Search Results

You are looking at 1 - 10 of 20 items for

  • Author or Editor: Jason C. Knievel x
  • All content x
Clear All Modify Search
Matthew D. Parker and Jason C. Knievel

Meteorologists and other weather enthusiasts sometimes lament that they live in weather holes—places that receive less exciting weather than do their surroundings . This belief seems to stem from countless hours spent gazing at thunderstorms on displays of radar reflectivity. To test objectively whether radar observations truly bear out this belief, the authors analyzed 6 yr of composite reflectivity from the Weather Surveillance Radar-1988 Doppler (WSR-88D) network. Statistics for 28 target cities, selected for their prominent meteorological communities, are compared with statistics for random points in the conterminous United States to see whether any of the targets is truly a weather hole or, perhaps, a hot spot—the counterpart to a hole. Holes and hot spots are defined by the frequency of convective echoes at a target relative to echoes in the surrounding region, and by the probability that convective echoes near a target were followed shortly by a convective echo at that target.

The data do, indeed, reveal mesoscale variability in occurrences of thunderstorms, as well as distinct signatures of storms' motion and the footprints of stormy regions at each target. However, although the data support the basic concept of convective weather holes and hot spots, only one of the meteorological targets fully met the authors' criteria for a hole and only one fully met their criteria for a hot spot. During the 6 yr studied, nearly all of the selected targets experienced convective storms about as often as their immediate surroundings did.

These results suggest that meteorologists are unnecessarily cranky about the frequency of storms in their hometowns. Meteorologists' belief that they live in weather holes may reveal the need to explore more deeply the statistical behavior of moist convection. The authors comment on some of the strengths and weaknesses of using composite reflectivity alone for that exploration and for determining weather holes and hot spots. Finally, the authors speculate that, with the proper quality control, statistics might serve in the near future as very powerful tools for probabilistic forecast guidance.

Full access
Jason C. Knievel and Richard H. Johnson

Abstract

The authors employ data from the NOAA Wind Profiler Network to present a scale-discriminating vorticity budget for a mesoscale convective vortex (MCV) that was generated by a mesoscale convective system (MCS) in Oklahoma and Kansas on 1 August 1996.

A spatial bandpass filter was used to divide observed wind into mesoscale and synoptic components. Then the authors sought sources and sinks of vorticity in those two components over 9 h of the MCV's lifetime.

The vorticity budget reveals that both the mesoscale and synoptic winds supplied significant vorticity to the MCV. The vortex's origin could not be proved, but data weakly suggest that tilting may have been mostly responsible. Convergence of absolute vorticity by the mesoscale wind was the reason the MCV grew deeper and stronger as the MCS weakened. Finally, tilting of synoptic and mesoscale vorticity by gradients in mesoscale vertical motion was responsible for a secondary deepening of the MCV as the MCS dissipated.

The budget suggests that, if the MCV of 1 August 1996 is representative, completely realistic simulations of MCVs should include planetary vorticity and a plausible, three-dimensionally heterogeneous synoptic wind.

Full access
Jason C. Knievel and Richard H. Johnson

Abstract

The authors present a unique, scale-discriminating study of the environment-relative circulations within a mesoscale convective system (MCS) and mesoscale convective vortex (MCV). The MCS, a leading convective line and trailing stratiform region that became asymmetric, passed through the National Oceanic and Atmospheric Administration (NOAA) Profiler Network (NPN) in Kansas and Oklahoma on 1 August 1996. The MCV appeared in the MCS's stratiform region just prior to the system's mature stage and grew to a depth of over 12 km as the MCS dissipated. The MCV did not apparently survive to the next day.

A spatial bandpass filter was used to divide observed wind into a component that was predominantly synoptic background wind and a component that was predominantly a mesoscale perturbation on that background wind.

A mesoscale updraft, mesoscale downdraft, and divergent outflows in the lower and upper troposphere were evident after the synoptic background wind was removed, so these four circulations were internal and fundamental to the MCS.

The mesoscale perturbation in wind in the middle troposphere extended farther behind the MCS than ahead of it, consistent with analytic studies and numerical simulations of gravity waves generated by heat sources characteristic of MCSs with leading convective lines and trailing stratiform regions.

Deepening of the MCV appeared to be reflected in the vertical wind shear at the vortex's center: as the MCV strengthened, the mesoscale shear through its lower part decreased, perhaps as wind became more vortical at increasing altitudes. Mesoscale and synoptic vertical shears were of similar magnitude, so an average of environmental soundings outside an MCS probably does not accurately represent the shear that affects an MCV. This suggests the need to reevaluate how the kinematical settings of MCVs are diagnosed.

Full access
Todd P. Lane and Jason C. Knievel

Abstract

Over the past decade, numerous numerical modeling studies have shown that deep convective clouds can produce gravity waves that induce a significant vertical flux of horizontal momentum. Such studies used models with horizontal grid spacings of O(1 km) and produced strong gravity waves with horizontal wavelengths greater than about 20 km. This paper is an examination of how simulated gravity waves and their momentum flux are sensitive to model resolution. It is shown that increases in horizontal resolution produce more power in waves with shorter horizontal wavelengths. This change in the gravity waves’ spectra influences their vertical propagation. In some cases, gravity waves that were vertically propagating in coarse simulations become vertically trapped in fine simulations, which strongly influences the vertical flux of horizontal momentum.

Full access
Jason C. Knievel and Richard H. Johnson

Abstract

By animating enhanced coarse surface pressure observations of 12 1985 Preliminary Regional Experiment for Storm-Scale Operational Research Meteorology (PRE-STORM) mesoscale convective systems (MCSs) the authors exposed 92 transitory highs and lows living within virtually all of the systems’ mesohighs and wake lows. A quasi-Lagrangian (feature following, not material following) analysis of the pressure fields produced five primary results.

First, these transients, with magnitudes of a few millibars, horizontal dimensions of order 100 km, and average lifetimes of about 2 h, collectively composed spatial and temporal envelopes that contributed at least part of the total pressure field within mesohighs and wake lows. Transients did not apparently favor formation or dissipation in any location of the envelopes. Second, as the MCSs matured, the difference between each complex’s transitory highs’ mean pressure and transitory lows’ mean pressure increased in 78% of the conclusive cases. Apparently, one frequent role of MCSs is locally to magnify storm-scale pressure gradients. Third, transient paths reflect the frequent symmetric-to-asymmetric metamorphoses of the MCSs. Fourth, the temporal fluctuations of the numbers and apparent sizes of transients within a composite MCS partially support theories of MCS upscale evolution. Finally, the composite’s transient numbers and apparent sizes varied almost identically with time in a pattern that closely resembles the fluctuation of stratiform and convective volumetric rain rates of MCSs.

Full access
Eric A. Hendricks, Jason C. Knievel, and Yi Wang

Abstract

The multilayer urban canopy models (UCMs) building effect parameterization (BEP) and BEP + building energy model (BEM; a building energy model integrated in BEP) are added to the Yonsei University (YSU) planetary boundary layer (PBL) parameterization in the Weather Research and Forecasting (WRF) Model. The additions allow for the first analysis of the detailed effects of buildings on the urban boundary layer in a nonlocal closure scheme. The modified YSU PBL parameterization is compared with the other 1.5-order local PBL parameterizations that predict turbulent kinetic energy (TKE), Mellor–Yamada–Janjić and Bougeault–Lacarerre, using both ideal and real cases. The ideal-case evaluation confirms that BEP and BEP+BEM produce the expected results in the YSU PBL parameterization because the simulations are qualitatively similar to the TKE-based PBL parameterizations in which the multilayer UCMs have long existed. The modified YSU PBL parameterization is further evaluated for a real case. Similar to the ideal case, there are larger differences among the different UCMs (simple bulk scheme, BEP, and BEP+BEM) than across the PBL parameterizations when the UCM is held fixed. Based on evaluation against urban near-surface wind and temperature observations for this case, the BEP and BEP+BEM simulations are superior to the simple bulk scheme for each PBL parameterization.

Restricted access
Jason C. Knievel, George H. Bryan, and Joshua P. Hacker

Abstract

Diffusion that is implicit in the odd-ordered advection schemes in early versions of the Advanced Research core of the Weather Research and Forecasting (WRF) model is sometimes insufficient to remove noise from kinematical fields. The problem is worst when grid-relative wind speeds are low and when stratification is nearly neutral or unstable, such as in weakly forced daytime boundary layers, where noise can grow until it competes with the physical phenomena being simulated. One solution to this problem is an explicit, sixth-order numerical diffusion scheme that preserves the WRF model’s high effective resolution and uses a flux limiter to ensure monotonicity. The scheme, and how it was added to the WRF model, are explained. The scheme is then demonstrated in an idealized framework and in simulations of salt breezes and lake breezes in northwestern Utah.

Full access
Daran L. Rife, Christopher A. Davis, and Jason C. Knievel

Abstract

The study describes a method of evaluating numerical weather prediction models by comparing the characteristics of temporal changes in simulated and observed 10-m (AGL) winds. The method is demonstrated on a 1-yr collection of 1-day simulations by the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) over southern New Mexico. Temporal objects, or wind events, are defined at the observation locations and at each grid point in the model domain as vector wind changes over 2 h. Changes above the uppermost quartile of the distributions in the observations and simulations are empirically classified as significant; their attributes are analyzed and interpreted.

It is demonstrated that the model can discriminate between large and modest wind changes on a pointwise basis, suggesting that many forecast events have an observational counterpart. Spatial clusters of significant wind events are highly continuous in space and time. Such continuity suggests that displaying maps of surface wind changes with high temporal resolution can alert forecasters to the occurrence of important phenomena. Documented systematic errors in the amplitude, direction, and timing of wind events will allow forecasters to mentally adjust for biases in features forecast by the model.

Full access
George H. Bryan, Jason C. Knievel, and Matthew D. Parker

Abstract

The authors evaluate whether the structure and intensity of simulated squall lines can be explained by “RKW theory,” which most specifically addresses how density currents evolve in sheared environments. In contrast to earlier studies, this study compares output from four numerical models, rather than from just one. All of the authors’ simulations support the qualitative application of RKW theory, whereby squall-line structure is primarily governed by two effects: the intensity of the squall line’s surface-based cold pool, and the low- to midlevel environmental vertical wind shear. The simulations using newly developed models generally support the theory’s quantitative application, whereby an optimal state for system structure also optimizes system intensity. However, there are significant systematic differences between the newer numerical models and the older model that was originally used to develop RKW theory. Two systematic differences are analyzed in detail, and causes for these differences are proposed.

Full access
Jason C. Knievel, David S. Nolan, and James P. Kossin

Abstract

The authors examine the degree of hydrostatic and gradient balances in a mesoscale convective vortex (MCV) in the stratiform region of a mesoscale convective system (MCS) that crossed Oklahoma on 1 August 1996. Results indicate that the MCV was partially unbalanced because the cool layer at the base of its core was too cool and too shallow to balance the tangential winds about the MCV's axis. The apparent imbalance may have been due to strong, unsteady forcing on the vortex; insufficient or unrepresentative data; approximations used in the analysis; or reasons that are unknown.

Full access