid-scale cloud parameterizations.

One approach to assess the impact of convective detrainment on the cloud fraction and optical properties is to use global variable-resolution meshes as the ones currently available with the Model for Prediction Across Scales (MPAS). Variable-resolution meshes spanning between nonhydrostatic and hydrostatic scales are shown to be ideal tools to evaluate the horizontal dependence of parameterized convective and grid-scale moist processes, including cloud liquid water and ice paths and precipitation. Earlier study (Fowler et al. 2016) shows that over the coarse and refined regions of a variable-resolution mesh, moist processes are driven by parameterized convection and grid-scale cloud microphysics, respectively.

We compare the cloud properties simulated with a 30 km uniform resolution mesh against those obtained with a 30 km-6 km variable-resolution mesh, focusing on the refined region of the mesh centered over the Central Pacific and across the transition region between the refined and coarse resolution of the variable-resolution of the mesh. We highlight the impact of convective detrainment on the simulated cloud liquid water and ice paths against satellite data using two scale-aware CPs, that of Grell and Freitas (2014) and that of Alapaty et al. (2014), paired with the cloud microphysics scheme of Thompson et al. (2008).

Experiments that explore the sensitivity of the cloud optical properties to the partitioning of the convective detrainment between the liquid and ice phase are described. Our results highlight the need to improve the parameterization of cloud microphysics processes in convective plume models to reduce biases between the fine and coarse areas of the variable-resolution mesh.

References:

Alapaty, K., J.S. Kain, J.A. Herwehe, O.R. Bullock, M.S. Mallard, T.L. Spero, and C.G. Nolte, 2014: Multi-scale Kain-Fritsch scheme: Formulations and tests. The 2014 Annual CMAS Conference, R.T.P., North Carolina: https://www.cmascenter.org/conference/2014/agenda.cfm#inline1503.

Fowler, L.D., W.C. Skamarock, G.A. Grell, S.R. Freitas, and M.G. Duda, 2016: Analyzing the Grell-Freitas convection scheme from hydrostatic to nonhydrostatic scales within a global model. Mon. Wea. Rev., 144, 2285-2306.

Grell, G.A., and S.R. Freitas, 2014: A scale and aerosol aware stochastic convection parameterization for weather and air quality modeling. Atmos. Chem. Phys., 14, 5233-5250.

Thompson, G. P.R. Field, R.M. Rasmussen, and W.D. Hall, 2008: Explicit forecasts of winter precipitation using an improved bulk cloud microphysics scheme. Part II: Implementation of a new snow parameterization. Mon. Wea. Rev., 136, 5095-5115.