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- Author or Editor: LANCE F. BOSART x
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Abstract
A case study is made of a period of exceptional warmth in the mid and upper troposphere over the Carib-bean from 19–21 February 1964. The case is a good example of intense winter interaction of mid-latitude and tropical circulations during which lateral forcing from higher latitudes appears to play a prominent role. For example, at 500 mb, temperatures of −1 to −2C which exhibited spacial and temporal continuity on the synoptic scale were noted. These values were more than three standard deviations warmer than monthly mean values and occurred in association with a strong low-latitude polar jet along the Gull Coast of the United States.
Isentropic trajectories document average 12-hr descent rates of up to 2 cm sec−1 in the vicinity of 15N in association with the warmth area. Criteria for inertial instability is satisfied over a portion of the anti-cyclonic flank of the strong jet stream leading to a speculative case for a transverse thermally indirect circulation such that a persistent anticyclone north of Puerto Rico is strengthened. Reed and Vleck's hypothesis on the annual temperature cycle of the tropical lower stratosphere is examined on a limited basis with daily data both directly and indirectly with rather inconclusive results.
Abstract
A case study is made of a period of exceptional warmth in the mid and upper troposphere over the Carib-bean from 19–21 February 1964. The case is a good example of intense winter interaction of mid-latitude and tropical circulations during which lateral forcing from higher latitudes appears to play a prominent role. For example, at 500 mb, temperatures of −1 to −2C which exhibited spacial and temporal continuity on the synoptic scale were noted. These values were more than three standard deviations warmer than monthly mean values and occurred in association with a strong low-latitude polar jet along the Gull Coast of the United States.
Isentropic trajectories document average 12-hr descent rates of up to 2 cm sec−1 in the vicinity of 15N in association with the warmth area. Criteria for inertial instability is satisfied over a portion of the anti-cyclonic flank of the strong jet stream leading to a speculative case for a transverse thermally indirect circulation such that a persistent anticyclone north of Puerto Rico is strengthened. Reed and Vleck's hypothesis on the annual temperature cycle of the tropical lower stratosphere is examined on a limited basis with daily data both directly and indirectly with rather inconclusive results.
Abstract
Average kinematic vertical motion and relative vorticity profiles are presented for a long-lived midlatitude convective complex. A breakdown into active convective and stratiform precipitation regions shows very good agreement in the vertical motion profiles with published results for tropical convective systems.
Abstract
Average kinematic vertical motion and relative vorticity profiles are presented for a long-lived midlatitude convective complex. A breakdown into active convective and stratiform precipitation regions shows very good agreement in the vertical motion profiles with published results for tropical convective systems.
Consensus (the average of all forecasts) skill levels in forecasting daily maximum and minimum temperature, precipitation probability across six class intervals, and precipitation amount at the State University of New York at Albany are reviewed for the period 1977–82. Skill is measured relative to a climatological control. Forecasts are made for four consecutive 24 h periods for Albany, N.Y., beginning at 1800 GMT of the current day.
For minimum temperature, the skill levels average 57%, 41%, 26%, and 15%, respectively, for 24, 48, 72, and 96 h in advance. For maximum temperature, a more limited sample yields corresponding skill levels of 84%, 49%, 34%, and 19% for 12, 36, 60, 84 h ahead. Linear regression analysis yields little in the way of a definitive trend, given the smallness of the explained variance. Comparison with other readily available objective and subjective operational guidance establishes the credibility of the consensus forecast.
Consensus (the average of all forecasts) skill levels in forecasting daily maximum and minimum temperature, precipitation probability across six class intervals, and precipitation amount at the State University of New York at Albany are reviewed for the period 1977–82. Skill is measured relative to a climatological control. Forecasts are made for four consecutive 24 h periods for Albany, N.Y., beginning at 1800 GMT of the current day.
For minimum temperature, the skill levels average 57%, 41%, 26%, and 15%, respectively, for 24, 48, 72, and 96 h in advance. For maximum temperature, a more limited sample yields corresponding skill levels of 84%, 49%, 34%, and 19% for 12, 36, 60, 84 h ahead. Linear regression analysis yields little in the way of a definitive trend, given the smallness of the explained variance. Comparison with other readily available objective and subjective operational guidance establishes the credibility of the consensus forecast.
Abstract
An argument is made that if human forecasters are to continue to maintain a skill advantage over steadily improving model and guidance forecasts, then ways have to be found to prevent the deterioration of forecaster skills through disuse. The argument is extended to suggest that the absence of real-time, high quality mesoscale surface analyses is a significant roadblock to forecaster ability to detect, track, diagnose, and predict important mesoscale circulation features associated with a rich variety of weather of interest to the general public.
Abstract
An argument is made that if human forecasters are to continue to maintain a skill advantage over steadily improving model and guidance forecasts, then ways have to be found to prevent the deterioration of forecaster skills through disuse. The argument is extended to suggest that the absence of real-time, high quality mesoscale surface analyses is a significant roadblock to forecaster ability to detect, track, diagnose, and predict important mesoscale circulation features associated with a rich variety of weather of interest to the general public.
Abstract
An analysis is presented of Tropical Storm Marco (1990), a storm that dropped copious amounts of rain over the southeast United States. Marco was noteworthy because of its role in the formation and evolution of two distinct episodes of cold-air damming and coastal frontogenesis over Georgia and the Carolinas. These mesoscale features led to greater than 300 mm of precipitation in 2 days over the near-coastal southeast United States; much of the rain occurred while Marco was over 400 km away. This case is further complicated by two other nearby tropical cyclones, which affected Marco’s track and the overall rainfall distribution. Synoptic and mesoscale analyses of the development of the coastal front and cold-air damming episodes show that the location of Marco helped to orient low-level winds toward the Appalachians. As rain developed inland, a pocket of relatively cool air, the “cool pool,” formed near the mountain slopes and was partially blocked by the higher terrain. Low-level analyses show that the coastal front on the oceanward edge of the cool pool became a focusing mechanism for ascent and precipitation, as moist, tropical air advected inland by Marco was forced upward at the density gradient. The results indicate that a weak tropical cyclone can directly effectuate intense precipitation distant from the storm center, both by causing moist tropical flow toward land and by inducing mesoscale features that focus the precipitation and lead to heavy rainfall and flooding.
Abstract
An analysis is presented of Tropical Storm Marco (1990), a storm that dropped copious amounts of rain over the southeast United States. Marco was noteworthy because of its role in the formation and evolution of two distinct episodes of cold-air damming and coastal frontogenesis over Georgia and the Carolinas. These mesoscale features led to greater than 300 mm of precipitation in 2 days over the near-coastal southeast United States; much of the rain occurred while Marco was over 400 km away. This case is further complicated by two other nearby tropical cyclones, which affected Marco’s track and the overall rainfall distribution. Synoptic and mesoscale analyses of the development of the coastal front and cold-air damming episodes show that the location of Marco helped to orient low-level winds toward the Appalachians. As rain developed inland, a pocket of relatively cool air, the “cool pool,” formed near the mountain slopes and was partially blocked by the higher terrain. Low-level analyses show that the coastal front on the oceanward edge of the cool pool became a focusing mechanism for ascent and precipitation, as moist, tropical air advected inland by Marco was forced upward at the density gradient. The results indicate that a weak tropical cyclone can directly effectuate intense precipitation distant from the storm center, both by causing moist tropical flow toward land and by inducing mesoscale features that focus the precipitation and lead to heavy rainfall and flooding.
Abstract
The explosively deepening oceanic cyclone or “bomb,” one which has central pressure falls of 12 mb (12 h)−1 or greater, has been studied using composites constructed from North Atlantic and Pacific weather ship rawinsonde data during the period October 1965 to May 1974, June through September excluded.
The composites revealed that the oceanic bomb evolved in a low-level baroclinic environment where the incipient circulation was confined to the lower troposphere. The cyclone subsequently developed into a deep vortex characterized by strong baroclinicity, strong low and midlevel ascent to the north and east of the cyclone, and lower tropospheric conditional instability found near and to the southeast of the cyclone center. A case study of a very intense cyclone that passed near ship 4YP revealed deep layers of conditional instability near the low center. It appears that explosively deepening cyclones are baroclinic phenomena whose development may be enhanced in some cases by the bulk effects of cumulus convention.
Abstract
The explosively deepening oceanic cyclone or “bomb,” one which has central pressure falls of 12 mb (12 h)−1 or greater, has been studied using composites constructed from North Atlantic and Pacific weather ship rawinsonde data during the period October 1965 to May 1974, June through September excluded.
The composites revealed that the oceanic bomb evolved in a low-level baroclinic environment where the incipient circulation was confined to the lower troposphere. The cyclone subsequently developed into a deep vortex characterized by strong baroclinicity, strong low and midlevel ascent to the north and east of the cyclone, and lower tropospheric conditional instability found near and to the southeast of the cyclone center. A case study of a very intense cyclone that passed near ship 4YP revealed deep layers of conditional instability near the low center. It appears that explosively deepening cyclones are baroclinic phenomena whose development may be enhanced in some cases by the bulk effects of cumulus convention.
Abstract
The results of an objective climatology of 500hPa trough merger (defined as the amalgamation of two initially separate vorticity maxima) and trough fracture (defined as the splitting of a single vorticity center into two separate vorticity centers) are presented for the Northern Hemisphere. The data source is the gridded National Meteorological Center (NMC; now the National Centers for Environmental Prediction) analyses available on CD-ROM on the NMC operational octagonal grid (381-km resolution at 60°N) for the period September 1957 to May 1989. A total of 2152 (3053) merger (fracture) events are identified. Merger and fracture events tend to occur preferentially in the band of middle-latitude westerlies. Merger events exhibit a tendency to avoid major mountainous regions and show a somewhat higher frequency of occurrence downstream of these regions. Fracture events cluster somewhat over eastern ocean basins, the southwestern United States, and northwestern Africa while also avoiding major mountainous regions. On average, trough genesis events exceed trough lysis events in northerly 500-hPa flow with the reverse being true for southerly flow. Genesis events maximize downstream of major mountain barriers such as the Rockies and the Stanovoi and Altai-Sayan Mountains of Mongolia, east of Greenland, over western Europe, and across much of the North Pacific and North Atlantic. An example of trough merger and associated cyclogenesis is presented to help offer a perspective on the climatological results and illustrate qualitatively the important role that large-scale confluent deformation frontogenesis plays in the merger and fracture process.
Abstract
The results of an objective climatology of 500hPa trough merger (defined as the amalgamation of two initially separate vorticity maxima) and trough fracture (defined as the splitting of a single vorticity center into two separate vorticity centers) are presented for the Northern Hemisphere. The data source is the gridded National Meteorological Center (NMC; now the National Centers for Environmental Prediction) analyses available on CD-ROM on the NMC operational octagonal grid (381-km resolution at 60°N) for the period September 1957 to May 1989. A total of 2152 (3053) merger (fracture) events are identified. Merger and fracture events tend to occur preferentially in the band of middle-latitude westerlies. Merger events exhibit a tendency to avoid major mountainous regions and show a somewhat higher frequency of occurrence downstream of these regions. Fracture events cluster somewhat over eastern ocean basins, the southwestern United States, and northwestern Africa while also avoiding major mountainous regions. On average, trough genesis events exceed trough lysis events in northerly 500-hPa flow with the reverse being true for southerly flow. Genesis events maximize downstream of major mountain barriers such as the Rockies and the Stanovoi and Altai-Sayan Mountains of Mongolia, east of Greenland, over western Europe, and across much of the North Pacific and North Atlantic. An example of trough merger and associated cyclogenesis is presented to help offer a perspective on the climatological results and illustrate qualitatively the important role that large-scale confluent deformation frontogenesis plays in the merger and fracture process.
