The system of trade wind cumulus clouds observed during the RICO field project was simulated by an LES model over a 50x50 km2 domain size. Parameters of latent heat release were analyzed with the goal of parameterizing their effects on grids typical for NWP and large-scale models. Over 2000 clouds were examined focusing on relationship between parameters of latent heat release (phase transition rates) and dynamical/microphysical cloud characteristics. The phase transition rates (Tr), which in warm tropical clouds are represented by processes of condensation/evaporation, were analyzed by stratifying the clouds by their size/stage of maturity. The analyzed parameters included, among others, integral mass and buoyancy fluxes, cloud and rain water parameters. In our previous investigation we found that a remarkably strong correlation exists between Tr and upward mass flux (ℳ). The strong dependence of phase transition rates on ℳ, as well as linear relationship between Tr and ℳ, was explained by applying the condensation theory and the concept of “quasi-steady” supersaturation. The LES derived slope of the linear fit agreed with its theoretically predicted value with an error less than 5%. This result implies that supersaturation in clouds, on average, varies within a few percentage points of its quasi-steady value. The theory, as well as LES data, show that the Tr - ℳ linear fit is valid for local variables, and, therefore, may be integrated to obtain horizontal mean parameters. Expanding the Tr - ℳ relationship for vertically dependent horizontal mean variables, may provide the framework for development of sub-grid scale (SGS) latent heat release parameterization. It was also suggested that calculating the slope of the linear fit from concurrent measurements of temperature and vertical velocity, and comparing it with the theoretical slope based on the quasi-steady supersaturation assumption, may offer a method for estimating the supersaturation in clouds.

The system of trade wind cumulus clouds observed during the RICO field project was simulated by an LES model in a domain of the size of a mesoscale model grid. More than 2000 clouds were analyzed by stratifying them by their cloud-top heights. The investigation was focused on phase transition rates (TR), which in warm tropical clouds are represented by the processes of condensation/evaporation. We previously demonstrated (Kogan, 2021a), based on LES data, that a nearly perfect correlation (R = 0.99) exists between upward mass flux (MFP) and condensation rate (CR), and that the correlation between MFP and evaporation rate (ER) is only slightly lower (R = 0.98). The strong dependence of TR on MFP and the linear relationship between them were explained by applying condensation theory and the concept of “quasi-steady” supersaturation. The LES-derived slope of the linear TR−MFP relationship agreed with its theoretical value, with an error of less than 5%. This result implies that supersaturation in clouds, on average, varies within a few percentage points of its quasi-steady value. In our analysis we considered parameters characterizing cloud as a whole; that is, parameters integrated over the cloud volume. However, condensation theory and LES data show that the linear fit is applicable to local variables and therefore may be integrated to obtain relationships for horizontally averaged variables. Expanding the TR−MFP relationship to vertically dependent variables may provide the framework for development of sub-grid scale latent heat release parameterization.

We performed LES simulation of tropical cumulus clouds initialized with soundings from the RICO field project (vanZanten et al. 2011, JAMES). Our analysis concentrated on thermodynamic characteristics of convective clouds, specifically on relationships between the condensation/evaporation rates and cloud micro-physical and dynamical parameters. Such relationships are important for developing parameterizations of the sub-grid latent heat release on the grid size typical for a mesoscale model. The simulation was conducted using an integration domain of 50km; about 2000 clouds were selected for analysis over the course of the 28 hour simulation. The condensation/evaporation rates were analyzed by stratifying the clouds by their size (cloud top). The analyzed parameters included, among others, integral mass and buoyancy flux, cloud and rain water, supersaturation. The results of the analysis revealed a rather remarkable relationship between integral latent heat released in a cloud and cloud’s integral mass flux. This relationship may form the basis for parameterization development of latent heat release.

The system of trade wind cumulus clouds observed during the RICO field project was simulated by an LES model over a domain size of a mesoscale model grid. More than 2000 clouds were analyzed by stratifying them by their size/stage of maturity. The investigation was focused on phase transition rates, which in warm tropical clouds are represented by the processes of condensation/evaporation. Data from LES simulations of shallow cumulus clouds demonstrated a nearly perfect correlation between condensation rate (CR) and upward (positive) mass flux (MFP). This strong correlation is explained using the condensation theory. The strong CR-MFP correlation implies that supersaturation in clouds varies within a few percent of its quasi-steady value. Calculating the slope of the CR-MFP linear fit from concurrent measurements of temperature and vertical velocity, and comparing it with the corresponding slope based on the quasi-steady supersaturation assumption, may provide a method for estimating the supersaturation in clouds. The strong dependence of condensation rates on vertical velocity may indicate the direction for development of SGS latent heat release parameterization.