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John J. Cassano and Thomas R. Parish

Abstract

A series of two-dimensional numerical experiments was conducted in order to describe the role of nonhydrostatic dynamics in simple Antarctic katabatic flows. The results presented include a comparison of the thermodynamic and dynamic fields produced by hydrostatic and nonhydrostatic numerical simulations. The source of the differences in the simulations was diagnosed based on an analysis of the model equation tendencies as well as calculated components of the pressure gradient force. Over most of the terrain slope, the nonhydrostatic effects were found to be insensitive to the model horizontal resolution, for a grid spacing ranging from 5 to 100 km.

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Thomas R. Parish and John J. Cassano

Abstract

Antarctica is known for its strong and persistent surface winds that are directed along topographic pathways. Surface winds are especially strong during the winter period. The high directional constancy of the wind and the close relationship of the wind direction to the underlying terrain can be interpreted as evidence of katabatic wind activity. Observations show that the directional constancy of the Antarctic surface wind displays little seasonal variation. Summertime winds cannot be expected to contain a significant katabatic component, owing to enhanced solar heating of the ice slopes. Observations also show that the coastal environs are subjected to wide variation in atmospheric pressure associated with frequent cyclone activity. The robust unidirectional nature of the Antarctic surface wind throughout the year implies that significant topographic influences other than those from katabatic forcing must be acting.

Idealized numerical simulations have been performed to illustrate the potential role of the Antarctic topography in shaping the wind. The presence of katabatic winds is dependent on radiative cooling of the ice slopes. Simulations without explicit longwave radiation show that the blocking influence of the Antarctic orography is a powerful constraint to the surface wind regime. Resulting low-level wind fields resemble katabatic winds, with directions being tied to the underlying terrain and speeds dependent on the slope of the ice surface. A numerical simulation of a strong wind event during austral autumn shows that the katabatic component is only a small fraction of the horizontal pressure gradient force for this case. This suggests that the role of katabatic winds in the Antarctic boundary layer may be overemphasized and that the adjustment process between the continental ice surface and the ambient pressure field may be the primary cause of the Antarctic wind field.

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Thomas R. Karl and Pamela J. Young

A long time series (1863–1984) of areal-average precipitation in the vicinity of the Great Salt Lake is shown to be highly correlated with the Great Salt Lake levels. This time series is used to assess the unusualnessof the recent episode of heavy precipitation (1981 through 1984). The Palmer Drought Severity Index (PDSI) is used to identify wet spells of weather. The analysis indicates that there were two very important wet spells in the time series, one beginning and ending in the 1860s and the most recent wet spell which began in late 1981. The cumulative excess precipitation during each wet spell was analyzed using stochastic frequency analysis. The analysis suggests that the recent heavy precipitation is not unexpected. Furthermore, if the climate of the past 100 years is representative of the climate over the next 100 years, another wet spell can be anticipated to be at least as severe, in terms of excess precipitation, as the 1981–84 wet spell. Whether lake levels can recede to sufficiently low levels to prevent new record high levels during the next severe wet period is uncertain, but it must be considered in risk-assessment strategies.

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Charles R. Hosler and Thomas J. Lemmons

Abstract

A ground-based 5-mm wavelength radiometer, named the Radiometric Thermasonde, has been developed. Typical vertical temperature profiles, derived from a recently developed radiometric data processing technique, are compared with profiles measured simultaneously by conventional techniques. Two years of performance testing of the operational prototype Thermasonde have demonstrated the feasibility of using radiometric remote probing techniques to describe thermal stability conditions within the first 1.5–2.0 km above the surface, during fair weather conditions. Preliminary tests indicate that temperature profiles derived from the Thermasonde can provide useful information, particularly for air pollution meteorology, for describing dispersion characteristics within the planetary boundary layer. The evaluation of a preliminary data reduction technique is described.

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Thomas R. Karl and Pamela J. Young

The dry and hot weather in the southeast United States during the first seven months of 1986 caused record drought. The agricultural and hydrological perspectives of this drought are examined via a climatological time series. Late nineteenth and twentieth century climate data from the most severely affected areas indicate that from an agricultural perspective the beginning and middle of the 1986 growing season was by far the worst on record. On the other hand, from a hydrological perspective the drought was not of sufficient duration to stand out as such an extreme anomaly. The 1986 drought is part of a change in recent years from the wet weather of the 1960s and much of the 1970s. At this time, there is no evidence to suggest that this change is anything more than another in a series of climate fluctuations typical throughout the climate records of many areas.

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Thomas R. Parish and John J. Cassano

Abstract

Antarctica is noted for strong and persistent winds in the lower atmosphere. The wind directions are controlled by the underlying ice terrain and are deflected in general 20°–50° to the left of the fall line. The Antarctic surface wind regime is thought to be the result of the dual influences of diabatic cooling of the terrain, responsible for the infamous katabatic winds, and the synoptic pressure gradient force in the free atmosphere. The relative importance of pressure gradients associated with katabatic and synoptic processes in forcing the wintertime Antarctic boundary layer winds is evaluated using output from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalysis program for June, July, and August of 1997. Both katabatic and synoptic forces are found to be significant in shaping the near-surface winter winds over the Antarctic ice slopes. Analyses show that the synoptic force is influenced by the underlying ice terrain such that the net force over Antarctica is directed primarily downslope. This result reflects the adjustment of the large-scale ambient pressure gradient to the continental orography. The synoptic force over Antarctica thus differs significantly in both magnitude and direction from that found over the oceanic regions to the north. The adjustment of the synoptic force over the ice sheets enable even the nonwinter Antarctic winds to attain a high directional constancy and resemble a gravity-driven flow. This process also suggests that direction alone is insufficient in classifying Antarctic flows as katabatic.

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Thomas J. Perrone and Paul R. Lowe

Abstract

A statistical forecasting experiment was performed to test the capability of predictors derived from observational data (analysis) fields at 950, 700, 500 and 200 mb to forecast tropical storm formation (genesis). National Oceanographic and Atmospheric Administration tropical mosaic visible satellite images and the Joint (United States Navy and Air Force) Typhoon Warning Center's Post-Season Best Track analyses of tropical storms were used to select a representative collection of tropical cloud clusters, some of which became tropical storms (GO cases), others of which did not (NO GO cases). Navy Fleet Numerical Oceanography Central archived analysis fields of surface pressure, winds, sea surface temperature, and moisture were accessed at locations and times corresponding to cloud cluster positions 24, 48 and 72 hours prior to tropical storm formation/nonformation, and candidate predictors were formed from these analysis data. The number of predictor candidates was increased by also calculating a predictor candidate's Laplacian, and the magnitude of its gradient. A Special local-maximum enhancement technique was also applied to some of the candidate predictors. Stepwise discriminant analysis was applied to these candidate predictors to select subsets with greatest predictive capability for forecasting tropical storm formation at projections of 24,48 and 72 hours. The resulting statistical forecast algorithms were evaluated on independent data, against climatology, and against a basic technique derived solely from latitude and longitude. The results show our forecast technique possesses considerable skill in predicting tropical storm formation with good pre-figurance, post-agreement, threat, and Brier scores.

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Thomas R. Karl and Pamela J. Young

Abstract

A long time series (1863–1984) of areal average precipitation in the vicinity of the Great Salt Lake is shown to be highly correlated with the Great Salt Lake levels. This time series is used to assess the unusual recent episode of heavy precipitation (1981 through 1984). The Palmer Drought Severity Index (PDSI) is used to identify wet spells of weather. The cumulative excess precipitation during each wet spell was analyzed using stochastic frequency analysis. The analysis indicates that there were two very important wet spells in the time series, the beginning and ending in the 1860s and the most recent wet spell which began in late 1981. The analysis suggests that the recent heavy precipitation is not unexpected. Furthermore, if the climate of the past 122 years is representative of the climate over the next 100 years, another wet spell can be anticipated to be at least as severe, in terms of excess precipitation, as the 1981–84 wet spell. Whether lake levels can recede to sufficiently low levels to prevent new record high levels during the next severe wet period is uncertain, but it must be considered in any long-term risk assessment strategies.

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William J. Eadie and Thomas R. Mee

Abstract

The influence of fall velocities of dry-ice pellets on the nucleation of slightly supercooled clouds is discussed and the conditions necessary for the production of ice crystals are examined. A theoretical argument is presented, which suggests that when cloud temperatures are warmer than about −5C, the number of ice crystals produced by a pellet of dry ice moving at its terminal velocity decreases rapidly as the temperature approaches 0C. In contrast to the pellet moving at terminal velocity, it is shown that the ice-crystal productivity of a slowly moving pellet remains high up to 0C. An experimental verification of this predicted dependence upon pellet velocity is described, data are presented, and the implications of these findings for future seeding experiments are discussed.

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Thomas R. Parish and John J. Cassano

Abstract

Katabatic winds have long been recognized as one of the key climatic variables of the low-level Antarctic environment. Antarctic surface winds display a high degree of persistence with mean directions related to the local topographic configuration of the ice sheet, consistent with katabatic forcing. Continental orography also constrains the atmospheric boundary layer motions through blocking and cold air damming. Finally, the coastal rim about the Antarctic continent is among the most active baroclinic zones on Earth. The establishment of the low-level wind field over Antarctica is thus potentially the result of a number of interacting processes.

To quantify the forcing of the wintertime surface wind field over the Antarctic continent, two numerical strategies are presented. First, idealized numerical simulations are conducted to illustrate the strong orographic control of the low-level wind field. Second, a series of daily numerical simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) has been performed for the midwinter month July 2001. The horizontal pressure gradient as depicted in MM5 was added to the standard output and an analysis was conducted to understand the forcing of the low-level wind field. Horizontal pressure gradients at the lowest sigma level (6 m above the surface) revealed a net forcing primarily down the local topographic fall line. Analyses of the katabatic forcing showed that it was a significant component of the total horizontal pressure gradient force over the interior of the continent. Near the coast and extending several hundred kilometers inland, however, effects of the ambient pressure gradient force were typically comparable to the katabatic forcing and often considerably more important. This suggests that the role of topography in shaping the Antarctic boundary layer winds through blocking and subsequent adjustment is critical to the establishment of the low-level wintertime Antarctic wind field.

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