On the Changes in Convection-Allowing WRF Forecasts of MCS Evolution due to Decreases in Model Horizontal and Vertical Grid Spacing. Part I: Changes in Cold Pool Evolution

Abstract

The degree of improvement in convective representation in NWP with horizontal grid spacings finer than 3 km remains debatable. While some research suggests sub-km horizontal grid spacing is needed to resolve details of convective structures, other studies have shown that decreasing grid spacing from 3-4 km to 1-2 km offers little additional value for forecasts of deep convection. In addition, few studies exist to show how changes in vertical grid spacing impact thunderstorm forecasts, especially when horizontal grid spacing is simultaneously decreased. The present research investigates how warm-season central U.S. simulated MCS cold pools for eleven observed cases are impacted by decreasing horizontal grid spacing from 3 to 1 km while increasing the vertical levels from 50 to 100 in WRF runs. 3 km runs with 100 levels produced the deepest and most negatively buoyant cold pools compared to all other grid spacings since updrafts were more poorly resolved, resulting in a higher flux of rearward-advected frozen hydrometeors, whose melting processes were augmented by the finer vertical grid spacing, which better resolved the melting layer. However, the more predominant signal among all eleven cases was for more expansive cold pools in 1 km runs, where the stronger and more abundant updrafts focused along the MCS leading line supported a larger volume of concentrated rearward hydrometeor advection and resultant latent cooling at lower levels.

A Climatological Study on the Two Types of Bow Echoes Over South China

“…The simulation results of two bow echoes among these cases (Liu et al., 2023; A. Zhou et al., 2020, 2022) indicate that the increasing system moving speed might be more related to the cold pool strength, which also displays the increasing trend during the same duration (not shown). This is consistent with previous studies that the cold pool strength reflects the development of bow echo systems, which is commonly related to the system moving speeds (Adams‐Selin et al., 2013; Meng et al., 2012; Parker & Johnson, 2000; Rotunno et al., 1988; Squitieri & Gallus, 2022). Notably, the moving speeds of MFBEs and CBEs disperse from 11 to 18 m s −1 , and the t test indicates that the differences of moving speeds between the two bow echo types are not significant (Figure 4d).…”

Spatial Dispersion and Statistical Description of Organized Cumulus Cloud Ensembles in Radiative Convective Equilibrium

“…Although the sensitivity of our results to vertical resolution was not tested, accurately representing the structure of cold pools and their bulk properties remains of secondary importance in this study as long as convective initiation along cold pool gust fronts and the subsequent organization of the cloud population are captured in a consistent manner across all five experiments. Note besides that Lebo and Morrison (2015) and Squitieri and Gallus (2022) found that simulations of cold pool driven Mesoscale Convective Systems using varying vertical grid lengths in the boundary layer produced qualitatively similar results, a conclusion that is expected to be particularly true over idealized surfaces where the sensitivity of the boundary layer structure to surface fluxes is reduced.…”

Comparison of Cold Pool Characteristics of Two Distinct Gust Fronts over Bohai Sea Bay in China

“…In conjunction with the ambient wind shear, cold pools contribute to the uplift of surrounding air parcels and facilitate the formation of new cells, thereby organizing mesoscale convective systems (MCSs) [37,38]. Consequently, cold pools play a pivotal role in organizing deep convection and are an indispensable component of MCSs [39,40].…”