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Steven Businger and Jong-Jin Baik

Abstract

A mesoscale “arctic hurricane” developed over the western Bering Sea on 7 March 1977 and traveled eastward parallel to the ice edge along a zone of large sea surface temperature gradient. Satellite imagery reveals spiral cloud bands of unusual symmetry and mesoscale dimension associated with the mature stage of the low. The track of the low pressure center passed over the rawinsonde station at St. Paul Island where time series of surface data show a pronounced maximum in equivalent potential temperature at the core of the low. The storm made landfall with surface winds >30 m s−1; at Cape Newenham, Alaska, on 9 March and rapidly dissipated thereafter.

Synoptic analyses show that the arctic hurricane formed at the leading edge of an outflow of arctic air that originated over the ice and passed over the open water of the western Bering 5u. In the mid- and upper troposphere a large cold-core low dominated the Bering Sea region. Quasi-geostrophic analysis at 0000 UTC 7 March 1977 reveals conditions conducive to synoptic-scale ascent over the region of the incipient low, as a sharp upper-level short wave crosses the Siberian coast. Conversely, during its mature stage little quasi-geostrophic forcing is seen over the low.

In order to investigate the ability of sea surface heat fluxes to develop and maintain the arctic hurricane, an analytical model based on the Carnot cycle, and an axisymmetric numerical model with the Kuo cumulus parameterization scheme are applied. The analytical calculation of the pressure drop from the outermost closed isobar to the storm center results in a central pressure of 973 mb, which agrees well with observation. When the initial environment of the numerical model is set to be similar to that observed with the arctic hurricane, the model correctly predicts the minimum sea-level pressure, strength of the wind circulation, and the magnitude of sensible beat fluxes observed with the storm. The dynamic and thermodynamic structures of the simulated storm are similar to those of tropical cyclones. The predicted development time of the storm is longer than observations suggest, and the diameter of the simulated anvil outflow is somewhat larger, pointing to the likely importance of baroclinic processes in the evolution of the disturbance, and the need for further numerical studies with mesoscale models that employ full three-dimensional primitive equations.

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Steven Businger and Richard J. Reed

Abstract

The small-scale and rapid development of polar lows over relatively data-sparse areas results in a special forecast challenge for the operational forecasting community. This paper constitutes a review of recent advances in our understanding of cyclogenesis in polar air masses. The review is primarily comprised of a survey of the observed features of polar lows as documented in a number of case studies presented in the recent literature. The review is organized on the basis of a combination of observational and physical considerations and is aimed at diagnosing common types of developments. Theoretical ideas concerning the origins of polar lows and results of numerical modeling experiments aimed at simulating their development are also summarized. Finally, a discussion of approaches to the operational problem of forecasting polar lows is given.

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Antti T. Pessi and Steven Businger

Abstract

Lightning data from the Pacific Lightning Detection Network (PacNet) and Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite were compared with TRMM precipitation radar products and latent heating and hydrometeor data. Three years of data over the central North Pacific Ocean were analyzed. The data were divided into winter (October–April) and summer (June–September) seasons. During the winter, the thunderstorms were typically embedded in cold fronts associated with eastward-propagating extratropical cyclones. Summer thunderstorms were triggered by cold upper-level lows associated with the tropical upper-tropospheric trough (TUTT). Concurrent lightning and satellite data associated with the storms were averaged over 0.5° × 0.5° grid cells and a detection efficiency correction model was applied to quantify the lightning rates. The results of the data analysis show a consistent logarithmic increase in convective rainfall rate with increasing lightning rates. Moreover, other storm characteristics such as radar reflectivity, storm height, ice water path, and latent heat show a similar logarithmic increase. Specifically, the reflectivity in the mixed-phase region increased significantly with lightning rate and the lapse rate of Z decreased; both of these features are well-known indicators of the robustness of the cloud electrification process. In addition, the height of the echo tops showed a strong logarithmic correlation with lightning rate. These results have application over data-sparse ocean regions by allowing lightning-rate data to be used as a proxy for related storm properties, which can be assimilated into NWP models.

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Steven A. Stage and Joost A. Businger

Abstract

The model for the cloud-topped marine boundary layer presented by Stage and Businger (1981) is discussed and compared with previous models. Our model gives a considerably different interpretation of the energetics of the layer and indicates that a much higher fraction (20%) of the layer turbulence kinetic energy production is available to drive entrainment than previously supposed. In a test case, the Lilly (1968) minimum entrainment model gives entrainment rates similar to ours; however, this model is based on physically unrealistic assumptions about layer energetics. It is noted that two soundings from the International Field Year for the Great Lakes (IFYGL) exhibit behavior not allowed by Deardorff’s (1976) model. In these cases our model gives a good fit to the data and Deardorff’s model predicted a boundary layer much deeper than observed. The depth of the layer of radiative cooling at cloud top is shown to be important only if it is a significant fraction of the mixed-layer depth zB. Layer energetics are shown to prevent the cloud-top entrainment instability condition from causing much difference in theentrainment rate

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Steven Businger, Selen Yildiz, and Thomas E. Robinson

Abstract

This study analyzes QuikSCAT surface wind data over the North Pacific Ocean to document the distribution of captured fetches in extratropical cyclones that produced hurricane force (HF) wind fields from January 2003 through May 2008. A case study is presented to introduce the datasets, which include surface wind analyses from the Global Forecast System (GFS) Global Data Assimilation System (GDAS), and wave hindcasts from the third-generation wave model (WAVEWATCH III; hereafter, WW3), in addition to the QuikSCAT surface wind data. The analysis shows significant interannual variability in the location of the captured fetches as documented by QuikSCAT, including a shift in the distribution of captured fetches associated with ENSO. GDAS surface winds over the ocean are consistently underanalyzed when compared to QuikSCAT surface winds, despite the fact that satellite observations of ocean surface winds are assimilated. When the WW3 hindcasts associated with HF cyclones are compared with buoy observations over the eastern and central North Pacific Ocean, the wave model significantly underestimates the large-swell events.

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Norbert Schörghofer, Steven Businger, and Matthias Leopold

Abstract

The coldest places on the Hawaiian island chain are not on the very summits of its tallest volcanoes, Mauna Kea (19.82°N; 4,207 m) and Mauna Loa (19.48°N; 4,169 m), but within craters and caves with perennial ice. Here, we explore unique microclimates in the alpine stone deserts of two tropical island volcanoes, report new temperature extremes for Hawaii, and study the role of microclimates in the preservation of perennial ice bodies. Nocturnal cold-air pools are common in the craters and are responsible for the coldest temperature ever reported from the Hawaiian Islands (–20°C). These cold-air pools are not frequent enough to substantially affect the annual heat budget of the ground, but cold air is frequently trapped between boulders and contributes to freezing conditions in this way. Perennial ice is found beneath even warmer environments in lava tube caves. The lowest annual-mean temperature (–0.7°C) was measured at the distal end of a spectacular ice cave on Mauna Loa, where the outside air temperature averages +8°C. In the current climate, the outside temperature rarely falls below freezing, so the air’s sensible heat cannot contribute to freezing conditions. Considering the effect of recent climate warming and the buoyancy of humid air, cold air that flowed down the lava tubes in winter nights, combined with sublimation cooling, is still a plausible explanation for the perennial ice ponds found there.

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Steven A. Stage and Joost A. Businger

Abstract

A model is presented for the growth and evolution of a cloud-topped marine boundary layer. In this model the entrainment rate is determined from the turbulence kinetic energy (TKE) budget. It is assumed that the TKE budget can be partitioned according to whether each process produces TKE or converts it into potential energy, and that dissipation is proportional to production. This leads to an entrainment relationship which is considerably different than used in previous cloud-topped models.

This model is used to study an episode of cold-air outbreak over Lake Ontario during the International Field Year for the Great Lakes (IFYGL). The model reproduces changes in potential temperature and dew point as the air crossed the lake and the associated time variation of these parameters at the down-wind shore with an accuracy of better than 1°C. Model and measured soundings closely match, especially with respect to the presence and location of such features as cloud layers. Depth of the mixed layer also was generally well modeled. Use of divergences measured by the lakewide IFYGL buoy network did not give good agreement with the data. It is believed that this indicates that mixed-layer depth is sensitive to divergences at a smaller scale than the size of the lake.

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James Foster, Michael Bevis, and Steven Businger

Abstract

The sliding-window technique uses a moving time window to select GPS data for processing. This makes it possible to routinely incorporate the most recently collected data and generate estimates for atmospheric delay or precipitable water in (near) real time. As a consequence of the technique several estimates may be generated for each time epoch, and these multiple estimates can be used to explore and analyze the characteristics of the atmospheric estimates and the effect of the processing model and parameters. Examples of some of the analyses that can be undertaken are presented. Insights into the phenomenology of the atmospheric estimates provided by sliding-window analysis permit the fine-tuning of the GPS processing as well as the possibility of both improving the accuracy of the near-real-time estimates themselves and constraining the errors associated with them. The overlapping data windows and the multiple estimates that characterize the sliding-window method can lead to ambiguity in the meaning of many terms and expressions commonly used in GPS meteorology. In order to prevent confusion in discussions of sliding-window processing, a nomenclature is proposed that formalizes the meaning of the primary terms and defines the geometric and physical relationships between them.

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William H. Bauman III and Steven Businger

Space shuttle launches and landings at Kennedy Space Center (KSC) are subject to strict weather-related launch commit criteria and landing weather flight rules. Complex launch commit criteria and end-of-mission landing weather flight rules demand very accurate forecasts and nowcasts (short-term forecasts of less than 2 h) of cloud, wind, visibility, precipitation, turbulence, and thunderstorms prior to shuttle launches and landings.

The challenges to the National Weather Service Spaceflight Meteorology Group forecasters at Johnson Space Center to nowcast and forecast for space shuttle landings and evaluate the landing weather flight rules are discussed. This paper focuses on the forecasts and nowcasts required for a normal end-of-mission and three scenarios for abort landings of a space shuttle at KSC. Specific weather requirements for a potential emergency landing are the dominant cause of weather-related delays to space shuttle launches. Some examples of meteorological techniques and technologies in support of space shuttle landing operations are reviewed. Research to improve nowcasting convective activity in the Cape Canaveral vicinity is discussed, and the particular forecast problem associated with landing a space shuttle during easterly flow regimes is addressed.

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Michael J. Murphy Jr. and Steven Businger

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On 2 April 2006, Oahu’s Ko‘olau Mountain Range endured more than 6 h of heavy rain with accompanying flash flooding along its northeast-facing slopes. The storm responsible for the event left a pattern of precipitation characteristic of orographic anchoring of convection with extreme rainfall gradients along the slopes and maxima along the crest of the mountain range. In fact, this was the third flash-flood event to impact the Ko‘olau Mountains in just over 1 month, with each event occurring under conditions of moist southeasterly flow at low levels and moderate conditional instability. Under these conditions persistent convection and localized heavy rainfall often occur over the Ko‘olau Mountain Range. Mesoscale analyses of the thunderstorm complex responsible for the 2 April 2006 heavy rain event and the results of a high-resolution numerical simulation employing the Weather Research and Forecasting (WRF) model are described in this study.

Key features of the convection that contributed to the longevity of the event include repeat formation of convective cells along the eastern side of the central Ko‘olaus, minimal horizontal cloud motion, and strong updrafts that sloped toward the northwest in the lower levels. The westerly shear of the low-level flow determined the pattern of accumulated precipitation by aligning the slope of the convective updrafts nearly parallel to the southeast-to-northwest-orientated Ko‘olau Mountain Range. The microphysical structure of the convection was complex, with the vertical advection of hydrometeors originating below the freezing level facilitating high concentrations of ice particles and an environment conducive to charge separation and lightning.

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