Louis W. Uccellini

The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms

Abstract Transverse circulations in the exit and entrance regions of jet streaks are investigated through numerical simulation, a case study, and an application of the isallobaric wind equation in isentropic coordinates, to study the interaction between upper and lower tropospheric jets and the development of severe convective storms. A hybrid isentropic-sigma coordinate numerical model is used to simulate the mass and momentum adjustments associated with a jet streak propagating in a zonal channel. The numerical results depict a two-layer mass adjustment in the exit and entrance region of the jet streak. The results also verify that the isallobaric wind on lower isentropic surfaces is a primary component of the return branches of transverse circulations and is foxed by the two-layer mass adjustment accompanying the propagating jet streak. Results from the case study of a severe weather out- break show that 1) a low-level jet (LLJ) beneath the exit region of an upper tropospheric jet streak is embedded in...

The Synoptic Setting and Possible Energy Sources for Mesoscale Wave Disturbances

Abstract Thirteen case studies of mesoscale wave disturbances (characterized by either a singular wave of depression or wave packets with periods of 1–4 h, horizontal wavelengths of 50–500 km, and surface pressure perturbation amplitudes of 0.2–7.0 mb) are reviewed to isolate common synoptic features for these cases and to shed light on possible energy sources for the waves. A strong thermal inversion in the lower troposphere (north of a frontal boundary) and a jet streak propagating toward a ridge axis in the upper troposphere are commonly observed in all the cases. In general, the area of wave activity is bounded by the jet axis to the west or northwest, a surface front to the southeast, an inflection axis (between the trough and ridge axes) to the southwest and the ridge axis to the northeast. The conditions specified by Lindzen and Tung as being necessary to form a wave duct, which include the existence of the lower-tropospheric inversion, seem to be met in many of these cases. This suggests that a du...

The Interaction of Jet Streak Circulations during Heavy Snow Events along the East Coast of the United States

Abstract The interaction of transverse vertical circulations associated with two separate jet steak/trough systems is found to be a common feature of cyclogenetic events which produce heavy snow along the East Coast of the United States. The transverse circulations are identified for eight cases that span the period from 1960 to 1987 utilizing an isentropic analysis of the operational radiosonde data. The analyses depict the interaction of 1) a direct circulation located within the confluent entrance region of an upper-level jet streak over the northeastern United States or southeastern Canada with 2) an indirect circulation in the diffluent exit region of a jet streak associated with a trough nearing the East Coast. This interaction contributes to differential moisture and temperature advections and vertical motions necessary to produce heavy snowfall along the coast. It is suggested that the circulation patterns associated with the jet streak establish an environment within which boundary layer processe...

Low-level water vapor fields from the VISSR Atmospheric Sounder (VAS) 'split window' channels

Abstract A simple physical algorithm is developed which calculates the water vapor content of the lower troposphere from the 11 and 12 μm (split window) channels on the VISSR Atmospheric Sounder (VAS) on the Geostationary Operational Environmental Satellites (GOES). The algorithm is applied to a time series of VAS split window radiances observed at 15 km horizontal resolution over eastern North America during a twelve hour period on 13 July 1981. Color coded images of the derived precipitable water (g cm−2) fields show vivid water vapor features whose broad structure and evolution are verified by the radiosonde and surface networks. The satellite moisture fields also reveal significant mesoscale features and rapid developments which are not resolved by the conventional networks. The VAS split window clearly differentiates those areas in which water vapor extends over a deep layer and is more able to support convective cells from those arms in which water vapor is confined to a shallow layer and is therefo...

A Case Study of Gravity Waves–Convective Storms Interaction: 9 May 1979

Abstract An analysis is presented of a series of severe storms which occurred in the north central United States on 9 May 1979 and whose spatial distribution and movement correlate well with observed gravity waves. Two gravity wave trains of 2.1–3 mb amplitude, 2.5–3.3 h period and 240–265 km horizontal wavelength were isolated through power spectra analysis and cross-correlation techniques applied to National Weather Service barograph traces. The wave trains propagated in the 200° direction, which coincided with the jet axis, with a phase velocity of 20–30 m s−1C and within a 300 km wide band. The storms were identified on enhanced infrared GOES satellite pictures with the help of radar summaries. These convective systems initially developed in Nebraska and propagated north–northeast at 25 m s−1, revealing wave-like characteristics with a separation of 300–400 km. The convective systems were closely linked to the observed wave trains with cell intensity, height and associated rainfall maximized at the wa...

AN EARTH-SYSTEM PREDICTION INITIATIVE FOR THE TWENTY-FIRST CENTURY

Some scientists have proposed the Earth-System Prediction Initiative (EPI) at the 2007 GEO Summit in Cape Town, South Africa. EPI will draw upon coordination between international programs for Earth system observations, prediction, and warning, such as the WCRP, WWRP, GCOS, and hence contribute to GEO and the GEOSS. It will link with international organizations, such as the International Council for Science (ICSU), Intergovernmental Oceanographic Commission (IOC), UNEP, WMO, and World Health Organization (WHO). The proposed initiative will provide high-resolution climate models that capture the properties of regional high-impact weather events, such as tropical cyclones, heat wave, and sand and dust storms associated within multi-decadal climate projections of climate variability and change. Unprecedented international collaboration and goodwill are necessary for the success of EPI.

Overview of the 12–14 March 1993 Superstorm

An extratropical cyclone of unusual intensity and areal extent affected much of the Gulf and East Coasts of the United States on 12–14 March 1993. In this paper, the many effects of the storm will be highlighted, including perhaps the most widespread distribution of heavy snowfall of any recent East Coast storm, severe coastal flooding, and an outbreak of 11 confirmed tornadoes. A meteorological description of the storm is also presented, including a synoptic overview and a mesoscale analysis that focuses on the rapid development of the cyclone over the Gulf of Mexico. This is the first part of a three-paper series that also addresses the performance of the operational numerical models and assesses the forecasting decisions made at the National Meteorological Center and National Weather Service local forecast offices in the eastern United States.

The possible influence of upstream upper-level baroclinic processes on the development of the QE II storm

Abstract An analysis of the QE II storm of 9–11 September 1978 presents evidence for the existence of upper-level baroclinic processes upstream of the rapidly developing cyclone. The analysis shows that a deepening shortwave trough was located 400 to 500 km upstream of the site of the storm 12 h prior to rapid cyclogenesis. The trough was associated with 1) a polar jet marked by 65 m s−1 winds in its core and significant vertical and horizontal wind shear, 2) positive vorticity advection and divergence at the 300 mb level, and 3) an intense frontal zone that extended from 300 mb down to the surface. It also appears that a tropopause fold likely extruded stratospheric air down to the 700–800 mb level, 400–500 km upstream of the surface low and 12 h prior to the explosive development phase of the cyclone. These findings raise questions about Gyakums assertion that the QE II storm developed in an area in which the baroclinic support was confined to the lower troposphere and the related assertion by Anthes e...

A snowfall impact scale derived from northeast storm snowfall distributions

Abstract A Northeast snowfall impact scale (NESIS) is presented to convey a measure of the impact of heavy snowfall in the Northeast urban corridor, a region that extends from southern Virginia to New England. The scale is derived from a synoptic climatology of 30 major snowstorms in the Northeast urban corridor and applied to the snowfall distribution of 70 snowstorms east of the Rocky Mountains. NESIS is similar in concept to other meteorological scales that are designed to simplify complex phenomena into an easily understood range of values. The Fujita scale for tornadoes and the Saffir–Simpson scale for hurricanes measure the potential for destruction to property and loss of life by wind-related damage (and storm surge for Saffir–Simpson) through use of a categorical ranking (0 or 1–5).

Regional Models: Emerging Research Tools for Synoptic Meteorologists

Abstract Although the development of limited-area, regional-scale numerical-weather prediction models has been driven largely by the practical need to improve quantitative precipitation forecasts, a related motivation has been scientific interest in investigating mesoscale phenomena and processes under “controlled” conditions. The establishment of mesoscale meteorology as a recognized discipline of the atmospheric sciences provides a context for exploring how regional models can be used effectively as research tools. It is argued that these models are sufficiently advanced and refined to provide investigators with four-dimensional, dynamically consistent data sets to supplement and extend those available from observations. Consequently, regional models combined with observations potentially are of considerable utility in conducting case studies of mesoscale phenomena. These contentions are illustrated with examples from the literature and with suggestions for future research involving the application of r...