Search

We present a new event-scale catalog of stable isotopic measurements from 5 years of storm events at 4 sites in southern California, which is used to understand the storm to storm controls on the isotopic composition of precipitation and validate the event-scale performance of an isotope-enabled GCM simulation ( IsoGSM) (Yoshimura et al. ). These analyses are motivated to improve the interpretation of proxy records from this region and provide guidance in testing the skill of GCMs in reproducing the hydrological variability in the western US. We find that approximately 40% of event-scale isotopic variability arises from the percentage of precipitation that is convective and the near surface relative humidity in the days prior to the storms landfall. The additional isotopic variability arises from the fact that storms arriving from different source regions advect moisture of distinct isotopic compositions. We show using both field correlation and Lagrangian trajectory analysis that the advection of subtropical and tropical moisture is important in producing the most isotopically enriched precipitation. The isotopic catalog is then used along with satellite-derived δD retrievals of atmospheric moisture to benchmark the performance of the IsoGSM model for the western US. The model is able to successfully replicate the observed isotopic variability suggesting that it is closely reproducing the moisture transport and storm track dynamics that drive the large storm-to-storm isotopic range. Notably, we find that an increase in moisture flux from the central tropical Pacific leads to a convergence of isotopically enriched water vapor in the subtropics and consequently an increase in δO of precipitation at sites along the entire west coast. Changes in poleward moisture flux from the central Tropical Pacific have important implications for both the global hydrological cycle and regional precipitation amounts and we suggest such changes can be captured through instrumental and proxy-reconstruction of the spatiotemporal isotopic patterns in the precipitation along the west coast of the US. [ABSTRACT FROM AUTHOR]

This observational study investigates statistical and synoptic-dynamic relationships between regime transitions, defined as a North Atlantic Oscillation (NAO) or Pacific-North American pattern (PNA) index change from at least a 1 standard deviation anomaly to at least a 1 standard deviation anomaly of opposite sign within 7 days, and cool-season (November-April) northeastern U.S. (NE) precipitation. A statistical analysis is performed of daily cool-season NE precipitation during all NAO and PNA transitions for 1948-2003, and a composite analysis and case study of a major cool-season NE precipitation event occurring during a positive-to-negative NAO transition are conducted. Datasets used are the 0.25° NCEP Unified Precipitation Dataset, the 2.5° NCEP-NCAR reanalysis, and the 1.125° 40-yr ECMWF Re-Analysis (ERA-40). Results of the statistical analysis suggest that cool-season NE precipitation tends to be enhanced during positive-to-negative NAO and negative-to-positive PNA transitions, and suppressed during negative-to-positive NAO and positive-to-negative PNA transitions. Of the four types of regime transitions, only the positive-to-negative NAO transition is associated with substantially more frequent major cool-season NE precipitation events compared to climatology. Results of the composite analysis and case study indicate that a surface cyclone and cyclonic wave breaking associated with the major NE precipitation event can help produce a high-latitude blocking pattern over the North Atlantic characteristic of a negative NAO pattern via thermal advection, potential vorticity transport, and diabatic processes. [ABSTRACT FROM AUTHOR]

In this paper, we first describe the current status of the Advanced Regional Prediction System of the Center for Analysis and Prediction of Storms at the University of Oklahoma. A brief outline of future plans is also given. Two rather successful cases of explicit prediction of tornadic thunderstorms are then presented. In the first case, a series of supercell storms that produced a historical number of tornadoes was successfully predicted more than 8 hours in advance, to within tens of kilometers in space with initiation timing errors of less than 2 hours. The general behavior and evolution of the predicted thunderstorms agree very well with radar observations. In the second case, reflectivity and radial velocity observations from Doppler radars were assimilated into the model at 15-minute intervals. The ensuing forecast, covering a period of several hours, accurately reproduced the intensification and evolution of a tornadic supercell that in reality spawned two tornadoes over a major metropolitan area. These results make us optimistic that a model system such as the ARPS will be able to deterministically predict future severe convective events with significant lead time. The paper also includes a brief description of a new 3DVAR system developed in the ARPS framework. The goal is to combine several steps of Doppler radar retrieval with the analysis of other data types into a single 3-D variational framework and later to incorporate the ARPS adjoint to establish a true 4DVAR data assimilation system that is suitable for directly assimilating a wide variety of observations for flows ranging from synoptic down to the small nonhydrostatic scales. [ABSTRACT FROM AUTHOR]

Presents information on a study which examined the development of an extratropical cyclone associated with a synoptic-scale trough merger in the United States. Methodology; Occurrence of synoptic-scale trough merger; Dynamics of surface cyclone.

Presents a case study on severe storm development along a dryline within a synoptically active environment in Oklahoma. Background on the development of the dryline; Analysis of aircraft and mobile sounding data relating to dryline motion; Structure and motion of the dryline.

A cyclonic storm that moved over the central United States on 8–9 March 1992 developed two convective rainbands, namely, a pre–dry trough rainband and a cold front aloft (CFA) rainband. This study extends the results of previous investigations of these two rainbands by examining their initiation with the use of a nested-grid mesoscale model simulation with spatial resolution down to 8.3 km. The model simulation reproduced the synoptic-scale setting in which the rainbands developed, as well as the mesoscale processes that initiated the rainbands. The pre–dry trough rainband was produced by the gradual ascent of a convectively unstable airstream above a gently sloping warm-frontal zone east of the dryline. After sufficient lifting, the instability was released through upright convection. The gradual ascent is well estimated by quasigeostrophic diagnosis, but the location and timing of the rainband are very sensitive to the convective stability characteristics within the airstream. The CFA rainband was initiated by a Pacific cold front that occluded with the warm-frontal surface. This mesoscale occlusion process produced a narrow region of enhanced ascent at the dryline, which resulted in the lifting of the western edge of an air mass with high convective available potential energy. The lower-tropospheric mesoscale occlusion process was not resolved by a quasigeostrophic vertical velocity diagnosis. Also, although an upper-level front and tropopause fold were present, the CFA was separate from that feature. [ABSTRACT FROM AUTHOR]

During March of 1948 Tinker Air Force Base was hit directly by two tornadoes during a period of only five days. The first tornado was the most destructive, to that point, ever to occur in Oklahoma. The second storm caused considerable additional damage and was remarkable in another, more significant, way. The first operational tornado forecast had been issued by Air Force Officers E. J. Fawbush and R. C. Miller a few hours before the tornado moved across the base. This extremely unusual meteorological situation, two tornadoes hitting the same location within five days, coupled with the fortuitous forecast of the event, had a profound impact on the evolution of operational severe weather forecasting in the United States. These events eventually stimulated the initiation of public severe thunderstorm forecasting by the Weather Bureau. Miller often presented anecdotal accounts of the events leading up to the landmark forecast, for example, in seminars and interviews during a visit to the National Severe Storms Laboratory during March 1994. He often stressed that the remarkable similarity of the synoptic settings on 21 and 25 March 1948 helped give him and Fawbush the courage to issue the now famous forecast. In this paper the synoptic environments that led to the two tornado occurrences at Tinker are analyzed and discussed. There were indeed similarities; however, it is surprising how different many aspects of the storm settings actually were. Similarities and important differences are illustrated with a series of synoptic surface and upper-air charts. It is likely that development of a base severe weather plan following the tornado disaster of 20 March, in addition to the presence and exhortations of General F. S. Borum at the base weather station on 25 March, provided as great a motivation for the first tornado forecast as did the similarity of the synoptic settings. [ABSTRACT FROM AUTHOR]

Between 25 and 27 June 1995, excessive rainfall and associated flash flooding across portions of western Virginia resulted in three fatalities and millions of dollars in damage. Although many convective storms occurred over this region during this period, two particular mesoscale convective systems that occurred on 27 June were primarily responsible for the severe event. The first system (the Piedmont storm) developed over Madison County, Virginia (eastern slopes of the Blue Ridge Mountains), and propagated slowly southward producing 100-300 mm of rain over a narrow swath of the Virginia foothills and Piedmont. The second system (the Madison storm) developed over the same area but remained quasi-stationary along the eastern slopes of the Blue Ridge for nearly 8 h producing more than 600 mm of rain. Analysis of this event indicates that the synoptic conditions responsible for initiating and maintaining the Madison storm were very similar to the Big Thompson and Fort Collins floods along the Front Range of the Rocky Mountains, as well as the Rapid City flood along the east slopes of the Black Hills of South Dakota. In all four events, an approaching shortwave aloft coupled with high-level difluence/divergence signaled the presence of local ascent and convective destabilization. A postfrontal ribbon of relatively fast-moving high-θ[sub e] air, oriented nearly perpendicular to the mountain range, provided a copious moisture supply and helped focus the convection over a relatively small area. Weak middle- and upper-tropospheric steering currents favored slow-moving storms that further contributed to locally excessive rainfall. A conceptual model for the Madison-Piedmont convective systems and their synoptic environment is presented, and the similarities and differences between the Madison County flood and the Big Thompson, Fort Collins, and Rapid City floods are highlighted. [ABSTRACT FROM AUTHOR]

The Kirchhofer technique for identifying spatial patterns in climate data depends on the threshold and minimum size used to define the groups. As a result, the analyst cannot be sure whether the groups defined by the Kirchhofer technique represent meaningful meteorological phenomena or whether they emerge due to random chance. Monte Carlo techniques are used to generate a three-dimensional surface that can be used to evaluate the statistical significance of the groups identified from the historical record for summertime maximum daily temperature in the mid-continental region of the US. Statistically meaningful groups can be identified using stringent criteria for threshold and minimum group size. Choosing among these values depends on how the groups are used and the importance of parsimony to that process. The reliability of critical values can be evaluated by comparing the statistical significance of results for individual groups generated by existing techniques. Copyright © 1999 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article reports on the move of Paul Yura, warning coordination meteorologist at the Charleston National Weather Service in South Carolina, in examining the synoptic and mesoscale situation involved in the outbreak of Hurricanes Frances and Jeanne in the U.S. in 2004. It is stated that Hurricane Jeanne followed the track of Hurricane Frances across Florida but just produced fewer tornadoes compared to the latter. According to Yura, the primary reason can be attributed to the less distribution of tornadoes triggered by Jeanne was due to a massive dry slot of air.

Clear quantitative differences (particularly with NH4+ and NO3-) were observed in the chemical composition of rain collected by an in situ, refrigerated event sampler versus using a non-refrigerated, composite, weekly sampling protocol, at a site in the upper Midwestern USA. There was a non-linear, parabolic relationship between pH and SO42- or NO3- concentrations. Such relationships could be stratified by the occurrences of three different classes of rainfall chemistry, governed by three different types of branching air trajectories preceding them. These overall processes were influenced by source-receptor issues. A Fourier modulated, three-way parabolic splineregression model was used to explain the non-linear relationships between H+ and SO42- or NO3-. This model performed much better (R2: 0.008--0.61) than the linear (R2: 0.008--0.31) and non-modulatedthree-way parabolic spline (R2: 0.22--0.36) regression analyses. Overall, the study allowed an integration of rainfall chemistry with synoptic meteorology and numerical modelling. (c) 1998 Elsevier Science Ltd. [ABSTRACT FROM AUTHOR]