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Mary A. Bedrick, Anthony J. Cristaldi III, Stephen J. Colucci, and Daniel S. Wilks

Abstract

An assessment was made of the 120-h predictions by the medium range forecast (MRF) run of the National Meteorological Center's (NMC's) global spectral model. The ability of the model to forecast surface cyclones and anticyclones was evaluated and compared to shorter range (48 h) forecasts by the NMC Nested Grid Model (NGM) verifying at the same time. The study covers a period from 15 April 1991 through 20 December 1991 over a domain extending from 30°N to 50°N and from 60°W to 130°W. Results showed that the MRF at the 120-h range tended to underforecast the frequency of surface cyclones and anticyclones in the Rocky Mountain region and, in general, to forecast anticyclones and cyclones farther away from the observed systems than did the 48-h NGM verifying at the same time. The forecasts for central sea level pressures averaged within 1.5 hPa of the forecasts of pressure by the NGM. Averaged over regions and seasons, the 120-h MRF errors in many cases were statistically indistinguishable from the 48-h forecast errors. To illustrate model performance, an example of an accurate 120-h forecast is contrasted with a less accurate 120-h forecast following similar initial conditions.

In addition, the accuracy of each 120-h MRF prediction for sea level pressure over a domain between 30°N and 60°N from 60°W to 150°W was subjectively determined for the period 14 March 1991 through 20 March 1992. Initial conditions of these forecasts were inspected for common features that could potentially be used to anticipate the accuracy of the 120-h MRF predictions. Typically, the best forecasts were characterized by below-normal 500-hPa heights at 40°N, 50°W in the initial conditions, corresponding to relatively weak 500-hPa geostrophic midlatitude (40°N–60°N) westerlies at 50°W. By comparison, the initial conditions of the worst forecasts featured, in most cases, above-normal 500-hPa heights at 40°N, 50°W corresponding to relatively stronger 500-hPa geostrophic midlatitude westerlies at 50°W.

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Lance F. Bosart, Gregory J. Hakim, Kevin R. Tyle, Mary A. Bedrick, W. Edward Bracken, Michael J. Dickinson, and David M. Schultz

Abstract

The results of a multiscale analysis of the 12–14 March 1993 superstonn (SS93) over eastern North America are presented. A time sequence of overlapping 10-day time-mean 5OO-hPa geopotential height and anomaly composites shows that the Northern Hemisphere (NH) flow pattern from 18 February to 15 March 1993 is characterized by 1) three persistent troughs situated over eastern Asia and the northwestern Pacific, over eastern North America, and over northwestern Africa and southwestern Europe eastward to central Russia; and 2) a massive blocking anticyclone located over the central and eastern Atlantic. Beginning 8–9 March 1993 the planetary-scale flow amplifies substantially. The explosive SS93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the NH available potential energy content, after peaking on 9 March 1993, decreases by about 6%–7%.

A dynamical tropopause analysis is used to track coherent transient potential vorticity (PV) anomalies and show their qualitative interaction with the planetary-scale flow. SS93 is attributed to the interaction and eventual merger of strong PV anomalies embedded in the northern and southern branches of the westerlies in a background confluent northwesterly flow associated with an amplifying positive-phase Pacific–North American flow pattern. The northern PV anomaly originates in southwestern Canada on 18 February and circumnavigates the NH at relatively high latitudes, a track that permits it to maintain arctic characteristics prior to merger. The southern PV anomalies, tracked from Europe and western Asia eastward across the Pacific, reach North America by 11 March 1993 where they become associated with widespread convention over southern Texas and the northwestern Gulf of Mexico beginning 12 March 1993.

The unique aspects of SS93 are attributed to 1) the near simultaneous amplification of the planetary-scale flow and the lateral and vertical interaction of individual PV anomalies cast of the Rockies during the merger process, and 2) the lag of the northern PV anomaly relative to the southern anomaly so that a baroclinic zone containing lower-tropospheric air of significant conditional instability is allowed to remain in place over southern Texas and the northwest Gulf of Mexico in the cyclogenetic environment ahead of the northern PV anomaly.

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Michael J. Dickinson, Lance F. Bosart, W. Edward Bracken, Gregory J. Hakim, David M. Schultz, Mary A. Bedrick, and Kevin R. Tyle

Abstract

The incipient stages of the 12–14 March 1993 “superstorm” (SS93) cyclogenesis over the Gulf of Mexico are examined. Noteworthy aspects of SS93 include 1) it is the deepest extratropical cyclone ever observed over the Gulf of Mexico during the 1957–96 period, and 2) existing operational prediction models performed poorly in simulating the incipient cyclogenesis over the northwestern Gulf of Mexico. A dynamic-tropopause (DT) analysis shows that SS93 is triggered by a potent potential vorticity (PV) anomaly as it crosses extreme northern Mexico and approaches the Gulf of Mexico. The low-level environment over the western Gulf of Mexico is warmed, moistened, and destabilized by a persistent southerly flow ahead of the approaching PV anomaly. Ascent and a lowering of the DT (associated with a lowering of the potential temperature) ahead of the PV anomaly contributes to further destabilization that is realized in the form of a massive convective outbreak.

An examination of the National Centers for Environmental Prediction (NCEP) Medium Range Forecast (MRF) model-initialized fields after convection begins shows that the MRF does not fully resolve important features of the potential temperature, pressure, and wind fields on the DT in the incipient SS93 environment. Similarly, the NCEP MRF 12-h/24-h forecasts verifying 1200 UTC 12 March and 0000 UTC 13 March are unable to simulate sufficient deep convection over the Gulf of Mexico, low-level PV growth in the incipient storm environment, high-level PV destruction and the associated warming and lifting of the DT over and downshear of the developing storm. Given that the MRF-initialized fields possess sufficient conditional instability, moisture, and ascent to trigger widespread deep convection, the poorly forecast incipient SS93 development appears to be associated with the failure of the model cumulus parameterization scheme. A comparison of the MRF forecasts with selected forecast fields derived from the European Centre for Medium-Range Weather Forecasts operational model supports this interpretation.

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David M. Schultz, W. Edward Bracken, Lance F. Bosart, Gregory J. Hakim, Mary A. Bedrick, Michael J. Dickinson, and Kevin R. Tyle

Abstract

In the wake of the eastern United States cyclone of 12–14 March 1993, a cold surge, originating over Alaska and western Canada, brought northerlies exceeding 20 m s−1 and temperature decreases up to 15°C over 24 h into Mexico and Central America. This paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale, focusing on 1) the structure and evolution of the leading edge of the cold surge, 2) the reasons for its extraordinary intensity and equatorward extent, and 3) the impact of the surge on the Tropics, specifically, on the strength of the trade winds and on the sea surface temperature in the eastern Pacific.

The cold surge was initiated as a developing cyclone over the Gulf of Mexico, and an upstream anticyclone east of the Rockies caused an along-barrier pressure gradient to form, forcing topographically channeled northerlies along the Rocky and Sierra Madre Mountains to transport cold air equatorward. On the mesoscale, the leading edge of the cold surge possessed nonclassical frontal structure. For example, as the cold surge entered Mexico, the coldest air and the strongest wind arrived at about 900 hPa before affecting the surface, suggestive of a tipped-forward leading edge to the surge. Also, satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air. The cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a gravity current, but none of these conceptual/dynamical models was fully applicable. Associated with the cold surge, gap winds up to 25 m s−1 were observed in the Gulfs of Tehuantepec (a tehuantepecer), Fonseca, Papagayo, and Panama, owing to the strong cross-mountain pressure gradient. In the case of the tehuantepecer, a rope cloud emanated from the Isthmus of Tehuantepec and turned anticyclonically, consistent with an inertial oscillation.

On the synoptic and planetary scales, the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold-air damming, by a low-latitude upper-tropospheric trough, and by the lower branch of the secondary circulation associated with a confluent jet-entrance region aloft. The cold surge also impacted the tropical atmosphere and ocean, by contributing to the strengthening of the northeast trade winds over the eastern Pacific Ocean and by inducing local cooling of the sea surface temperature in the Gulfs of Tehuantepec and Papagayo by about 4°–8°C.

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