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Charles Warner and Richard H. Grumm

Abstract

A monsoon depression over the Bay of Bengal on 7 July 1979 has been studied using a variety of observations, in particular, cloud photographs from aircraft. Ascent in the lower troposphere was concentrated in mesoscale features of cumulus clouds covering ∼1% of the inner area (3 × 105 km2) of the depression. Across these mesoscale features discontinuities in thermal fields were found along with abrupt wind shifts.

Much of the volume of the depression featured thin fragmentary layers of stratus, implying an absence of strong vertical motion. Observed by photography, individual rising cumulus towers were of width up to a few kilometers, increasing with height; rise rates of towers reached 9 m s−1. Measured with aircraft instruments, mean updrafts in cumulus clouds were ∼2.5 m s−1. In cumulus populations scattered throughout the storm, number densities of cumulus ranged from ∼1 km−2 for fractus to ∼1 per 1500 km2 for Congestus. Congestus penetrating the 500 (300) hPa level were ∼1 per 3200 (13 000) km2. Fractional area coverage by cumulus updrafts was ∼0.5% in humilis, less in other categories. Coverage by cumulus updrafts was roughly 20 times less than coverage by inert remnants of cumulus. Cloudy ascending motion in populations of cumulus was generally on the order of hundredths of Pascals per second. It appeared to be mostly compensated by local subsidence. Great number densities of humilis were found moistening the central area following subsidence and drying.

Total cloud cover was dominated by mid-level thin fragmentary status layers and cumulus debris. There was extensive anvil cloud based at ∼400 hPa, apparently arising from cumulus.

Detailed observations were made of a cloud line growing out of the southwesterly flow south of the center of the depression. The line was followed for 3 h on GOES-1 visible imagery. It propagated faster than the low-level winds. Aircraft altimetry showed an abrupt height drop from 6097 to 6090 m at 483 hPa, over a distance of 50 km from southeast to northwest through the line. Southwesterly momentum was lifted from 900 to 600 hPa and from southeast to northwest through the line. Other colocated singularities in convection and wind fields were found.

Ascent in the lower troposphere over the depression as a whole (1066 km2) was assessed from aircraft and dropwindsonde data to be approximately −0.3 Pa s−1.

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Richard H. Grumm and John R. Gyakum

Abstract

An examination is made of the current National Meteorological Center (NMC) operational models’ ability to forecast surface anticyclones. A study of the 1981–82 cold season reveals systematic underprediction of the phenomenon on the part of both the Limited Area Fine Mesh (LFM) and spectral models. However, the LFM forecasts weaker anticyclones than does the spectral model. This difference is apparent in the region of eastern North America and the western Atlantic Ocean. The systematic underprediction found in this study is as great as Colucci and Bosart found for NMC's six-layer primitive equation model.

No overall systematic forecast bias is found for the 1000–500 mb mean temperatures over the surface anti-cyclones. However, excessively warm temperatures are forecast in the Pacific northwest region of both models, and the LFM forecasts erroneously cold temperatures in the western Atlantic basin south of 40°N. The spectral model shows a significant improvement over the LFM in this latter region.

The mean anticyclone displacement error for both models at 48-h range is about 500 km. There is also a tendency for both models to place anticyclones erroneously to the south and east of their observed positions, suggesting the models' translation of these anticyclones to be too fast. Colucci and Bosart also found a fast bias, but this study suggests an overall improvement in anticyclone placement.

Finally, a case of a recent poorly forecasted anticyclone-cyclone complex illustrates the deleterious effects those forecasts can have in the attempt to correctly forecast significant precipitation events. Our study shows an unforecasted precipitation event to have occurred in a lower troposphere warm advection region associated with a poorly forecasted surface anticyclone.

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Robert E. Hart and Richard H. Grumm

Abstract

A method for ranking synoptic-scale events objectively is presented. NCEP 12-h reanalysis fields from 1948 to 2000 are compared to a 30-yr (1961–90) reanalysis climatology. The rarity of an event is the number of standard deviations 1000–200-hPa height, temperature, wind, and moisture fields depart from this climatology. The top 20 synoptic-scale events from 1948 to 2000 for the eastern United States, southeast Canada, and adjacent coastal waters are presented. These events include the “The Great Atlantic Low” of 1956 (ranked 1st), the “superstorm” of 1993 (ranked 3d), the historic New England/Quebec ice storm of 1998 (ranked 5th), extratropical storm Hazel of 1954 (ranked 9th), a catastrophic Florida freeze and snow in 1977 (ranked 11th), and the great Northeast snowmelt and flood of 1996 (ranked 12th).

During the 53-yr analysis period, only 33 events had a total normalized anomaly (M TOTAL) of 4 standard deviations or more. An M TOTAL of 5 or more standard deviations has not been observed during the 53-yr period. An M TOTAL of 3 or more was observed, on average, once or twice a month. October through January are the months when a rare anomaly (M TOTAL ≥ 4 standard deviations) is most likely, with April through September the least likely period. The 1960s and 1970s observed the fewest number of monthly top 10 events, with the 1950s, 1980s, and 1990s having the greatest number. A comparison of the evolution of M TOTAL to various climate indices reveals that only 5% of the observed variance of M TOTAL can be explained by ENSO, North Atlantic oscillations, or Pacific–North American indices. Therefore, extreme synoptic-scale departures from climatology occur regardless of the magnitude of conventional climate indices, a consequence of a necessary mismatch of temporal and spatial scale representation between the M TOTAL and climate index measurements.

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John H. E. Clark, Richard P. James, and Richard H. Grumm

Abstract

The processes responsible for a banded snowfall region during a December 1997 East Coast storm are examined. Conventional data plus a numerical simulation with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used. Calculations of slantwise potential area near the bands suggest that the release of conditional symmetric instability played a role in their formation. The location and timing for the appearance of negative moist potential vorticity (MPV) cannot, however, be reconciled with band formation. A balanced MPV model based on the geostrophic momentum approximation is developed. It provided new insights into the mechanisms of MPV generation. A swath of negative balanced MPV now coincides with the snowbands. MPV sources are proposed that are linked to a vigorous mesoscale updraft near the bands. The updraft occurred on the warm, moist side of a zone of midtroposphere frontogenesis. Negative MPV develops through differential ageostrophic transports of geostrophic momentum and equivalent potential temperature. Of these, differential vertical equivalent potential temperature transport was the most efficient and accounted for the largest fraction of model-produced negative MPV tendencies near the bands. This mechanism was particularly strong in the lower troposphere near the mesoscale updraft.

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John R. Gyakum, John R. Anderson, Richard H. Grumm, and Elissa L. Gruner

Abstract

An eight year sample of cold-season (1 October through 31 March) extratropical cyclones in the, Pacific Ocean basin is used to study central pressure changes and life cycle characteristics.

We find that over 90% of the cyclones passing through the area of the Kuroshio Current intensify in this region. Corresponding percentages in excess of 60% extend from the Kuroshio, south of 45°N, eastward to 130°W. Mean 24-h central pressure falls of all cyclones exceed 9 mb through the entire basin west of 140°W in the latitude band 30° to 50°N.

A statistical analysis of 24-h central pressure changes is performed on all cyclones within our domain. A frequency distribution of 1996 cases of 24-h maximum deepening reveals statistically significant departures from a Gaussian distribution, with the coefficient of skewness substantially negative. We also find similarly significant departures from normal in a frequency distribution of all 24-h central pressure changes, in spite of the fact that this distribution would be expected to have relatively fewer nonlinear interactions of processes associated with maximum deepening. A stratification of these data into ten degree latitude bands reveals that the ocean-dominated areas south of 60°N all have significant departures from the normal distributions with significantly large negative values of skewness. The land and ice-dominated region between 60° and 70°N has a deepening rate distribution that is approximately Gaussian with coefficients of skewness and kurtosis within the confidence limits of a normal distribution. These results suggest that the underlying ocean surface may be responsible for the significant departures of the pressure change distribution from a normal distribution.

We find that explosively developing cyclones (defined as those systems whose central pressure falls at least 24 mb in 24 h at 45°N) have longer lifetimes than the more conventional lows. Approximately 74% of the explosive cyclones last for at least four days. Only 21% of the nonexplosive cases exist for as long as four days. The vast majority of rapid deepeners commence their maximum intensification within 24 h of their initial formation. Thus, a correct analysis and forecast of a newly formed cyclone appears crucial to a successful explosive cyclone simulation.

Although cyclone formation areas cover vast areas of the Pacific, especially those east of Japan, south of Alaska, and the surroundings of the Kamchatka Peninsula, explosive cyclone formation positions are almost exclusively south of 50°N, concentrated east of the Asiatic continent, and in an area between 150° and 160°W. The “bomb” maximum deepening positions are located in areas slightly to the north and east of their formation positions. Dissipation positions, while concentrated in the Gulf of Alaska, the northeast Pacific, and in an area west of Kamchatka for all systems, are almost exclusively confined to areas north of 50°N for the rapidly deepening cyclones.

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