Mesoscale simulations of organized convection: Importance of convective equilibrium

Abstract

SUMMARYThe validity of convective parametrization breaks down at the resolution of mesoscale models, and the success of parametrized versus explicit treatments of convection is likely to depend on the large-scale environment. In this paper we examine the hypothesis that a key feature determining the sensitivity to the environment is whether the forcing of convection is sufficiently homogeneous and slowly varying that the convection can be considered to be in equilibrium. Two case studies of mesoscale convective systems over the UK, one where equilibrium conditions are expected and one where equilibrium is unlikely, are simulated using a mesoscale forecasting model. The time evolution of area-average convective available potential energy and the time evolution and magnitude of the timescale of convective adjustment are consistent with the hypothesis of equilibrium for case 1 and nonequilibrium for case 2.For each case, three experiments are performed with different partitionings between parametrized and explicit convection: fully parametrized convection, fully explicit convection and a simulation with significant amounts of both. In the equilibrium case, bulk properties of the convection such as area-integrated rain rates are insensitive to the treatment of convection. However, the detailed structure of the precipitation field changes; the simulation with parametrized convection behaves well and produces a smooth field that follows the forcing region, and the simulation with explicit convection has a small number of localized intense regions of precipitation that track with the mid-level flow. For the non-equilibrium case, bulk properties of the convection such as area-integrated rain rates are sensitive to the treatment of convection. The simulation with explicit convection behaves similarly to the equilibrium case with a few localized precipitation regions. In contrast, the cumulus parametrization fails dramatically and develops intense propagating bows of precipitation that were not observed. The simulations with both parametrized and explicit convection follow the pattern seen in the other experiments, with a transition over the duration of the run from parametrized to explicit precipitation. The impact of convection on the large-scale flow, as measured by upper-level wind and potential-vorticity perturbations, is very sensitive to the partitioning of convection for both cases.

On the forecast skill of a convection-permitting ensemble

“…All other days, i.e., 87 days on which rain was reported, are assigned to the bins of weak (23 days) or strong synoptic forcing (64 days). For the classification, a method described by Done et al (2006) and successfully applied by Kühnlein et al (2014) is used, which is based on the temporal variability of CAPE (convective available potential energy) as a measure of atmospheric instability. According to Done et al (2006), the approach helps to distinguish between days on which convection is predominantly at equilibrium or at non-equilibrium.…”

“…For the classification, a method described by Done et al (2006) and successfully applied by Kühnlein et al (2014) is used, which is based on the temporal variability of CAPE (convective available potential energy) as a measure of atmospheric instability. According to Done et al (2006), the approach helps to distinguish between days on which convection is predominantly at equilibrium or at non-equilibrium. This means that the destabilization of the atmosphere by large-scale synoptic forcing is balanced or unbalanced, respectively, by the stabilization through convection.…”

“…is calculated hourly from AROME-EPS CAPE forecasts using t = 1 h. Following the suggestion of Done et al (2006), a specific day is assigned to weak synoptic forcing if the areal mean of t c exceeds a threshold of 6 h at least once a day by at least three ensemble members. In order to test the method of Done et al (2006), we compared the classification with alternative approaches, such as the temporal change of midtropospheric vorticity and convection related to pat-terns in 500 hPa geopotential using archived ECMWF forecast and ERA-Interim reanalyses.…”

“…In order to test the method of Done et al (2006), we compared the classification with alternative approaches, such as the temporal change of midtropospheric vorticity and convection related to pat-terns in 500 hPa geopotential using archived ECMWF forecast and ERA-Interim reanalyses. The results were comparable to those of the equilibrium method.…”

Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

“…In principle, the predictability of convective precipitation should be increased over orography (Paegle et al, 1990), where vertical motions owing to specific topographic features can locally remove the convective inhibition and initiate cells (Done et al, 2006). However, for this enhanced predictability to be realized the initial and boundary conditions on the larger scales must be sufficiently accurate, and high-resolution grids are needed to represent the fine-scale terrain details.…”