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Steven Businger

Abstract

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Steven Businger
and
Bernard Walter

Abstract

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

Abstract

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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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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Antti T. Pessi
and
Steven Businger

Abstract

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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Terrence J. Corrigan Jr.
and
Steven Businger

Abstract

A series of extreme cloudbursts occurred on 14 April 2018 over the northern slopes of the island of Kaua‘i, Hawaii. The storm inundated some areas with 1262 mm (∼50 in.) of rainfall in a 24-h period, eclipsing the previous 24-h U.S. rainfall record of 1100 mm (42 in.) set in Texas in 1979. Three periods of intense rainfall are diagnosed through detailed analysis of National Weather Service operational and special datasets. On the synoptic scale, a slowly southeastward propagating trough aloft over a deep layer of low-level moisture (>40 mm of total precipitable water) produced prolonged instability over Kaua‘i. Enhanced northeast to east low-level flow impacted Kaua‘i’s complex terrain, which includes steep north- and eastward-facing slopes and cirques. The resulting orographic lift initiated deep convection. The wind profile exhibited significant shear in the troposphere and streamwise vorticity within the convective storm inflow. Evidence suggests that large directional shear in the boundary layer, paired with enhanced orographic vertical motion, produced rotating updrafts within the convective storms. Mesoscale rotation is manifest in the radar data during the latter two periods, and reflectivity cores are observed to propagate both to the left and to the right of the mean shear, which is characteristic of supercells. The observations suggest that the terrain configuration in combination with the wind shear separates the area of updrafts from the downdraft section of the storm, resulting in almost continuous heavy rainfall over Waipā Garden.

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Michael J. Murphy Jr.
and
Steven Businger

Abstract

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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Steven Businger
,
William H. Bauman III
, and
Gerald F. Watson

Abstract

An investigation was conducted of the mesoscale evolution of a quasi-stationary front, termed the Piedmont front owing to its location through the central Carolinas, and an associated outbreak of severe weather on 13 March 1986. Space-time relationships between mesoscale processes associated with the evolution of the surface front and the initiation of severe thunderstorms were studied utilizing the enhanced surface and upper-air observation networks deployed during the field phase of GALE. Surface streamline patterns, frontogenesis, and moisture-flux divergence were computed employing an objective analysis scheme.

Following the arrival at the Carolina coast of a coastal front, the Piedmont front rapidly developed along an axis of dilatation over the eastern margin of the Piedmont, while the coastal front gradually dissipated over the nearshore waters. A differential cloud cover across the Piedmont front resulted in enhanced solar insolation on the warm side of the front that strengthened frontogenesis and acted to further destabilize the atmosphere. On the afternoon of 13 March four severe thunderstorms formed in the vicinity of the Piedmont front. Three of the storms were located in the vicinity of mesolows that formed on the front Subsequently, convection organized into a squall line along the front as synoptic-scale forcing associated with a short-wave trough and cold front aloft (CFA) overtook the Piedmont front from the west.

Stability analyses indicate that on the synoptic scale only a weak to moderate potential for severe weather existed over portions of eastern North and South Carolina. However, fields of moisture-flux divergence show a mesoscale pattern of enhanced convergence well correlated with the locations of the severe thunderstorm cells. A schematic is presented that summarizes the principal factors involved in the development of the severe weather in this complex case.

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Steven Businger
,
Michael E. Adams
,
Steven E. Koch
, and
Michael L. Kaplan
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Steven Businger
,
Michael E. Adams
,
Steven E. Koch
, and
Michael L. Kaplan

Abstract

Mesoscale height and temperature fields can be extracted from the observed wind field by making use of the full divergence equation. Mass changes associated with irrotational ageostrophic motions are retained for a nearly complete description of the height field. Above the boundary layer, in the absence of friction, the divergence equation includes terms composed of the components of the wind and a Laplacian of the geopotential height field. Once the mass field is determined, the thermal structure is obtained through application of the hypsometric equation.

In this paper an error analysis of this divergence method is undertaken to estimate the potential magnitude of errors associated with random errors in the wind data. Previous applications of the divergence method have been refined in the following ways. (i) The domain over which the method is applied is expanded to encompass the entire STORM-FEST domain. (ii) Wind data from 23 profiler and 38 rawinsonde sites are combined in the analysis. (iii) Observed profiler and rawinsonde data are interpolated to grid points through a modified objective analysis, and (iv) the variation in elevation of the profiler sites is taken into account.

The results of the application of the divergence method to the combined wind data from profiler and rawinsonde sites show good agreement between the retrieved heights and temperatures and the observed values at rawinsonde sites. Standard deviations of the difference between the retrieved and observed data lie well within the precision of the rawinsonde instruments. The difference field shows features whose magnitude is significantly larger than the errors predicted by the error analysis, and these features are systematic rather than random in nature, suggesting that the retrieved fields are able to resolve mesoscale signatures not fully captured by the rawinsonde data alone.

The divergence method is also applied solely to the profiler data to demonstrate the potential of the divergence method to provide mass and thermal fields on a routine basis at synoptic times when operational rawinsonde data are not available. A comparison of the heights derived from the profiler winds with those independently measured by rawinsondes indicates that valuable information on the evolution of atmospheric height and temperature fields can be retrieved between conventional rawinsonde release times through application of the divergence method. The implications of the results for applications of the method in weather analysis and in numerical weather prediction are discussed.

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