Comparison of the Diurnal Precipitation Cycle in Convection-Resolving and Non-Convection-Resolving Mesoscale Models

Abstract

The diurnal cycles of rainfall in 5-km grid-spacing convection-resolving and 22-km grid-spacing nonconvection-resolving configurations of the Weather Research and Forecasting (WRF) model are compared to see if significant improvements can be obtained by using fine enough grid spacing to explicitly resolve convection. Diurnally averaged Hovmöller diagrams, spatial correlation coefficients computed in Hovmöller space, equitable threat scores (ETSs), and biases for forecasts conducted from 1 April to 25 July 2005 over a large portion of the central United States are used for the comparisons. A subjective comparison using Hovmöller diagrams of diurnally averaged rainfall show that the diurnal cycle representation in the 5-km configuration is clearly superior to that in the 22-km configuration during forecast hours 24-48. The superiority of the 5-km configuration is validated by much higher spatial correlation coefficients than in the 22-km configuration. During the first 24 forecast hours the 5-km model forecasts appear to be more adversely affected by model "spinup" processes than the 22-km model forecasts, and it is less clear, subjectively, which configuration has the better diurnal cycle representation, although spatial correlation coefficients are slightly higher in the 22-km configuration. ETSs in both configurations have diurnal oscillations with relative maxima occurring in both configurations at forecast hours corresponding to 0000-0300 LST, while biases also have diurnal oscillations with relative maxima (largest errors) in the 22-km (5-km) configuration occurring at forecast hours corresponding to 1200 (1800) LST. At all forecast hours, ETSs from the 22-km configuration are higher than those in the 5-km configuration. This inconsistency with some of the results obtained using the aforementioned spatial correlation coefficients reinforces discussion in past literature that cautions against using "traditional" verification statistics, such as ETS, to compare high-to low-resolution forecasts. ABSTRACTThe diurnal cycles of rainfall in 5-km grid-spacing convection-resolving and 22-km grid-spacing nonconvection-resolving configurations of the Weather Research and Forecasting (WRF) model are compared to see if significant improvements can be obtained by using fine enough grid spacing to explicitly resolve convection. Diurnally averaged Hovmöller diagrams, spatial correlation coefficients computed in Hovmöller space, equitable threat scores (ETSs), and biases for forecasts conducted from 1 April to 25 July 2005 over a large portion of the central United States are used for the comparisons. A subjective comparison using Hovmöller diagrams of diurnally averaged rainfall show that the diurnal cycle representation in the 5-km configuration is clearly superior to that in the 22-km configuration during forecast hours 24-48. The superiority of the 5-km configuration is validated by much higher spatial correlation coefficients than in the 22-km configuration. During the first 24 forecast hours the 5-km model for...

On the forecast skill of a convection-permitting ensemble

“…As a consequence, convectively generated cold pools that drive convective system propagation cannot be properly simulated, resulting in simulated system movement that is too slow. In weak synoptic forcing, for example, organized mesoscale convection systems (MCSs) are particularly challenging for convection-parameterizing models (Clark et al, 2007;Liu et al, 2006). Another drawback is that the inadequate descriptions of buoyancy and updrafts in a convection-parameterizing model often cause convection to initiate too early.…”

“…Another drawback is that the inadequate descriptions of buoyancy and updrafts in a convection-parameterizing model often cause convection to initiate too early. This premature initiation of convection often results in timing and location errors as well as difficulty to simulate the diurnal cycle of rainfall (Clark et al, 2007). Detailed discussion on the convection initiation in a convection-parameterizing model can be found in Davis et al (2003) and Bukovsky et al (2006).…”

Diurnal cycle of precipitation over the British Isles in a 0.44° WRF multiphysics regional climate ensemble over the period 1990–1995

“…Many studies (e.g. Dai et al 1999a;Brockhaus et al 2008;Clarke et al 2007;Diro et al 2012) have attributed this to an overly sensitive precipitation trigger mechanism in the models. Another well known feature of all models is that the diurnal cycle of warm season precipitation is more difficult to simulate than the winter diurnal cycle.…”

“…This is often attributed to the enhanced representation of complex terrain and land surface processes (Kendon et al 2012). Furthermore, reducing the grid spacing to 'convection resolving' values (i.e., <~5km) can also improve the simulation of precipitation through improved representation of small scale convective processes (Brockhaus et al 2008;Clarke et al 2007;Kendon et al 2012). Although increasing the horizontal resolution has been shown to reduce the bias in the simulated diurnal cycle, some bias still remains.…”

Assessment of a Regional-Scale Weather Model for Hydrological Applications in South Korea

“…Generally, cumulus parameterization is not necessary for a grid size (grid scale) less than 3 or 4 km because mesoscale processes and motion can be approximately resolved by the grid size (Gilliland & Rowe, 2007). However, in order to avoid the energy accumulation at grid points, cumulus parameterization is commonly activated simultaneously with microphysics parameterization for rainfall prediction for simulations with grid size of 4 km or even smaller, especially when the grid size falls in the so-called "no-man's land" or "gray zone" (1 -10 km, based on Gerard, 2007).. On the other hand, Clark et al (2007) demonstrated that the diurnal precipitation cycle was better simulated using convection resolving microphysics schemes than non-convection-resolving cumulus parameterization scheme. Thus, in this study, we will test both approaches for finer-scale grid intervals, namely activating cumulus and microphysics parameterizations or using microphysics parameterization alone.…”