A new bimodal diagnostic cloud fraction parameterization
Van Weverberg, Kwinten — Met Office - UK
Area of research:
The parameterization of subgrid cloud fraction remains important in numerical weather prediction (NWP) models, even at convection-permitting resolution. The subgrid variability of moisture and temperature are still significant at these scales, so that a grid box can be partly cloudy even when the grid-mean state is subsaturated. Complex schemes have been developed over the past decade to explicitly predict the higher-order moments of the subgrid variability to infer subgrid cloudiness. While these schemes are mathematically elegant, they are still computationally expensive and require a number of assumptions to obtain closure. Simpler, diagnostic approaches could help improve NWP models at a fraction of the cost.
Scientists at the UK Met Office developed a new bimodal diagnostic cloud fraction parameterization for use in convection-permitting NWP simulations. The scheme first identifies entrainment zones near sharp inversions. For each level within the entrainment zone, a univariate, bimodal subgrid distribution of saturation departure is then reconstructed from air parcels originating from the top and the bottom of the entrainment zone. Each mode has variances derived from scale-aware turbulent properties, and a weight so that the total saturation-departure in each grid box remains conserved. Indeed, this bimodal behavior is often seen in lidar and aircraft observations, as pockets of dry air from above the inversion are drawn into the entrainment zone without readily mixing. The new parameterization allows for skewed, bimodal subgrid saturation-departure distributions, similar to observations. Using a wealth of observations from the Midlatitude Continental Convective Clouds Experiment (MC3E) at the Southern Great Plains ARM site, it is shown that the new parameterization outperforms existing and more complex approaches, in terms of cloud cover, optical thickness, cloud water content and surface radiation. It is also shown that the new cloud scheme scales with resolution in a way similar to observations.
A new diagnostic bimodal cloud fraction parameterization has been developed for convection-permitting NWP simulations. The scheme allows for bimodal and skewed subgrid saturation departure distributions, similar to those observed with lidar and aircraft. Cloud cover, water content, and optical properties are better represented and more scale-aware with this new scheme, compared to current operational approaches and even more complex prognostic cloud fraction schemes.