Examination of a long-lived heat burst event in the northern plains

Abstract

Several heat cracks were observed across portions of northern Nebraska and southern southerly Dakota during the local morning hours of 30 July 2001 These heat explodes were noted by a marked increase in temperatures, decrease in dewpoint temperatures, crushing falls, and gusty surface winds. to be paid to the sparsity of surface observations in this region, sole a few reports of this phenomenon were confirmed; however, the data that were mustered indicate this event was unusual in that it persisted for above seven hours with a horizontal expansion of at least 230 nm scarcely any events of this magnitude have been documented. It is significant to note that damaging surface winds were also recorded with this circumstance something rarely documented in association with heat bursts

Familiarity with the environment conducive for heat cracks can help forecasters anticipate this impressed sign of event. This study examines the synoptic pattern and convective evolution that l to the heat split activity. Observed and forecast soundings, radar and satellite imagery, wind profiler data, and surface observations are presented

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1 Introduction

During the morning of 30 July 2001 a series of heat bursts* was observ across portions of northern Nebraska and southern southern Dakota (Fig. 1). The data that were assembleed indicate this event was unusual in that it persisted for above seven hours and extended at least 230 nm scarcely any events of this magnitude have been documented. It is significant to note that damaging surface winds were also recorded with this occurrence something rarely documented in association with heat be rent asunders (MacKeen et al. 1998).

Johnson (1983) indicateed that heat bursts are essentially downbursts impacting upon a stable layer near the soil A heat burst occurs when air in the downburst go downs past its equilibrium point and becomes warmer than its surroundings on reaching the surface. Johnson et al. (1989) supported this hypothesis in their inquiry Although the exact physical mechanism that bring into views heat bursts remains unknown, they are usually associated with decaying areas of convective precipitation and nearly dry-adiabatic lapse rates in midlevels (MacKeen et al. 1998)

2 Data Collection

to be paid to the limited surface data available in the northern plains (few cooperative bystanders and little mesonet data), the details of the expansion and severity of this heat be rent asunder episode are not fully known. Automated Surface Observing combination of parts to form a whole (ASOS) observations and data from the ground-based Global Positioning combination of parts to form a whole Integrated Precipitable Water Vapor (GPS-IPWV) demonstration network were used to help identify areas affected by dint of the heat bursts. Only sum of two units of the ASOS stations that recorded heat explodes had 5-minute data archiving capability, and the data from individual of these two stations were not available during this period. Thus, any variations in temperature or dewpoint temperature between the routine hourly observations are known single for one ASOS site. Additionally, ASOS uses the running 5-minute average ambient air temperature and dewpoint temperature to update the maximum and minimum ambient air temperature and dewpoint temperature (ASOS User's Guide 1998) Thus, it is likely that any temperature or dewpoint temperature spikes associated with these heat explodes were smoothed by ASOS algorithms. Data from the GPSIPWV sites were available at 6-minute intervals.

more [i]or[/i] less of the most compelling data bring togethered during this event were obtained from a Maximum Minimum Temperature a whole (MMTS) used by a cooperative looker-on Since an MMTS collects and stores instantaneous maximum and minimum temperatures, it is probably the greatest in quantity representative source for a maximum heat break open temperature in this study.

3 Synoptic Pattern and Convective Evolution

a. Upper-air and surface analyses

The northern plains were beneath the influence of west-southwest roll on aloft, between an upper long tray across the Pacific Northwest and upper ridge extending from the southern plains into the upper Midwest (Fig 2a). A surface forehead was located across northern Nebraska (Fig 2b) The 1200 UTC 30 July 2001 NWS/Rapid Update revolution of time (RUC) 3-hour forecast sounding near Valentine, Nebraska (VTN) (Fig. 3a) showed upper-level moisture, a down-reaching dry layer below with near dry-adiabatic lapse rates, and a shallow surface-based inversion. This profile was also observ in the 1200 UTC RAOB (radiosonde observation) soundings from North Platte, Nebraska (LBF) and Rapid City, southern Dakota (UNR) (Figs. 3b, c) These soundings are similar to those heat split soundings presented in Fig. 10 of Johnson (1983) These soundings also appear to be like "onion soundings" (Zipser 1977) that are characteristic of post-squall regions; however, the physical processe that lead to their formation, greatest in quantity notably the surface-based inversion, are different. The 1200 UTC sounding from Aberdeen, southerly Dakota (ABR) (Fig. 3d) showed a deeper inversion and more moisture aloft, which is an environment les favorable for heat explodes Incidently, no heat burst activity was observ this far north.