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David E. Kingsmill

Abstract

The initiation of convection associated with a sea-breeze front, a gust front, and their collision is analyzed using data collected in east-central Florida during the Convection and Precipitation/Electrification project. In conjunction with satellite, surface, and rawinsonde information, dual-Doppler radar-derived winds are used to determine the three-dimensional kinematic factors critical to storm development. The gust front, which emanated from storms on the western half of the peninsula, propagated more rapidly and was deeper than the sea-breeze front, which originated from the east coast and was characterized by a distinctly scalloped appearance. Convection associated with the sea-breeze front appeared to develop preferentially at the vertices of this scalloped pattern where there were enhanced regions of convergence and upward motion. On the gust front, a Helmholtz shearing instability produced an organized configuration of convergence and updraft maxima along its length. However, these were not favored areas for convection initiation as storms originated ahead of the gust front in the form of low intensity reflectivity maxima (believed to be clouds forming on horizontal convective rolls), which rapidly grew in size and intensity once intercepted by the boundary. Contrary to past studies, convective activity and frontal updrafts were not enhanced after the collision of the gust front and sea-breeze front. While the magnitude of convergence at low levels increased, its depth decreased, which led to updraft intensifies similar to those before the collision. The implications of this study to the nowcasting of convection and possible areas of future research are discussed.

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Roger M. Wakimoto and David E. Kingsmill

Abstract

A case study of a sea-breeze front originating from the east coast of Florida colliding with a gust front moving toward the southeast is presented. Single- and multi-Doppler radar analyses combined with serial balloon ascents suggest that the denser sea-breeze flow undercut the cold pool behind the gust front and may have generated a westward-propagating undular bore. In addition, another undular bore was generated by the collision and propagated eastward through the CaPE network. The latter bore propagated upstream, against the southeasterly flow behind the sea-breeze front and was apparent as a narrow band, 5–6 km wide, of near-zero radial velocities embedded within an overall flow that was positive as seen by the CP-4 Doppler radar.

As the eastern bore propagated through the dual-Doppler lobe defined by the NCAR CP-3 and CP-4 radars, a detailed and unique wind synthesis revealed its three-dimensional kinematic structure. Significant along-bore variability was shown in the convergence and vertical velocity fields. Vertical cross sections of the synthesized wind were similar to past studies of wavelike phenomena embedded within an ambient flow. A sounding was launched within the leading edge of the undular bore as it passed over a CLASS site. The thermodynamic profile revealed a change from a frontal stable layer to a well-mixed zone indicative of forced uplift of surface air parcels within, the cold air behind the sea-breeze front. Theoretical calculations of the propagation speed and wavelength of an undular bore closely matched the observations.

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Raul A. Valenzuela and David E. Kingsmill

Abstract

This study documents orographic precipitation forcing along the coastal mountains of Northern California during the landfall of a significant winter storm over the period 16–18 February 2004. The primary observing asset is a scanning X-band Doppler radar deployed on the coast at Fort Ross, California, which provides low-level (e.g., below 1 km MSL) horizontal and vertical scans of radial velocity and reflectivity to characterize airflow and precipitation structures. Further context is provided by a wind-profiling radar, a radio acoustic sounding system (RASS), balloon soundings, buoys, a GPS receiver, and surface meteorological sensors. The winter storm is divided into two episodes, each having pre-cold-frontal low-level jet (LLJ) structures and atmospheric river characteristics. Episode 1 has a corridor of terrain-trapped airflow (TTA) that forms an interface with the LLJ. The interface extends ~25 km offshore in a ~0.5-km vertical layer, and the western edge of this interface near the ocean surface advances toward the coast over the course of ~5 h. The TTA acts as a dynamically driven barrier, so that the incoming LLJ slopes upward offshore below 1.5 km MSL and precipitation is enhanced over the ocean and near the coast. The absence of a TTA in episode 2 allows the cross-barrier flow to slope upward and enhance precipitation directly over the coastal mountains. A theoretical analysis favors the hypothesis that a gap flow exiting the Petaluma Gap forces the TTA.

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Raul A. Valenzuela and David E. Kingsmill

Abstract

This study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15 m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478 m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500 m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320°–140°) and altitude of nearby topography, mean wind direction in the lowest 500 m MSL () between 0°–140° is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between and orographic rainfall reveals that an upper threshold of 150° more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.

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Raul A. Valenzuela and David E. Kingsmill

Abstract

This study documents the mean properties and variability of kinematic and precipitation structures associated with orographic precipitation along the coast of Northern California in the context of terrain-trapped airflows (TTAs). TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. Seven land-falling winter storms are examined with observations from a scanning X-band Doppler radar deployed on the coast at Fort Ross, California. Additional information is provided by a 915-MHz wind-profiling radar, surface meteorology, a GPS receiver, and balloon soundings. The composite kinematic structure during TTA conditions exhibits a significant horizontal gradient of wind direction from the coast to approximately 50 km offshore and a low-level jet (LLJ) that surmounts a weaker airflow offshore corresponding to the TTA, with a zone of enhanced precipitation evident between ~5 and 25 km offshore and oriented nearly parallel to the coastline. Conversely, the composite kinematic structure during NO-TTA conditions exhibits a smaller offshore horizontal gradient of wind direction and precipitation structures are generally enhanced within km of the coastline. Interstorm variability analysis reveals significant variations in kinematic structures during both TTA and NO-TTA conditions, whereas significant variations in precipitation structures are only evident during TTA conditions. The interstorm analysis also illustrates more clearly how LLJ vertical structures evident during NO-TTA conditions exhibit ascent along the coast and over the coastal mountains, which is in contrast to TTA conditions where the ascent occurs offshore and over the TTA.

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David E. Kingsmill and Roger M. Wakimoto

Abstract

A kinematic, dynamic, and thermodynamic analysis of a weakly sheared, airmass thunderstorm observed over northern Alabama is presented. Most notable is the fact that the dominant cell in this storm closely resembles the Byers and Braham model for warm-based, airmass storms. Several phenomena never documented for this storm-type are discussed. One of these is a strong and deep downdraft observed at midlevels with an associated “weak-echo” trench. Its origin appears to be related to a wake entrainment process. A midlevel inflow which causes a visible constriction in the storm cloud is also observed. This inflow results in a division of the thermal buoyancy into two maxima in the vertical: one associated with the precipitation core and the other with strong positive vertical motions in the growing cumulus turret. In addition, a downdraft separate from that seen at midlevels develops at low levels and causes a microburst outflow at the surface. This downdraft appears to be initiated by precipitation loading and intensified by negative thermal buoyancy. The Byers and Braham model of the cumulus, mature and dissipating stages, is discussed in light of these new features.

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David E. Kingsmill and N. Andrew Crook

Abstract

Observations from east-central Florida during the Convection and Precipitation/Electrification (CaPE) experiment are used to investigate the factors that influence atmospheric bore formation from colliding density currents. Ten cases involving the collision of a gust front with a sea-breeze front are analyzed with Doppler radar and sounding and surface mesonet data. The gust fronts in these collisions were generally deeper, denser, and faster propagating than their sea-breeze-front counterparts. Seven of the 10 cases produced dual boundaries that moved away from each other in a relative sense after the collision. Post-collision boundaries moving in the same direction and oriented similarly to the pre-collision gust front occurred in all 10 cases. They transported mass in the manner of a density current in six cases, while the others behaved more like bores or bore–density current hybrids, as they were characterized by little or no mass transport. Similarity of gust-front and sea-breeze-front propagation speed prior to the collision was the best indicator of bore character in post-collision boundaries induced by gust fronts. Sea-breeze-front-induced post-collision boundaries occurred in the seven cases in which the pre-collision sea-breeze front was nonstationary. These post-collision boundaries exhibited no purely mass transport behavior and were all categorized as bores or bore/density current hybrids. The potential ability to predict dual zones of convergence emanating from boundary collisions as demonstrated in this study may be of value to convective nowcasting in Florida and perhaps other locales, as 70% of the observed post-collision boundaries initiated new convection or enhanced existing convection.

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Justin R. Minder and David E. Kingsmill

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Observations from several mountain ranges reveal that the height of the transition from snowfall to rainfall, the snow line, can intersect the terrain at an elevation hundreds of meters below its elevation in the free air upwind. This mesoscale lowering of the snow line affects both the accumulation of mountain snowpack and the generation of storm runoff. A unique multiyear view of this behavior based on data from profiling radars in the northern Sierra Nevada deployed as part of NOAA’s Hydrometeorology Testbed is presented. Data from 3 yr of storms show that the mesoscale lowering of the snow line is a feature common to nearly all major storms, with an average snow line drop of 170 m.

The mesoscale behavior of the snow line is investigated in detail for a major storm over the northern Sierra Nevada. Comparisons of observations from sondes and profiling radars with high-resolution simulations using the Weather Research and Forecasting model (WRF) show that WRF is capable of reproducing the observed lowering of the snow line in a realistic manner. Modeling results suggest that radar profiler networks may substantially underestimate the lowering by failing to resolve horizontal snow line variations in close proximity to the mountainside. Diagnosis of model output indicates that pseudoadiabatic processes related to orographic blocking, localized cooling due to melting of orographically enhanced snowfall, and spatial variations in hydrometeor melting distance all play important roles. Simulations are surprisingly insensitive to model horizontal resolution but have important sensitivities to microphysical parameterization.

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Ellen M. Sukovich, David E. Kingsmill, and Sandra E. Yuter

Abstract

Empirical characterization of graupel and snow in precipitating tropical convective clouds is important for refining satellite precipitation retrieval algorithms and cloud-resolving and radiative transfer models. Microphysics data for this analysis were collected by the University of North Dakota (UND) Citation and the National Aeronautics and Space Agency (NASA) DC-8 aircraft during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) in the western tropical Pacific Ocean. An ice particle identification algorithm was applied to two-dimensional optical array probe data for the purpose of identifying ice particle ensembles dominated by graupel or snow particles. These ensembles were accumulated along 1-km flight segments at temperatures below 0°C. A third category, mixed graupel/snow, has characteristics between those of the predominately graupel and snow ensembles and can be used either in combination with the other two categories or separately. Snow particle ensembles compose 80% of UND Citation and 98% of NASA DC-8 ensemble data. For the UND Citation, graupel ensembles compose ∼5% of the total with mixed graupel/snow ensembles composing ∼15%. There were no graupel ensembles in the NASA DC-8 data, which were collected primarily at temperatures <−35°C. Particles too small to classify (<150-μm maximum dimension) compose 56% of UND Citation and 64% of NASA DC-8 particle images. Nearly all these “tiny” particles occur coincident with particles >∼150 μm. Combining data from both aircraft, snow and mixed graupel/snow ensembles were evident over the full range of subfreezing temperatures (from 0° to −65°C) sampled by the aircraft. In contrast, graupel ensembles were present primarily at temperatures >−10°C. Accurate graupel identification was further supported by all graupel ensembles observed either coincident with or within a 10-km horizontal distance of radar-identified convective precipitation structures.

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Roger M. Wakimoto, Cathy J. Kessinger, and David E. Kingsmill

Abstract

On 9 July 1987, a series of low-reflectivity microbursts were studied over Colorado using dual-Doppler analyses, cloud photogrammetry, and in situ measurements collected by aircraft. These types of wind-shear events are particularly hazardous to the aviation community since the parent cloud and pendant virga shafts appear innocuous. The microburst downdrafts are shown to develop at the location where the virga shafts are, visually, the lowest and opaque. As the downdraft intensifies, sublimation and evaporation (to a smaller extent) rapidly deplete the hydrometeors and result in a shift of the axis of maximum negative vertical velocities into a relatively low reflectivity and transparent region of the virga shafts. Comparisons with weak downdrafts or null null cases reveal that the maximum radar reflectivities within the parent clouds for the two cases are comparable; however, the microburst storm consistently exhibits a larger horizontal area encompassed by the 10-dBZ contour at midlevels prior to downdraft formation.

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