Contrasting Spring and Summer Large-Scale Environments Associated with Mesoscale Convective Systems over the U.S. Great Plains

Abstract

Mesoscale convective systems (MCSs) are frequently observed over the U.S. Great Plains during boreal spring and summer. Here, four types of synoptically favorable environments for spring MCSs and two types each of synoptically favorable and unfavorable environments for summer MCSs are identified using self-organizing maps (SOMs) with inputs from observational data. During spring, frontal systems providing a lifting mechanism and an enhanced Great Plains low-level jet (GPLLJ) providing anomalous moisture are important features identified by SOM analysis for creating favorable dynamical and thermodynamic environments for MCS development. During summer, the composite MCS environment shows small positive convective available potential energy (CAPE) and convective inhibition (CIN) anomalies, which are in stark contrast with the large positive CAPE and negative CIN anomalies in spring. This contrast suggests that summer convection may occur even with weak large-scale dynamical and thermodynamic perturbations so MCSs may be inherently less predictable in summer. The two synoptically favorable environments identified in summer have frontal characteristics and an enhanced GPLLJ, but both shift north compared to spring. The two synoptically unfavorable environments feature enhanced upper-level ridges, but differ in the strength of the GPLLJ. In both seasons, MCS precipitation amount, area, and rate are much larger in the frontal-related MCSs than in nonfrontal MCSs. A large-scale index constructed using pattern correlation between large-scale environments and the synoptically favorable SOM types is found to be skillful for estimating MCS number, precipitation rate, and area in spring, but its explanatory power decreases significantly in summer. The low predictability of summer MCSs deserves further investigation in the future.

The Detection of Mesoscale Convective Systems by the GPM Ku-Band Spaceborne Radar

“…In April, MCSs produced under strong baroclinic forcing usually feature broad stratiform rain region, which results in the largest precipitation area, but the number is relatively low compared to the midsummer, when the CONUS has the weakest baroclinic instability and minimal frontal forcing. However, due to the favorable thermodynamic conditions and possible influence from sub-synoptic disturbances, local convection can nevertheless frequently grow upscale into MCSs (Wang et al 2011;Song et al 2019;. The combination of spring-like baroclinic waves and a continuously warming surface possibly favors the peak number of MCSs in late spring and early summer (i.e., June), but the MCS precipitation areas are generally smaller than in spring.…”

Spatiotemporal Characteristics and Large-Scale Environments of Mesoscale Convective Systems East of the Rocky Mountains

“…Observational studies have documented various large-scale environment features important to MCS genesis, such as a low-level jet (LLJ) of air with low static stability and high convective available potential energy (CAPE), baroclinic frontal zone, vertical wind shear, low-level convergence and upper-level divergence (Coniglio et al 2010;Laing and Fritsch 2000;Song et al 2019). It is unclear if climate models, either global or regional scale, are able to simulate the diverse large-scale environmental conditions that are associated with observed MCSs in different seasons (Song et al 2019).…”

Can the Multiscale Modeling Framework (MMF) Simulate the MCS‐Associated Precipitation Over the Central United States?

“…Previous studies show that the MMF with the coarse host‐GCM grid is able to capture MCSs in the Tropics (Tao & Chern, ; Randall et al, ). However, our study shows the MMF model has difficulty in simulating MCS‐associated precipitation in midlatitude continental conditions during spring where most MCSs were supported by strong baroclinic forcing (Feng et al, ; Song et al, ). The different model behaviors reflect the different mechanisms driving the MCSs between tropical and midlatitude regions.…”

“…Investigating how resolving topography variations within the CRM domains would help the MMF model simulate MCSs can be an interesting future work. Further, summer MCSs in this region often occur under weak baroclinic forcing with favorable thermodynamic environments (Feng et al, ; Song et al, ). Future work similar to this study is needed to examine how well the MMF can simulate observed summer MCSs.…”