Impact of resolution on simulation of closed mesoscale cellular convection identified by dynamically guided watershed segmentation

Abstract

Organized mesoscale cellular convection (MCC) is a common feature of marine stratocumulus clouds that forms in response to interactions among dynamic, microphysical, and radiative processes at the mesoscale. Cloud resolving models begin to resolve some of these processes, but using high resolutions is extremely costly. To understand the impact of limited resolution on MCC, we use the Weather Research and Forecasting model with chemistry and fully coupled cloud-aerosol interactions to simulate MCC over the southeast Pacific during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). A suite of experiments with 3 and 9 km grid spacing indicates that the simulations with finer grid spacing have smaller liquid water paths and cloud fractions, while cloud tops are higher. The observed diurnal cycle is reasonably well simulated. To isolate organized MCC characteristics, we develop a new automated method that uses the watershed segmentation combining the detection of cloud boundaries with coincident vertical velocities. This ensures that the detected cloud fields are dynamically consistent for closed MCC, a common feature within the VOCALS-REx region. We demonstrate that the 3 km simulation is able to reproduce the scaling between horizontal cell size and boundary layer depth seen in satellite observations for the conditions of 900-1400 m deep boundary layers. However, the 9 km simulation is unable to resolve smaller circulations corresponding to shallower boundary layers, instead producing MCC with an invariant horizontal scale for all simulated boundary layers depths. The results demonstrate that the grid spacing needed for proper simulation of the MCC structure in marine stratocumulus regions depends on the boundary layer depth.

Mechanisms of convective cloud organization by cold pools over tropical warm ocean during the AMIE/DYNAMO field campaign

“…The watershed technique segments a given 2‐D data field into regions, analogous to “watersheds” or catchment basins in topography, and marks their boundaries. Similar applications of this technique have been used to perform cloud classification for precipitation retrieval algorithms using satellite infrared images [ Hong et al ., ] and to identify organized mesoscale cellular convective cells in stratocumulus clouds [ Martini et al ., ]. A minimum threshold area of 1 km 2 (four contiguous grid points) is required for a cold pool to be identified.…”

Cloud regimes as phase transitions

“…In other words, the open-and closed-cell regimes are robust states in the sense that the mean cloud area fraction̄in each case is essentially the same over a broad range of values of D and F; on the other hand, shallow cumulus clouds and unrobust phase or disorganized clouds have greater variability in the cloud area fraction. This model behavior here is an idealization, since, in nature, these properties are less clear-cut, and even a single cloud regime such as open-cell stratocumulus can exhibit different properties under different conditions; for instance, Martini et al [2014] show that the depth of the boundary layer can influence the size of the cells which in turn influences the cloud albedo, and such effects of the details of individual cells are not currently included in the present results, in which the open-cell regime is idealized as a state with negligible cloud cover. Nevertheless, the present idealized model provides a simple basis to which additional physical processes could be included in the future.…”

Coupling spectral‐bin cloud microphysics with the MOSAIC aerosol model in WRF‐Chem: Methodology and results for marine stratocumulus clouds

“…For all simulations, the initial and boundary meteorological conditions are produced from the National Center for Environmental Prediction (NCEP) Final Analysis (FNL) data with 1-degree grid spacing, and the meteorological boundary conditions are updated in every 6 h. We use 1 km horizontal grid spacing and 64 vertical layers as in Yang et al [2011Yang et al [ , 2012 and Martini et al [2014]. The layer thickness increases from 30 m at the lowest level to 50 m at around 1 km altitude, and about 90 m at 2 km altitude.…”