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Steven Businger

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Steven Businger

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This paper makes a case for why Hawaii is the rainbow capital of the world. It begins by briefly touching on the cultural and historical significance of rainbows in Hawaii. Next it provides an overview of the science behind the rainbow phenomenon, which provides context for exploring the meteorology that helps explain the prevalence of Hawaiian rainbows. Last, the paper discusses the art and science of chasing rainbows.

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Steven Businger and Joost A. Businger

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In this note the magnitude of the viscous dissipation of turbulence kinetic energy in the surface layer of storms is investigated. It is shown that the layer-integrated dissipative heating is a cubic function of the wind speed. The magnitude of the estimated heating at higher wind speeds confirms the importance to storm evolution of this term in the turbulence kinetic energy equation and suggests that dissipative energy should be included in numerical weather prediction models, particularly in models that resolve mesoscale structures in storms. A general discussion of the implications of the results for the energetics of a range of storm systems is provided.

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Steven J. Caruso and Steven Businger

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The occurrence of subtropical cyclones over the central North Pacific Ocean has a significant impact on Hawaii’s weather and climate. In this study, 70 upper-level lows that formed during the period 1980–2002 are documented. In each case the low became cut off from the polar westerlies south of 30°N over the central Pacific, during the Hawaiian cool season (October–April). The objectives of this research are to document the interannual variability in the occurrence of upper-level lows, to chart the locations of their genesis and their tracks, and to investigate the physical mechanisms important in associated surface development. Significant interannual variability in the occurrence of upper-level lows was found, with evidence suggesting the influence of strong El Niño–Southern Oscillation events on the frequency of subtropical cyclogenesis in this region. Of the 70 upper-level lows, 43 were accompanied by surface cyclogenesis and classified as kona lows. Kona low formation is concentrated to the west-northwest of Hawaii, especially during October and November, whereas lows without surface development are concentrated in the area to the east-northeast of Hawaii. Kona low genesis shifts eastward through the cool season, favoring the area to the east-northeast of Hawaii during February and March, consistent with a shift in the climatological position of the trough aloft during the cool season. Consistent with earlier studies, surface deepening is well correlated with positive vorticity advection by the thermal wind. Static stability and advection of low-level moisture are less well correlated to surface deepening. These results suggest that kona low formation, to first order, is a baroclinic instability that originates in the midlatitudes, and that convection and latent-heat release play a secondary role in surface cyclogenesis.

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Roland Stull and Steven Businger

To document the inner workings of graduate degree programs, the authors surveyed the 67 American and Canadian universities that grant Doctor of Philosophy (Ph.D.) and/or Master of Science (M.S.) degrees in the atmospheric sciences and related fields. Topics included (a) admission standards such as graduate record exam scores and grade point averages; (b) start-up issues such as course requirements and computer programming skills; (c) M.S. attributes such as thesis length, years until graduation, and thesis versus nonthesis options; (d) Ph.D. procedures such as exam sequences and timing, thesis page length, workplace ethics and teamwork, and development of teaching skills; and (e) employment after graduation. This information could aid university departments in their future program planning.

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Ian Morrison and Steven Businger

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A subtropical cyclone or kona low affected the island of Hawaii on 24–28 February 1997 and brought with it record winds at Hilo, large hail, blizzard conditions at higher elevations, and high surf. Damage estimates for the storm due to crop loss, property damage, and utility line destruction exceed $4 million. A detailed case study of the storm was conducted using all available operational data and data from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. The kona low formed on 23 February 1997 along a stalled trough northeast of the Hawaiian Islands and is investigated during five evolutionary stages: (i) incipient, (ii) intensifying, (iii) mature, (iv) weakening, and (v) dissipating.

The system’s initial development is linked to dynamics at the 250-mb level. The maximum circulation, absolute vorticity, divergence, and height anomalies all occurred at 250 mb during the period of most rapid deepening. Cold anomalies occurred in a deep layer between 850 and 250 mb that tilted eastward with height. Quasigeostrophic analysis showed enhanced vorticity to the west of a thickness trough, a configuration that maintained an area of positive vorticity advection to the west of the surface low and over new convection east and southeast of the low. The vorticity tendency is dominated by the advection of vorticity aloft in this case, especially during the incipient and intensifying stages. The vorticity tendency is dominated by the generation of vorticity by divergence in the lower troposphere.

Cloud bands with embedded convective cells formed on the low’s eastern side and propagated eastward, eventually leaving the area of synoptic-scale ascent and losing their convective properties. Areas where the best-lifted index values were less than zero and areas of positive low-level advection of equivalent potential temperature coincided with regions of deep convection, as inferred from satellite imagery.

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Steven Businger and Bernard Walter

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The NOAA P-3 aircraft was used to collect data in a genesis region for mesoscale comma clouds over the Gulf of Alaska. Aircraft measurements in the genesis region showed that rainbands with spacings of 65–75 km and orientations along the mean wind shear were present. Possible mechanisms for the formation of the rainbands, including conditional symmetric instability (CSI) and modified wave-CISK were investigated, but the data did not allow the formation of the rainbands to be conclusively ascribed to a particular mechanism. The existence of low static stability in the genesis region was also documented and its role in mesoscale comma-cloud development explored.

Careful analysis of images from NOAA polar orbiter and GOES satellites together with synoptic analyses made it possible to trace the life cycles of several mesoscale comma clouds as the genesis region moved across the Gulf of Alaska. As the genesis region approached a preexisting polar frontal cloud band, a wave cyclone formed on the front and absorbed one of the comma clouds. The resulting cyclone central pressure dropped 25 mb in 12 hours. The intensity of this development was underestimated by operational forecast models.

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Steven Businger and Peter V. Hobbs

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Satellite and synoptic data are used to establish the environments in which two comma cloud systems occurred over the Pacific Ocean; serial rawinsonde, aircraft, and single- and dual-Doppler radar data provide information on the mesoscale and microscale structures of the systems.

The disturbances formed within polar air masses in regions of moderately strong cyclonic vorticity. A surface low-pressure center was associated with the comma cloud, and a surface-pressure trough was situated under the tail of the comma cloud. In both cases, there was a wind maximum near 850 mb, located on the southeast flank of the comma cloud, just ahead of the short-wave trough.

Well-defined rainbands were present in both comma cloud systems. The average width of the rainbands was ∼20 km and their average separation ∼30 km. The rainbands were aligned along the direction of the mean wind and perpendicular to the thermal wind over the depth of the rainbands. Precipitation cores, produced by embedded convection, within the rainbands had an average spacing along the length of the rainbands of ∼17 km. The precipitation cores contained updraft speeds of several meters per second and relatively high liquid water contents; they retained their identities over periods of several hours. Wind shifts, lines of convergence and associated updrafts occurred at low levels toward the rear of the rainbands. At higher levels, cloud particles moved from the rear toward the front of the rainbands, where they fell out as precipitation through a low-level flow of moist air. The precipitation was augmented by convective elements in an unstable layer near the top of the rainband, which produced ice crystals that grew by riming and aggregation as they fell through the low-level, moist inflow.

The spacing and orientation of the rainbands can be explained by the theory for mixed dynamic/convective instability developed by Sun. The precipitation cores embedded in the rainbands may have been the result of enhanced updrafts at the points where infection-point instability rolls, oriented nearly perpendicular to the length of the rainbands, intersected the rainbands.

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Antti T. Pessi and Steven Businger

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In this paper, the potential of lightning data assimilation to improve NWP forecasts over data-sparse oceans is investigated using, for the first time, a continuous, calibrated lightning data stream. The lightning data employed in this study are from the Pacific Lightning Detection Network/Long-Range Lightning Detection Network (PacNet/LLDN), which has been calibrated for detection efficiency and location accuracy. The method utilizes an empirical lightning–convective rainfall relationship, derived specifically from North Pacific winter storms observed by PacNet/LLDN. The assimilation method nudges the model’s latent heating rates according to rainfall estimates derived from PacNet/LLDN lightning observations. The experiment was designed to be employed in an operational setting. To illustrate the promise of the approach, lightning data from a notable extratropical storm that occurred over the northeast Pacific Ocean in late December 2002 were assimilated into the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The storm exhibited a very electrically active cold front with most of the lightning observed 300–1200 km away from the storm center. The storm deepened rapidly (12 hPa in 12 h) and was poorly forecast by the operational models. The assimilation of lightning data generally improved the pressure and wind forecasts, as the validation of the model results using available surface and satellite data revealed. An analysis is presented to illustrate the impact of assimilation of frontal lightning on the storm development and dynamics. The links among deep convection, thermal wind along the front, and cyclogenesis are explicitly explored.

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Steven Businger and Jong-Jin Baik

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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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