Proximity soundings of thundersnow in the central United States

Abstract

[1] Proximity balloon soundings for snow events with lightning and thunder during the period 1961 through 1990 reveal a less statically stable environment than similar nonthundering snow events. When thundersnow is present, a less stable environment (and in some cases subsequent upright convection) is found aloft in all of the thundering cases examined here; all of the events feature their most unstable parcel originating above a frontal inversion. In fact, only events in the cold air north of an extratropical cyclone are included in this study. Events with a lake effect or orographic enhancement are eliminated from the sample. The basic composite derived by averaging temperatures at an established interval reveals a nearly saturated lower atmosphere, below 0°C throughout its depth, with the frontal inversion present and its most unstable parcel occurring just above the top of the inversion. The feature-preserving composite approach of R. A. Brown (1993) better defines the frontal inversion bottom and top as well as the level and temperature of the most unstable parcel; these are the features in need of preservation, and a less statically stable environment emerges by doing so. Other salient features include the most unstable parcel originating some 30-50 mbar above the top of the frontal inversion and significant drying $100 mbar above the level of the most unstable parcel. The bulk sounding characteristics also favor the existence of lightning. The composite temperature at the level of the most unstable parcel is À8.7°C, which allows for enhanced amounts of supercooled water to enter any updraft that may form. The temperature of the most unstable parcel at its origin is also warmer than the charge reversal temperature; therefore convection of any appreciable depth will span that level. Moreover, the height of the composited À10°C level is 2959 m above ground level, which previous investigators have shown is sufficiently high to favor lightning production. Yet no convective available potential energy (CAPE) appears with either composite approach, which concurs with previous studies. While several of the composite members feature CAPE for elevated layers, the majority do not, suggesting that other processes (e.g., the release of symmetric instability), which are difficult to assess from a single sounding, tend to be at work.

A Case Study of Anomalous Snowfall with an Alberta Clipper

“…The NARR soundings for Buffalo immediately prior (0000 UTC and 0300 UTC, not shown) are less impressive, having lifting parcel levels of 650 hPa (both), Lifted Index values (Lifted Index is the algebraic difference between the temperature of a adiabatically lifted parcel (based on a near-surface mixed layer) and that of the environment at 500 hPa; positive values indicate a parcel that is cooler than the environment, indicating static stability) of +4 (0000 UTC) and +2 (0300 UTC), and 700-500-hPa lapse rates of 5.7 K km −1 (0000 UTC) and 6.4 K km −1 (0300 UTC). However the 0600 UTC sounding is quite similar to the model suggested by Market et al [37] for midwestern United States convective snow cases, with a Lifted Index of 0, and a 700-500-hPa lapse rate of 7.2 K km −1 . However, no most unstable convective available potential energy (MUCAPE) is diagnosed at any of these times.…”

A Case Study of Severe Winter Convection in the Midwest

“…While most thundersnow events in the Midwest occur in the presence of an extratropical cyclone (Market et al 2002) resulting from some type of elevated instability, this event appeared to maintain a quality more related to warm season and warm sector severe weather rooted in the planetary boundary layer, of the like associated with squall lines. Previous snow event studies involving wintertime convection, such as Kocin et al (1985) and Schneider (1990), have been completed with the aid of mesoscale models.…”

“…Previous snow event studies involving wintertime convection, such as Kocin et al (1985) and Schneider (1990), have been completed with the aid of mesoscale models. Yet, these events were part of large synoptic-scale systems more typical of a cold season extratropical cyclone scenario described by established climatologies (Holle et al 1998;Market et al 2002).…”

Lightning and Rainfall Characteristics in Elevated vs. Surface Based Convection in the Midwest that Produce Heavy Rainfall

“…The Mann-Whitney test was used as opposed to a two-sample t-test because the variables tested are not normally distributed. This was done in a similar manner by [27] for proximity soundings of thunder snow and non-thundering snow events in the central United States. All surface based thunderstorm cases (n = 8) were considered one set of data and the elevated thunderstorm cases (n = 8) were considered the other set of data for the Mann-Whitney tests.…”

“…In a further effort to minimize bias between the two groups of thunderstorms, sounding parameters were calculated, including the depth of the warm cloud layer, for both elevated and surface based convection. This was done for both individual soundings, as well as their composites as suggested by [26] and employed by [27]. In addition, the two sets were examined for the dominant mode of convection.…”