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Robert J. Oravec and Lance F. Bosart

Abstract

A case study of a transitory anticyclone is conducted as part of the Genesis of Atlantic Lows Experiment (GALE) for the Intensive Observing Period (IOP) of 7–9 March 1986. The special GALE data networks were activated in anticipation of possible Appalachian cold air damming. Cold air damming did not occur and the purpose of the case study is to diagnose a nonevent so that processes associated with cold air damming can be better isolated.

An important finding is that the configuration of the antecedent synoptic-scale flow is crucial in determining whether cold air damming will evolve and be sustained. In the 7–9 March 1986 case a surface anticyclone moved steadily southeastward from west of the Great Lakes to the Middle Atlantic coast and offshore as part of a progressive westerly flow regime aloft. The typical cold air damming signature consisting of 1) a slow moving surface anticyclone along the United States-Canada border with a “U” shaped pressure ridge in the sea level isobars east of the Appalachian Mountains, 2) a split-flow regime in the middle and upper troposphere with a leading northern trough and a lagging southern trough, 3) a tendency for widespread precipitation within the shallow dome of trapped cold air with cooling locally reinforced by evaporation, 4) a mountain-parallel low-level jet down the pressure gradient. and 5) a moist southeasterly flow from the Atlantic Ocean above the trapped cold air dome—was not observed.

Instead, a transient sea-level anticyclone crossed the central Appalachians and moved rapidly offshore in response to synoptic-scale forcing aloft. This prevented the establishment of a sustained low-level northerly flow of cool air cast of the mountains. While the lower third of the troposphere stabilized along the coast in response to initial cold air advection near the surface and subsidence warming near 700 mb, further cooling and stabilization was precluded by the absence of sufficient moisture to trigger the generation of widespread precipitation and resultant rain-cooled air.

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Robert J. Oravec and Richard H. Grumm

Abstract

A study of rapidly deepening cyclones (RDC) produced by the National Meteorological Center's (NMC) Nested Grid Model (NGM) was conducted over a three-year period from the winter of 1988/89 through the autumn of 1991. The axis of RDCs was observed over the western Atlantic 0cean from off the mid-Atlantic coast northeastward to the southern tip of Greenland, and a smaller axis was observed over the eastern Pacific over the Gulf of Alaska and southern Alaska. Relatively few RDCs occurred over the eastern Pacific during the winter and spring, with the maximum distribution occurring during the autumn.

The ability of the NGM to forecast RDCs was a function of both the forecast length and the season. The NGM's ability to simulate RDCs was greatest at 12 h and steadily decreased with forecast length. Similarly, the false alarm rate (FAR) was lowest at 12 h and steadily increased with forecast length through 48 h. The ability of the NGM to detect RDCs was greatest in the winter and decreased during the spring and fall.

There was a smaller decrease in probability of detection in the middle of the forecast cycle during the winter and spring that may be attributed to spinup problems in the NGM during the initialization of the model. Similar trends in the FAR were noted, with a decrease in FAR beyond 12 h during the winter seasons.

The results from this study showed the NGM was too slow to deepen RDCs at all 12-h forecast periods, with the pressure errors increasing with forecast length. The NGM also had a cold bias in the 1000–500-mb thickness forecasts over the RDCs. However, the NGM showed exceptional skill in correctly forecasting the sign of the 12-h pressure change for the RDCs. During the three winter seasons the NGM rarely misforecast the sign of the 12-h forecast pressure change during rapid cyclogenesis.

Over the western Atlantic the NGM was too slow to move the RDCs to the east The overall position errors for RDCs were approximately 10% smaller than the position errors for all cyclones in the NGM at all forecast periods.

An examination of two RDC events revealed significant differences in the NGM's ability to forecast the rapid deepening. During the ERICA IOP 4 cyclone, the NGM forecast the cyclone fairly well, showing its bias of being too slow to deepen the RDCs and too slow to move it eastward. Much poorer skill in forecasting the 4 January 1992 cyclone off the coast of the Carolinas occurred with the NGM having significant problems resolving subgrid-scale processes as the storm deepened rapidly as it crossed the warm Gulf Stream waters.

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Richard H. Grumm, Robert J. Oravec, and Anthony L. Siebers

Abstract

Systematic errors in the nested-grid model (NGM) forecasts of surface cyclones are examined over a two-year period from 1 December 1988 through 30 November 1990. The parameters examined include the location, central pressure, 850-mb temperature, and the 1000-500-mb thickness over the center of the surface cyclone. The mean cyclone position error was typically 150 km at 12 h and 225 km at 24 h, and grew to about 350 km by 48 h. The overall mean cyclone pressure error was −0.57 and −0.68 mb at 24 and 48 h, respectively.

The results show that the skill of the NGM forecasts of surface cyclones displayed both seasonal and annual variability. The seasonal variability is represented by overall smaller errors in the summer and larger errors in the winter.

The NGM tended to overdeepen surface cyclones in all but the summer months. A large part of the pressure error was due to the model's inability to fill cyclones properly and a tendency to forecast systems to deepen when they were observed to fill. About 15% of the time in the winter months, the NGM forecast cyclones to deepen when they were observed to fill.

The NGM had difficulty detecting the initial development of surface cyclones, especially near the elevated terrain of western North America and along the track of transient cyclones. In these same regions, the NGM tended to forecast cyclones that were not observed. There was a preponderance of both nonobserved and nonforecast cyclones over the elevated terrain of North America, indicating that the NGM has difficulty with orographic effects.

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