Using ARM Nauru Observations to Evaluate a Cloud-Base Mass Flux Parameterization for Shallow Marine Cumuli

Description

The parameterizations of fair-weather cumuli in climate models are frequently based on cumulus mass flux approaches. One critical component of these parameterizations is the formulation of the cloud-base mass flux. In this study the ARM Nauru observations obtained during a prolonged period of suppressed conditions dominated by shallow cumuli are used to evaluate a parameterization of the cumulus cloud-base mass flux. The parametrization evaluated was formulated originally for use in bulk models of shallow cumuli and considers the mass budget of the transition layer (at the top of the mixed layer) that separates the cumulus layer from the subcloud layer. It is shown that in this formulation of the cloud-base mass flux normalized by the convective velocity scale w*is proportional to the subloud turbulence kinetic energy (TKE) and inversely proportional the convective inhibition (CIN) energy associated with the stable layer that forms the transition layer at the top of the mixed layer. Application of a mixed layer model to the subcloud layer using monthly averaged near surface temperature, moisture, and wind speed measurements and cloud base height estimates from a ceilometer indicate that the subcloud layer is in radiative-convective equilibrium. The associated surface virtual temperature fluxes from these steady conditions provide monthly estimates of the convective velocity scale w*. The CIN is calculated from the night-time soundings made at the island. The CIN and w* monthly values obtained from these calculations show a good correlation with the normalized cloud-base mass flux and provide one of the first direct verifications of a cloud-base mass flux parametrization of shallow cu using observations. The cloud-base mass flux formulation evaluated in this study is compared with other mass flux parameterizations used in large-scale numerical models.