The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

Abstract

Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly low-level jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 140) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature e advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surface-based stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximum-e CAPE values of more than 1000 J kg 1 in the vicinity of the MCS site and higher values upstream. Maximum-e convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg 1) while to the south convection is limited by large values of CIN (greater than 60 J kg 1). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.

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

“…Nocturnal mesoscale convective systems (MCSs) are responsible for a large percentage of warm season rainfall in the central United States (Fritsch et al 1986), and are capable of producing extreme rainfall events and flash flooding (e.g., Moore et al 2003;Schumacher and Johnson 2005). Whereas daytime convection frequently derives convective energy from the planetary boundary layer (PBL) (these types of systems are referred to as ''surface based''), nocturnal MCSs often thrive in conjunction with a statically and convectively stable PBL, and derive the majority of their energy from layers of air above the PBL (Corfidi et al 2008; these types of systems are referred to as ''elevated'').…”

Instability indices and forecasting thunderstorms: the case of 30 April 2009

“…It is, therefore, necessary to study the behaviour of thunderstorms in mesoscale conditions. Moore et al (2003) have investigated the environment of warmseason elevated thunderstorms associated with heavy rainfall over the central United States. Jacobs and Maat (2005) have presented numerical guidance methods for aiding the decision making related to aviation meteorological forecasts.…”

25 Years Inter-Research 1979-2004

“…Skill (46%) and R 2 (32.5%) at the station level (Table 7B) are still low possibly because summer precipitation in Iowa is associated with intense mesoscale convective systems (e.g. Moore et al 2003). Nevertheless, at daily timescales (Fig.…”

“…It should be noticed, however, that in some locations like the grid cell in Iowa, where the low-level jet can be an important precipitation mechanism during the summer (e.g. Giorgi et al 1996, Moore et al 2003, the best results were obtained adding the meridional wind component at the 850 hPa level (v 8) rather than u8. Thus, it appears that the role of the low-level wind component in the mid-latitudes during the summer season is regionally dependent.…”

“…This is possibly associated with the important role of the meridional low-level jet and low-level moisture during the warmseason precipitation in this region (e.g. Giorgi et al 1996, Moore et al 2003. Skill (46%) and R 2 (32.5%) at the station level (Table 7B) are still low possibly because summer precipitation in Iowa is associated with intense mesoscale convective systems (e.g.…”