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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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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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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

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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Lindsay Parker
,
R. M. Welch
, and
D. J. Musil

Abstract

Aircraft observations and high resolution Landsat Multispectral Scanner digital data are used to determine the sizes of spatial inhomogeneities (“holes”) in cumulus clouds. The majority of holes are found near cloud edges, but the larger holes tend to be found in cloud interiors. Aircraft measurements show these cloud spatial inhomogeneities in the range of 100 to 500 m, while Landsat data show them in the range of 100 m to 3 km.

The number of holes per cloud decreases exponentially with increasing hole diameter. Small clouds not only have smaller holes, but also fewer holes than large clouds. Large clouds have large holes in them, as well as large numbers of the smaller holes. The total cloud area occupied by holes increases with increasing cloud size.

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Wenshou Tian
,
Douglas J. Parker
, and
Charles A. D. Kilburn

Abstract

Radar and satellite images provide observations of convective rolls and other structures in the convective boundary layer (CBL), but the data are intermittent, and neither radar nor satellite gives a complete picture of roll circulation in the observed cases. As a consequence, numerical modeling is a useful complement to the observations, to investigate the temporal and spatial details of convective rolls. In this paper, observations of convective rolls over the south of England are described. Numerical simulations have been performed to investigate these rolls using the Boundary Layer Above Stationary Inhomogeneous Uneven Surfaces (BLASIUS) model, a relatively simple boundary layer code for flow over topography. The numerical results indicate that most of the features of the convective structures can be successfully reproduced, notably the roll orientation and spacing and the basic features of the cloud field. These features are in good agreement for two case studies, one with distinct rolls and the other with more dispersed convective structures and a time-dependent basic state. The model tends to predict the initial occurrence of rolls later than observed, and this time of occurrence is found to be influenced by model resolution.

The presence of low topography (with maximum height on the order of 30% of the CBL depth) may have a small influence on the average orientation and spacing, and the time of initial occurrence of modeled rolls. Local flow anomalies related to the hills are much more pronounced. These anomalies appear to be related to coherent patterns in the model cloud fields, with a tendency for more cloud cover upstream and over hills, and cloud clearing in the lee as a result of descent suppressing convective eddies. When the satellite imagery is combined with topography data, this kind of orographic control of the shallow convection by the topography is evident. The CBL height varies considerably in the early stages of CBL evolution over hilly topography, but when the convection is fully developed the CBL height is almost constant over the domain.

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Johannes M. L. Dahl
,
Matthew D. Parker
, and
Louis J. Wicker

Abstract

This study addresses the sensitivity of backward trajectories within simulated near-surface mesocyclones to the spatiotemporal resolution of the velocity field. These backward trajectories are compared to forward trajectories computed during run time within the numerical model. It is found that the population of backward trajectories becomes increasingly contaminated with “inflow trajectories” that owe their existence to spatiotemporal interpolation errors in time-varying and strongly curved, confluent flow. These erroneous inflow parcels may mistakenly be interpreted as a possible source of air for the near-surface vortex. It is hypothesized that, unlike forward trajectories, backward trajectories are especially susceptible to errors near the strongly confluent intensifying vortex. Although the results are based on model output, dual-Doppler analysis fields may be equally affected by such errors.

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Michael T. Kiefer
,
Matthew D. Parker
, and
Joseph J. Charney

Abstract

Fire lines are complex phenomena with a broad range of scales of cross-line dimension, undulations, and along-line variation in heating rates. While some earlier studies have examined parcel processes in two-dimensional simulations, the complexity of fire lines in nature motivates a study in which the impact of three-dimensional fire line details on parcel processes is examined systematically. This numerical modeling study aims to understand how fundamental processes identified in 2D simulations operate in 3D simulations where the fire line is neither straight nor uniform in intensity. The first step is to perform simulations in a 3D model, with no fire line undulations or inhomogeneity. In general, convective modes simulated in the 2D model are reproduced in the 3D model. In one particular case with strong vertical wind shear, new convection develops separate from the main line of convection as a result of local changes to parcel speed and heating. However, in general the processes in the 2D and 3D simulations are identical. The second step is to examine 3D experiments wherein fire line shape and along-line inhomogeneity are varied. Parcel heating, as well as convective mode, is shown to exhibit sensitivity to fire line shape and along-line inhomogeneity.

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A. N. Ross
,
A. M. Tompkins
, and
D. J. Parker

Abstract

Gravity-current models have been used for many years to describe the cold pools of low-level air that are generated by cumulonimbus precipitation. More recently, it has been realized that surface fluxes of heat and water vapor can be important in modifying these flows, through turbulent mixing of buoyancy by convection, and through direct modification of the cold pool buoyancy. In this paper, simple models describing the role of surface fluxes in depleting the negative buoyancy of a gravity current and the consequences of this for the flow dynamics are discussed.

It is pointed out that the depletion of cold pool buoyancy by surface fluxes is analogous to the depletion of buoyancy in a turbidity current through particle sedimentation, and in one regime of parameter values the analogy is exact. This analogy allows one to use simple flow models that have been tested extensively against laboratory experiments on turbidity currents. A simple “box model” and a more sophisticated shallow water model are each developed. It is shown how these models can give relatively simple expressions for cold pool “runout length” and buoyancy distributions. These runout lengths compare well with maximum cold pool sizes previously observed in cloud-resolving model simulations of unorganized tropical deep convection.

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Michael T. Kiefer
,
Matthew D. Parker
, and
Joseph J. Charney

Abstract

Wildfires are capable of inducing atmospheric circulations that result predominantly from large temperature anomalies produced by the fire. The fundamental dynamics through which a forest fire and the atmosphere interact to yield different convective regimes is still not well understood. This study uses the Advanced Regional Prediction System (ARPS) model to investigate the impact of the environmental (i.e., far upstream, undisturbed by fire) wind profile on dry convection above a prescribed heat source of an intensity and spatial scale comparable to a wildfire. Dimensional analysis of the fire–atmosphere problem provides two relevant parameters: a surface buoyancy parameter that addresses the amount of heat a parcel of air receives in transiting above the fire and an advection parameter that addresses the degree to which the environmental wind advects updrafts away from the fire. Two-dimensional simulations are performed in which the upstream surface wind speed and mixed layer mean wind speed are varied independently to better understand the fundamental processes governing the organizational mode and updraft strength.

The result of these experiments is the identification of two primary classes of dry convection: plume and multicell. Simulated plume cases exhibit weak advection by the mean wind and are subdivided into intense plume and hybrid classes based on the degree of steadiness within the convection column. Hybrid cases contain columns of largely discrete updrafts versus the more continuous updraft column associated with the intense plume mode. Multicell cases develop with strong mixed layer advection and are subdivided into strong and weak classes based on the depth of convection. Intense plume and strong multicell (hybrid and weak multicell) cases occur when the surface buoyancy is large (small). Parcel analyses are performed to more closely examine the forcing of convection within different areas of the parameter space. The multicell (strong and weak) and intense plume modes are forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the perturbation wind field, whereas the hybrid mode is forced by a combination of buoyancy and dynamic pressure gradient forcing associated with the strong background shear.

The paper concludes with a discussion of the degree of nonlinearity that is likely to exist at the fire front for each of the convective modes; nonlinear fire behavior is most likely for the hybrid mode and least likely for the weak multicell mode. Knowledge of the sensitivity of the convective mode to upstream conditions can provide information about the degree of nonlinear or erratic fire behavior expected for a given wind profile upstream of the fire.

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