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Adam J. French and Matthew D. Parker

Abstract

On 30 March 2006, a convective episode occurred featuring isolated supercells, a mesoscale convective system (MCS) with parallel stratiform (PS) precipitation, and an MCS with leading stratiform (LS) precipitation. These three distinct convective modes occurred simultaneously across the same region in eastern Kansas. To better understand the mechanisms that govern such events, this study examined the 30 March 2006 episode through a combination of an observation-based case study and numerical simulations. The convective mode was found to be very sensitive to both the environmental thermodynamic and wind shear profiles, with variations in either leading to different convective modes within the numerical simulations. Strong vertical shear and moderate instability led to the development of supercells in western Oklahoma. Strong shear oriented parallel to a surface dryline, coupled with dry air in the middle and upper levels, led to the development of the PS linear MCS in central Kansas. Meanwhile, moderate wind shear coupled with high instability and strong linear forcing led to the development of the LS MCS in eastern Kansas. Absent linear forcing, the moderate shear environment in eastern Kansas was supportive of isolated supercells in the numerical experiments. This suggests that the linear initiation mechanism was key to the development of the LS linear MCS. From the results of this study it is concuded that, for this event, localized environmental variations were largely responsible for the eventual convective mode, with the method of storm initiation having an impact only within the weaker shear environment of eastern Kansas.

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Richard G. French and Peter J. Gierasch

Abstract

We examine a propagating wave interpretation of the temperature profile features observed in the Jovian upper atmosphere by Veverka et al. Inertia-gravity waves with frequencies on the order of 3 × 10−3 sec−1 are consistent with the data. If the interpretation is correct, and if the waves carry energy upward, it implies 1) that there is excitation of such waves at lower levels, 2) that eddy diffusivities on the order of 106 cm2 sec−1 are probably generated by the waves, and 3) that the energy carried by waves is important to the upper atmospheric heat balance.

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Adam J. French and Matthew D. Parker

Abstract

Output from idealized numerical simulations is used to investigate the storm-scale processes responsible for squall-line evolution following a merger with an isolated supercell. A simulation including a squall line–supercell merger is compared to one using the same initial squall line and background environment without the merger. These simulations reveal that while bow echo formation is favored by the strongly sheared background environment, the merger produces a more compact bowing structure owing to a locally enhanced rear-inflow jet. The merger also represents a favored location for severe weather production relative to other portions of the squall line, with surface winds, vertical vorticity, and rainfall all being maximized in the vicinity of the merger.

An analysis of storm-scale processes reveals that the premerger squall line weakens as it encounters outflow from the preline supercell, and the supercell becomes the leading edge of the merged system. Subsequent localized strengthening of the cold pool and rear-inflow jet produce a compact, intense bow echo local to the merger, with a descending rear-inflow jet creating a broad swath of damaging surface winds. These features, common to severe bow echoes, are shown to be a direct result of the merger in the present simulations, and are diminished or absent in the no-merger simulation. Sensitivity tests reveal that mergers in a weaker vertical wind shear environment do not produce an enhanced bow echo structure, and only produce a localized region of marginally enhanced surface winds. Additional tests demonstrate that the details of postmerger evolution vary with merger location along the line.

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Adam J. French and Matthew D. Parker

Abstract

Some recent numerical experiments have examined the dynamics of initially surface-based squall lines that encounter an increasingly stable boundary layer, akin to what occurs with the onset of nocturnal cooling. The present study builds on that work by investigating the added effect of a developing nocturnal low-level jet (LLJ) on the convective-scale dynamics of a simulated squall line. The characteristics of the simulated LLJ atop a simulated stable boundary layer are based on past climatological studies of the LLJ in the central United States. A variety of jet orientations are tested, and sensitivities to jet height and the presence of low-level cooling are explored.

The primary impacts of adding the LLJ are that it alters the wind shear in the layers just above and below the jet and that it alters the magnitude of the storm-relative inflow in the jet layer. The changes to wind shear have an attendant impact on low-level lifting, in keeping with current theories for gust front lifting in squall lines. The changes to the system-relative inflow, in turn, impact total upward mass flux and precipitation output. Both are sensitive to the squall line–relative orientation of the LLJ.

The variations in updraft intensity and system-relative inflow are modulated by the progression of the low-level cooling, which mimics the development of a nocturnal boundary layer. While the system remains surface-based, the below-jet shear has the largest impact on lifting, whereas the above-jet shear begins to play a larger role as the system becomes elevated. Similarly, as the system becomes elevated, larger changes to system-relative inflow are observed because of the layer of potentially buoyant inflowing parcels becoming confined to the layer of the LLJ.

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Adam J. French and Matthew D. Parker

Abstract

A set of 21 cases in which an isolated supercell merged with a squall line were identified and investigated using analyses from the Rapid Update Cycle (RUC) model, archived data from the Weather Surveillance Radar-1988 Doppler (WSR-88D) network, and severe storm reports. This analysis revealed two primary environments associated with these mergers: a weak synoptic forcing, weak to moderate shear environment (WF) and a strong synoptic forcing, strong shear environment (SF). These environments bear a strong resemblance to those identified for progressive (WF) and serial (SF) derechoes in past studies. Radar reflectivity data revealed a spectrum of storm evolution patterns that generally lead to the merged system organizing as a bow echo. At one extreme, observed exclusively in the WF environment, the entire squall line evolved into a large bow echo following the merger. At the other extreme, observed for several cases in the SF environment, a localized bowing segment developed embedded within the larger squall line. The remaining cases exhibited characteristics best described as a hybrid of these extremes. Storm rotation generally weakened and became concentrated in low levels following the merger, although the exact evolution differed between the two background environments. Finally, an analysis of storm reports revealed that hail reports were maximized premerger and severe wind reports postmerger in both environments, while the distribution of tornado reports varied. In the WF environment a larger fraction of tornado reports occurred postmerger, while tornado production was maximized premerger in the SF environment. This suggests an evolving severe weather threat during the course of the merger, the details of which depend on the background environment.

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Casey E. Letkewicz, Adam J. French, and Matthew D. Parker

Abstract

Base-state substitution (BSS) is a novel modeling technique for approximating environmental heterogeneity in idealized simulations. After a certain amount of model run time, base-state substitution replaces the original horizontally homogeneous background environment with a new horizontally homogeneous environment while maintaining any perturbations that have developed during the preceding simulation. This allows the user to independently modify the kinematic or thermodynamic environments, or replace the entire sounding without altering the structure of the perturbation fields. Such an approach can provide a powerful hypothesis test, for example, in a study of how an isolated convective storm would respond to a different environment within a horizontally homogeneous setting. The BSS modifications can be made gradually or instantaneously, depending on the needs of the user. In this paper both the gradual and instantaneous BSS procedures are demonstrated for simulations of deep moist convection, using first a wholly idealized setup and then a pair of observed near-storm soundings. Examination of domainwide model statistics demonstrates that model stability is maintained following the introduction of the new background environment. Following BSS, domain total mass and energy exhibit the expected instantaneous jumps upward or downward as a result of the imposed changes to the mean thermal and wind profiles, after which they remain steady during the subsequent simulation. The gridded model fields are well behaved and change gradually as the simulated storms respond meteorologically to their new environments. The paper concludes with a discussion of several unique aspects of the BSS approach.

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William P. Kustas, Thomas J. Jackson, Andrew N. French, and J. Ian MacPherson

Abstract

The 1997 Southern Great Plains Hydrology Experiment (SGP97) was designed and conducted to extend surface soil moisture retrieval algorithms based on passive microwave observations to coarser resolutions, larger regions with more diverse conditions, and longer time periods. The L-band Electronically Scanned Thinned Array Radiometer (ESTAR) on an airborne platform was used for daily mapping of surface soil moisture over an area of approximately 40 km × 260 km for a 1-month period. Results showed that the soil moisture retrieval algorithm performed the same as in previous investigations, demonstrating consistency of both the retrieval and the instrument. This soil moisture product at 800-m pixel resolution together with land use and fractional vegetation cover information is used in a remote sensing model for computing spatially distributed fluxes over the SGP97 domain. Validation of the model output is performed at the patch scale using tower-based measurements and at regional scale using aircraft flux observations. Comparisons at the patch scale yielded discrepancies between model- and tower-based sensible and latent heat fluxes of 40% and 20%, respectively. At regional scales, differences between modeled and aircraft-based sensible and latent heat fluxes were less, on the order of 30% and 15%, respectively. A preliminary comparison of regional average energy fluxes with a model using remotely sensed temperatures was conducted and yielded good agreement. The utility of spatially distributed energy flux and model-simulated surface temperature maps over the SGP97 region is discussed.

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J. Galloway, A. Pazmany, J. Mead, R. E. McIntosh, D. Leon, J. French, R. Kelly, and G. Vali

Abstract

This paper presents airborne W-band polarimetric radar measurements at horizontal and vertical incidence on ice clouds using a 95-GHz radar on the University of Wyoming King Air research aircraft. Coincident, in situ measurements from probes on the King Air make it possible to interpret polarimetric results in terms of hydrometeor composition, phase, and orientation. One of the key polarimetric measurements recently added to those possible with the W-band radar data system is the copolar correlation coefficient ρ HV. A discussion of the relation between cloud scattering properties and ρ HV covers a test for isotropy of the distribution of observed hydrometeors in the plane of polarization and qualitative evaluation of the possible impact of Mie (resonant) scattering on ρ HV measurements made at W band. Prior measurements of ρ HV at S band and Ku band are compared with the W-band results. The technique used to measure ρ HV, including the real-time and postprocessing steps required, is explained, with a discussion of the expected measurement error for the magnitude and phase of ρ HV.

Cloud data presented include melting-layer observations at vertical incidence, observation of a convective snow cell at vertical incidence, and observations of needle crystals at both horizontal and vertical incidence. The melting layer observations provide a consistency check for the measurements of ρ HV and linear depolarization ratio (LDR) at W band through the test for isotropy. The vertical incidence measurements of a convective snow cell displayed significant mean orientation of the hydrometeors observed in the features evident in Z DR and the phase of ρ HV. Data taken on needle crystals provided clear indication of particle alignment in the measurements of Z DR and LDR for the horizontal incidence case and equally clear indication of a lack of orientation for the vertical incidence case.

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J. Galloway, A. Pazmany, J. Mead, R. E. McIntosh, D. Leon, J. French, S. Haimov, R. Kelly, and G. Vali

Abstract

Investigation of precipitation formation requires measurements of the drop size distribution in a cloud. These measurements have usually been made using ground-based radar systems or aircraft in situ probes. Difficulties encountered in practice using these systems include accounting for the air motion at points remote from the radar systems and small sample volumes measured using the aircraft probes. An airborne W-band radar system provides a measurement from a much larger sample volume, close to the aircraft, with a correction for air motion possible using the data from the aircraft inertial navigation system. The Coastal Stratus Experiment conducted off the coast of Oregon in late 1995 provided W-band radar and microphysical probe data sampled from much of the same region of a marine stratus cloud. The unique combination of cloud probes and W-band radar on board the University of Wyoming King Air allowed the radar sampling to be only 60 m away from the probe sampling region. Doppler spectrum data from the W-band radar were used to produce estimates of the drop size spectrum density N(D). These estimates were compared to measurements of N(D) taken by the Particle Measuring Systems forward scattering spectrometer, 1D, and 2DC probes. This comparison suggests that a vertically pointing airborne W-band radar is a viable remote sensing tool for measuring N(D) in clouds and precipitation. This radar provides information on drop size distribution variation on a much smaller horizontal scale than the probes as a result of the much higher sample rate and larger measurement sample volume.

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Timothy A. Supinie, Youngsun Jung, Ming Xue, David J. Stensrud, Michael M. French, and Howard B. Bluestein

Abstract

Several data assimilation and forecast experiments are undertaken to determine the impact of special observations taken during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) on forecasts of the 5 June 2009 Goshen County, Wyoming, supercell. The data used in these experiments are those from the Mobile Weather Radar, 2005 X-band, Phased Array (MWR-05XP); two mobile mesonets (MM); and several mobile sounding units. Data sources are divided into “routine,” including those from operational Weather Surveillance Radar-1988 Dopplers (WSR-88Ds) and the Automated Surface Observing System (ASOS) network, and “special” observations from the VORTEX2 project.

VORTEX2 data sources are denied individually from a total of six ensemble square root filter (EnSRF) data assimilation and forecasting experiments. The EnSRF data assimilation uses 40 ensemble members on a 1-km grid nested inside a 3-km grid. Each experiment assimilates data every 5 min for 1 h, followed by a 1-h forecast. All experiments are able to reproduce the basic evolution of the supercell, though the impact of the VORTEX2 observations was mixed. The VORTEX2 sounding data decreased the mesocyclone intensity in the latter stages of the forecast, consistent with observations. The MWR-05XP data increased the forecast vorticity above approximately 1 km AGL in all experiments and had little impact on forecast vorticity below 1 km AGL. The MM data had negative impacts on the intensity of the low-level mesocyclone, by decreasing the vertical vorticity and indirectly by decreasing the buoyancy of the inflow.

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