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Kevin Knupp

Abstract

The evolution of a gust front to bore to solitary wave transition, and comprehensive information on the evolving nocturnal boundary layer (NBL) associated with this change, are described with analysis of radar and profiler measurements. The observations were obtained on 21 June 2002 in the Oklahoma panhandle during the International H2O Project. The evolution of this system, from a strong bore (initiated by a vigorous gust front) to a solitary wave, was observed over a 4-h period with Doppler radar and surface measurements. Detailed information on the mature bore structure was obtained by a cluster of profiling instruments including two boundary layer wind profilers, a lidar ceilometer, and a microwave profiling radiometer.

A strong bore was initiated by an extensive gust front that perturbed an incipient NBL whose development (prior to sunset) was enhanced by shading from the parent mesoscale convective system. At the time of bore formation, the NBL was about 300 m deep and exhibited a surface temperature about 4 K less than the afternoon maximum. Initially, the bore assumed kinematic properties similar to those of a gust front. As the NBL stabilized, the bore matured and exhibited undular formations over 30–60-km segments along the bore axis. A 30-km-wide cloud field accompanied the mature bore system within three hours of its formation. System-relative airflow within the cloud field was front-to-rear and exhibited a primary hydraulic jump updraft (4–5 m s−1 magnitude) within the bore core. The bore core exhibited a low, smooth cloud base, a cloud depth of 2.5 km, nearly adiabatic liquid water content, and pronounced turbulence. The maximum parcel displacements within the bore were about 2 km (sufficient for marginal convective initiation), and the net parcel displacement from before to after bore passage was 0.6–0.9 km.

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Kevin R. Knupp

Abstract

This paper presents results from a comprehensive investigation in which observations from several case studies an integrated with three-dimensional cloud model results to examine the general kinematic structure of downdrafts associated with High Plains precipitating convection. One particular downdraft type, the low-level precipitation-associated downdraft, is the focus of this paper.

General airflow and trajectory patterns within low-level downdrafts are convergent from 0.8 km upwards to downdraft top, typically less than 5 km AGL. Observed mass flux profiles often increase rapidly with height as a result of strong buoyancy forcing below the melting level. Inflow to the low-level downdraft although vertically continuous, can be separated into two branches. The up-down branch originating within the planetary boundary layer initially rises up to 4 km and then descends within the main precipitation-associated downdraft. The midlevel branch, usually more pronounced during early downdraft stages, originates from above the PBL and transports low-valued θ e air to low levels.

The depth of the low-level downdraft can be approximated by the environmental sounding transition level, defined as the vertical height interval separating conditionally unstable to neutral air below from roughly stable air above. The low-level downdraft is most frequently located along the upshear storm flank relative to the updraft, although a downshear downdraft may become common under higher environmental wind shear.

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Kevin R. Knupp

Abstract

The dynamical and thermodynamical properties of precipitation-associated downdrafts are examined using a Lagrangian trajectory analysis approach applied to parcels passing through the low-level downdraft of precipitating convection. Both observations and three-dimensional cloud model results for one particular case presented in Part I are included. For this case, negative buoyancy within the low-level downdraft is very rapidly produced over the lowest 2 km by melting and evaporation of precipitation. Cooling profiles from the melting and evaporation cooling components show a significant overlap in the case considered. Both diagnosed and modeled low perturbation pressure located near the 2 km level appear to be generated by the rapid onset of negative buoyancy.

The diagnosed and modeled behavior along two low-level downdraft branches defined in Part I are examined for this particular case. It is found that the low-level downdraft is forced in varying amounts by condensate loading, negative buoyancy produced by precipitation evaporation and melting, and pressure forces. The relative role of cooling by evaporation and melting varies according to trajectory type. Along midlevel trajectories, evaporation is most significant, whereas along up–down trajectories melting is more important. For the particular case examined, the maximum amplitude in downward acceleration was found in the 1–2 km layer, where effects of loading, melting, evaporation/sublimation and negative vertical pressure gradient contributed to downward acceleration.

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Kevin R. Knupp
and
William R. Cotton

Abstract

The evolution of the turbulent structure of an intense, quasi-steady thunderstorm is examined using Doppler radar estimates of turbulent kinetic energy dissipation rates (ε) and radial shears of raw radial velocity (ΔV rR). A comparison of turbulent patterns with mean storm airflow is made.

Observations taken during the quasi-steady mature stage reveal that turbulent intensity and activity peaked at mid to upper storm levels. The primary storm updraft was nearly turbulence-free at low levels, but exhibited increasingly turbulent activity at higher levels indicating a transition from quasi-laminar flow to bubble-like flow. Comparison of ε and ΔV rR patterns with environmental parameters such as equivalent potential temperature and momentum suggests that buoyancy and wind shear acted together to generate turbulent eddies, some greater than 500 m in size, at middle storm levels. At mid to upper storm levels, patterns of ε and ΔV rR exhibited considerable spatial and temporal variability, and maximum estimated dissipation rate estimates approached 0.15 m2 s−3. During one particular time period, 11 local ε maxima were estimated, some with magnitudes exceeding 0.07 m2 s−3.

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Timothy A. Coleman
and
Kevin R. Knupp

Abstract

The “impedance relation” between the wind perturbation within an ageostrophic atmospheric disturbance and its pressure perturbation and intrinsic propagation speed has been in use for decades. The correlation between wind and pressure perturbation was established through this relation. However, a simple Lagrangian model of an air parcel traversing an idealized sinusoidal wave in the pressure field indicates that the impedance relation produces significant errors. Examination of the nonlinearized horizontal momentum equation with a sinusoidal disturbance in pressure reveals an additional nonlinear term in the impedance relation, not previously included.

In this paper, the impedance relation is rederived, with the solution being the original equation with the addition of the nonlinear term. The new equation is then evaluated against the Lagrangian model of an air parcel traversing an idealized gravity wave, as well as three observed cases. It is shown that the new impedance relation is indeed more accurate in predicting wind perturbations in disturbances based on pressure perturbations and intrinsic speed than the accepted equation. Implications for determination of the intrinsic phase speed of a disturbance when pressure and wind perturbations are known (another widely used application of the impedance relation) are also discussed.

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Kevin R. Knupp
,
Justin Walters
, and
Michael Biggerstaff

Abstract

Detailed observations of boundary layer structure were acquired on 14 September 2001, prior to and during the landfall of Tropical Storm Gabrielle. The Mobile Integrated Profiling System (MIPS) and the Shared Mobile Atmospheric Research and Teaching Radar (SMART-R) were collocated at the western Florida coastline near Venice, very close to the wind center at landfall. Prior to landfall, the boundary layer was rendered weakly stable by a long period of evaporational cooling and mesoscale downdrafts within extensive stratiform precipitation that started 18 h before landfall. The cool air mass was expansive, with an area within the 23°C surface isotherm of about 50 000 km2. East-northeasterly surface flow transported this cool air off the west coast of Florida, toward the convergent warm core of the Gabrielle, and promoted the development of shallow warm and cold fronts that were prominent during the landfall phase.

Airflow properties of the boundary layer around the coastal zone are examined using the MIPS and SMART-R data. Wind profiles exhibited considerable temporal variability throughout the period of observations. The stable offshore flow within stratiform precipitation exhibited a modest jet that descended from about 600 to 300 m within the 20-km zone centered on the coastline. In contrast, the onshore flow on the western side of the wind center produced a more turbulent boundary layer that exhibited a well-defined top varying between 400 and 1000 m MSL. The horizontal variability of each boundary layer is examined using high-resolution Doppler radar scans at locations up to 15 km on either side of the coastline, along the mean flow direction of the boundary layer. These analyses reveal that transitions in boundary layer structure for both the stable and unstable regimes were most substantial within 5 km of the coastline.

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Kevin R. Knupp
and
William R. Cotton

Abstract

An analysis of an intense, quasi-steady thunderstorm which developed over mountainous terrain is presented. This storm, extensively analyzed using multiple Doppler radar and surface mesonet data, formed within an environment having strong low-level wind shear. The evolution and characteristics of the mesoscale systems prior to storm formation are presented in Part I (Cotton et al., 1982). Such an environment was responsible for several unique storm features, including a quasi-steady primary updraft circulation and movement 50° to the left of the cloud layer (2–8 km AGL) environmental winds.

Several interactions were observed or inferred near and within the storm. Vertical transport of northerly low-level momentum within the updraft imparted a significant blocking on mid-level flow having southerly momentum. Such a blocking affected the movement and characteristics of adjacent, less organized storms. Additional storm-environment interactions produced an organized recirculation of precipitation particles from the mid-level updraft to the low-level updraft.

It is concluded that the steadiness of the storm depended on two factors: 1) the introduction of low-level flow which was directed opposite to mid-level flow, 2) formation of persistent downdrafts of sufficient magnitude to sustain an active gust front.

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Charles A. Knight
and
Kevin R. Knupp

Abstract

The growth trajectories of precipitation particles that attain diameters from 0.5 to 2.0 cm are modeled within the wind field of a small, relatively steady-state, southeastern Montana thunderstorm. The trajectories are calculated backwards, from systematic arrays of particles of specified sizes at a level near cloud base. Using a simple set of criteria for rejecting the obviously impossible trajectories, the patterns of accepted trajectory end-points are compared with the radar echo patterns. Good agreement lends credence to the qualitative aspects of the trajectories. For a given size of precipitation particle, the method helps one to assign different trajectory types to specific regions within the horizontal plant on which the calculations were started. The relative importance of the different types of trajectories can thus be estimated. Particle origin mechanisms are discussed in terms of the regions in which the trajectories are found to start. The variety of successful trajectories leading to 1 cm and larger hail in a storm of considerable structural simplicity is noteworthy.

Sensitivity tests indicate that the liquid water content is by far the most important specification in this framework. Ongoing work is directed toward improving this specification and deriving estimates of particle concentration.

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Robert M. Rauber
,
Joseph Wegman
,
David M. Plummer
,
Andrew A. Rosenow
,
Melissa Peterson
,
Greg M. McFarquhar
,
Brian F. Jewett
,
David Leon
,
Patrick S. Market
,
Kevin R. Knupp
,
Jason M. Keeler
, and
Steven M. Battaglia

Abstract

This paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral width within the temperature range from −10° to −20°C, while microphysical measurements showed the simultaneous presence of graupel, ice particles, and supercooled water at the same temperatures, together supporting noninductive charging as an important charging mechanism in these storms.

A climatology of lightning flashes across the comma head of 16 winter cyclones shows that lightning flashes commonly occur on the southern side of the comma head where dry-slot air is more likely to overrun lower-level moist air. Over 90% of the cloud-to-ground flashes had negative polarity, suggesting the cells were not strongly sheared aloft. About 55% of the flashes were associated with cloud-to-ground flashes while 45% were in-cloud flashes.

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