Hydrological and meteorological experimentation at the mesoscale

2000

Monthly Weather Review, 1998

On 8-9 March 1992, a long-lived squall line traversed the state of Kansas, producing hail and damaging winds. It was shown previously that this squall line was part of a synoptic-scale rainband 2000 km in length that was associated with a cold front aloft (CFA). The present study is concerned with the detailed mesoscale structure of this squall line and its relationship to the CFA. Examination of synoptic-scale cross sections based on rawinsonde ascents, and a mesoscale cross section of winds derived from dual-Doppler radar measurements, shows that the squall line was exactly coincident with the ''nose'' of the CFA. The dual-Doppler analysis also shows that the inflow of air to the squall line was elevated, drawing in air from the potentially unstable layer within the weak warm frontal-like feature that was being occluded by the CFA. The stability analysis of the air in the pre-squall-line environment shows that when the CFA overtook the surface position of the drytrough, the thermal and moisture structure of the atmosphere was such that a moderate amount of lifting provided by the CFA could have released convective instability within an elevated layer approximately 1-2 km above ground. The mesoscale structure of the squall line, derived from the radar reflectivity and dual-Doppler wind fields, differs substantially from the ''leading line/trailing stratiform'' conceptual model for midlatitude squall lines. The lack of a strong cold pool, and the presence of strong low-level shear, indicates that the squall line described here was able to persist in its mature stage in an environment that was ''greater than optimal'' in terms of the balance of the vorticity of the cold pool to that of the low-level shear. However, in view of 1) the weakness of the surface cold pool, 2) the elevated inflow and convergence associated with the convection, and 3) the collocation of the large rainband in which the squall line was embedded and the CFA, it seems likely that the CFA (rather than the cold pool) provided the driving force for the squall line.

Monthly Weather Review

A satellite-based climatology is presented of 9607 mesoscale convective systems (MCSs) that occurred over the central and southeastern United States from 1996 to 2017. This climatology is constructed with a fully automated algorithm based on their cold cloud shields, as observed from infrared images taken by GOES-East satellites. The geographical, seasonal, and diurnal patterns of MCS frequency are evaluated, as are the frequency distributions and seasonal variability of duration and maximum size. MCS duration and maximum size are found to be strongly correlated, with coefficients greater than 0.7. Although previous literature has subclassified MCSs based on size and duration, we find no obvious threshold that cleanly categorizes MCSs. The Plains and Deep South are identified as two regional modes of maximum MCS frequency, accounting for 21% and 18% of MCSs, respectively, and these are found to differ in the direction and speed of the MCSs (means of 16 and 13 m s−1), their distribut...

Convective storms that generate hail, lightning, and damaging winds have been identified as a formidable hazard to life and property. Even more impactful are stronger storms that generate and loft liquid-phase hydrometeors to high altitudes where freezing occurs and collisions between drops, graupel, and ice crystals lead to electrification. Condensate loading, sometimes combined with the lateral entrainment of subsaturated air in the storm middle level, initiates the convective downdraft. The subsequent melting of frozen hydrometeors and subcloud evaporation of liquid precipitation, in conjunction with precipitation loading, result in the cooling and negative buoyancy that accelerate the downdraft in the unsaturated layer. A downburst, in general, is defined as a strong downdraft that induces an outburst of damaging winds at or near the ground, and a microburst as a very small downburst with an outflow diameter of less than 4 km and a lifetime of less than 5 minutes. Previous studies of the microphysical structure of downburst-producing convective storms have entailed analysis of polar and geostationary satellite imagery and derived products, meteorological Doppler radar, and in-situ surface wind observations. The current study expands upon previous analysis by incorporating lower tropospheric vertical wind and temperature profile data generated by the Cooperative Agency Profilers (CAP) system that consists of Boundary Layer Profiler (BLP) instruments, operating at a frequency of 915 MHz, and Radio Acoustic Sounding System (RASS) instruments. In addition, Geostationary Lightning Mapper (GLM) data from Geostationary Operational Environmental Satellite (GOES)-16 will also be displayed and analyzed to better explain the role of lightning in a downburst-producing convective storm. Selected thunderstorm events that demonstrate the physical process of downburst generation as observed simultaneously by the GOES-16 Advanced Baseline Imager (ABI) and GLM, Doppler radar (NEXRAD), and boundary layer profilers will be analyzed in this paper. Vertical sounding profile data from the CAP system has been applied as a supplement to Sounding/Hodograph Analysis and Research Program in Python (SHARPpy)-generated thermodynamic profiles to further study the favorable environment for severe convective storm winds. On the afternoon of 1 August 2017, severe downburst-producing thunderstorms occurred in the United States Mid-Atlantic region and over southern California that resulted in tree damage, downed power lines, and traffic disruptions. As shown in Figure 1, for both of these events, GOES-16 ABI water vapor (WV) – thermal infrared (IR) channel brightness temperature difference (BTD) imagery at 2-km resolution displayed a high level of detail in storm structure and was effective in identifying storm-scale features, including cold cloud tops (red shading) and dry-air intrusions (white arrows). Corresponding GLM imagery displayed lightning events in close proximity to downburst events at the time of downburst occurrence.

Weather and Forecasting, 2007

This study documents the convective storm structures and ambient conditions associated with severe storms (wind, hail, and tornado) over the northeastern United States for two warm seasons (May–August), including 2007 and a warm season comprising randomly selected days from 2002 to 2006. The storms were classified into three main convective organizational structures (cellular, linear, and nonlinear) as well as several sub-categories. The same procedure was applied to the highly populated coastal zone of the northeastern United States, including New Jersey, Connecticut, Rhode Island, and New York. The coastal analysis included six warm seasons from 2002 to 2007. Over the Northeast, severe wind events are evenly distributed among the cellular, linear, and nonlinear structures. Cellular structures are the primary hail producers, while tornadoes develop mainly from cellular and linear structures. Over the coastal zone, primarily cellular and linear systems produce severe wind and hail, while tornadoes are equally likely from all three convective structures. Composites were generated for severe weather days over the coastal region for the three main convective structures. On average, severe cellular events develop during moderate instability [most unstable CAPE (MUCAPE) ;1200 J kg 21 ], with low-level warm-air advection and frontogenesis at the leading edge of a thermal ridge collocated with an Appalachian lee trough. Severe linear events develop in a similar mean environment as the cellular events, except that most linear events occur with a surface trough upstream over the Ohio River valley and half of the linear events develop just ahead of progressive midlevel troughs. Nonlinear severe events develop with relatively weak mean convective instability (MUCAPE ;460 J kg 21), but they are supported by midlevel quasigeostrophic (QG) forcing for ascent.

Downbursts are strong downdrafts that induce an outburst of damaging winds at or near the ground, and a microburst is a very small downburst with an outflow diameter of less than 4 km and a lifetime of fewer than 5 minutes. The dangers posed by convective storm-generated downbursts have been extensively documented. Severe windstorms (i.e., widespread convective wind gusts > 25.7 m s−1 (50 kt)) resulting from mesoscale convective systems (MCS) cause significant disruption to society, including widespread power outages, tree and structural damage, and transportation accidents that affect multi-state regions and metropolitan areas along their track. A derecho, defined as a long-lived, widespread severe convective windstorm, is composed of numerous downbursts (intense localized storm downdrafts) organized into clusters or families of clusters. Derechos can produce winds above hurricane force along a track that may exceed several hundred kilometers. This dissertation shows how ground and satellite-based instrumentation can be combined to monitor windstorms over the American Midwest and Atlantic coastal region. Convective windstorm potential has been expressed as a grouping of stability parameters that are relevant for downburst generation. These include the lower-to-mid-tropospheric temperature and equivalent potential temperature (theta-e) lapse rates, vertical relative humidity differences, and the amount of convective available potential energy (CAPE) in the troposphere. Accordingly, this paper will provide background on convective windstorms and a three-step process that monitors the ambient environment that leads to the formation of severe storms and the specific characteristics of those that produce downburst winds. We found that downburst monitoring and subsequent prediction is essentially a three-step process with an objective to build a three-dimensional model of the thermodynamic structure of the ambient environment and conceptual model of downburst-producing convective storms in mid-latitude continental regions. Collection and exploitation of surface-based observations, ground-based microwave and radio profiler measurements, and satellite-based 2-D plan view images all serve as building blocks in the conceptual modelling process. Modification of sounding profiles with surface observations of temperature and humidity is an additional step that results in improved representation of the ambient environment.

Monthly Weather Review, 1973

Detailed hourly precipitation patterns are can also be tracked in the wind and pressure fields. Finally, analyzed for two major winter U.S. east coast storms that some thoughts are offered on the possible dynamic signifiexhibited considerable mesoscale features. Pronounced cance of organized mesoscale precipitation patterns, spatial and temporal continuity is noted for individual along with comments relevant to the forecasting of such convective rainfalls within the cold air. Such features patterns.

Bulletin of the …, 2007

Cumulus clouds being observed with the Chilbolton radar during CSIP.