Observational Analysis of the Predictability of Mesoscale Convective Systems

Abstract

OBSERVATIONAL ANALYSIS OF THE PREDICTABILITY OF MESOSCALE CONVECTIVE SYSTEMSMesoscale convective systems (MCSs) have a large influence on the weather over the central United States during the warm season by generating essential rainfall and severe weather. To gain insight into the predictability of these systems, the precursor environment of several hundred MCSs were thoroughly studied across the U.S. during the warm seasons of 1996-98. Surface analyses were used to identify triggering mechanisms for each system, and North American Regional Reanalyses (NARR) were used to examine dozens of parameters prior to MCS development. Statistical and composite analyses of these parameters were performed to extract valuable information about the environments in which MCSs form. Similarly, environments that are unable to support organized convective systems were also carefully investigated for comparison with MCS precursor environments.The analysis of these distinct environmental conditions led to the discovery of significant differences between environments that support MCS development and those that do not support convective organization. MCSs were most commonly initiated by frontal boundaries; however, such features that enhance convective initiation are often not sufficient for MCS development, as the environment needs to lend additional support for the development and organization oflong-lived convective systems. Low-level warm air advection, low-level vertical wind shear, and convective instability were found to be III the most important parameters in determining whether concentrated convection would undergo upscale growth into a MCS.Based on these results, an index was developed for use in forecasting MCSs. The MCS index is comprised of conditional terms to ensure that the index is only defined in regions where convective initiation and development are possible. The MCS index assigns a likelihood of MCS development based on three terms: 700 mb temperature advection, 0-3 Ian vertical wind shear, and the lifted index eLI).Each of these parameters promotes convective development and organization through the enhancement of vertical lifting. An analysis of the MCS index showed that it exhibits similar diurnal, episodic, and seasonal characteristics to MCSs. In addition, an objective evaluation of the MCS index revealed that it possesses significant skill in forecasting MCSs, especiallygiven that convective initiation has occurred, offering the possibility of usefulness in operational forecasting.iv

The Impact of Low-Level Moisture Errors on Model Forecasts of an MCS Observed during PECAN

“…In this article, we use an observational analysis of an elevated MCS's near environment to demonstrate how numerical weather prediction (NWP) models' moisture errors within an MCS's EIL influence the ability of these models to accurately predict the MCS. Horizontal and temporal variations in CAPE and CIN influence an MCS's intensity, what direction it will move, the likelihood of the MCS maintaining itself (e.g., Crook and Moncrieff 1988;Coniglio et al 2007;Jirak and Cotton 2007;Trier et al 2014a,b;Peters and Schumacher 2016), and a storm's ability to produce hail (e.g., Brooks et al 2003;Edwards and Thompson 1998;Jewell and Brimelow 2009), tornadoes (e.g., Smith et al 2012; Thompson et al 2012), and damaging winds (e.g., Mahoney et al 2009). The presence of nonzero CIN implies that a parcel is negatively buoyant prior to reaching its level of free convection (LFC; the level after which an ascending parcel attains positive buoyancy relative to its surrounding environment).…”

The 2015 Plains Elevated Convection at Night Field Project

“…Other studies have shown that daytime heating over the elevated terrain of the Rockies generates mesoscale potential vorticity (PV) anomalies that persist even in the absence of latent heating and are advected over the plains at night (Li and Smith 2010). The vertical motion associated with these elevated PV anomalies may trigger convection and help to maintain MCSs (e.g., Jirak and Cotton 2007).…”

The Dependence of Storm Longevity on the Pattern of Deep Convection Initiation in a Low-Shear Environment

“…A number of additional experiments could be developed, steered by how different patterns might affect the depth of the cold pool and therefore storm longevity. Such experiments could involve changing the orientation of the initial line of convection, changing the number/ spacing of thermals in multiple thermal experiments, or initializing thermals in clusters instead of lines (e.g., Jirak and Cotton 2007;Jewett and Wilhelmson 2006).…”