Kinematics and thermodynamics of a midlatitude, continental mesoscale convective system and its mesoscale vortex

Also issued as author's dissertation (Ph.D.) -- Colorado State University, 2001.Includes bibliographical references.The author examines a mesoscale convective system (MCS) and the mesoscale convective vortex (MCV) it generated. The MCS, which comprised a leading convective line and trailing stratiform region, traversed Kansas and Oklahoma on 1 August 1996, passing through the NOAA Wind Profiler Network, as well as four sites from which soundings were being taken every three hours during a field project. The unusually rich data set permitted study of the MCS and MCV over nine hours on scales between those of operational rawinsondes and Doppler radars. The author used a spatial bandpass filter to divide observed wind into synoptic and mesoscale components. The environment-relative, mesoscale wind contained an up- and downdraft and divergent outflows in the lower and upper troposphere. The mesoscale wind was asymmetric about the MCS, consistent with studies of gravity waves generated by heating typical of that in many MCSs. According to a scale-discriminating vorticity budget, both the synoptic and mesoscale winds contributed to the prominent resolved sources of vorticity in the MCV: tilting and convergence. Unresolved sources were also large. The author speculates that an abrupt change in the main source of vorticity in an MCV may appear as an abrupt change in its altitude of maximum vorticity. Distributions of temperature and humidity in the MCS were consistent with its mesoscale circulations. In the terminus of the mesoscale downdraft, advection of drier, potentially warmer air exceeded humidifying and cooling from rain, so profiles of temperature and dewpoint exhibit onion and double-onion patterns. The mesoscale updraft was approximately saturated with a moist adiabatic lapse rate. Mesoscale drafts. and convective drafts vertically mixed the troposphere, partially homogenizing equivalent potential temperature. The MCV contained a column of high potential vorticity in the middle troposphere, with a cold core below the freezing level and a warm core above-a pattern characteristic of profiles of heating by stratiform regions. The cold core was 2 km too shall w to be in pure gradient balance with wind in the MCV. Ongoing forcing during the observed lifetime of the MCV may have prevented it from achieving balance, even if that was its tendency.Sponsored by the National Science Foundation under grants ATM-0071371 and ATM-9618684; and NASA grant NCCS-288

Surface pressure transients in mesoscale convective systems

Spring 1996.Also issued as author's thesis (M.S.) -- Colorado State University, 1996.Includes bibliographical references.For decades meteorologists have observed that mesoscale convective systems (MCSs) increase surface pressure beneath and immediately behind their leading cumulonimbi ( the mesohigh) and reduce surface pressure at the rear edge of their anvils (the wake low). By enhancing coarse surface pressure observations of 12 PRE-STORM MCSs, I exposed transitory highs and lows living within mesohighs and wake lows. I propose that these transients are the more elemental MCS surface pressure perturbations; mesohighs and wake lows are merely temporal and spatial envelopes of transients. Moreover, existing theories of mesohigh and wake low origins readily apply to the ephemeral transients. A quasi-Lagrangian analysis of 92 transients produced five primary results. First, 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. Second, there is no clear evidence that transitory highs consistently strengthened before their partner transitory lows. Third, transient paths reflect MCSs' occasional. symmetric-to-asymmetric metamorphoses. Fourth, composites of the time-evolution of the numbers and apparent sizes of transients partially support theories of MCS upscale evolution. Fifth, composite transient numbers and apparent sizes vary almost identically with time in a pattern that closely resembles the fluctuation of stratiform and convective volumetric rain rates of MCSs studied by McAnelly and Cotton (1992).Sponsored by the National Science Foundation ATM-9313716

DO METEOROLOGISTS SUPPRESS THUNDERSTORMS? Radar-Derived Statics and the Behavior of Moist Convection

Most meteorologists are acquainted with the no- tion of a weather hole—that is, a place that receives less exciting weather than does its surroundings. Exciting weather takes many forms, but when people use the term weather hole, they tend to mean a place that thunderstorms often barely miss, or near which approaching storms often dissipate. For this paper, that is the meaning we adopt. In our experience, many meteorologists and lay weather enthusiasts genuinely believe that they live in weather holes, and this belief, almost without fail, seems to stem from countless hours spent gazing at displays of radar reflectivity. We have generally presumed that such people simply relish thunderstorms, are memorably disappointed whenever storms miss them, and erroneously conclude that their locations are subject to some kind of meteorologic disfavor. The recent availability of multiple years\u27 worth of national radar composites from the Weather Surveillance Radar-1988 Doppler (WSR-88D) network makes it possible to address objectively, if not definitively, whether meteorologists appear to live in weather holes and whether such an appearance is physical or artificial

DO METEOROLOGISTS SUPPRESS THUNDERSTORMS? Radar-Derived Statistics and the Behavior of Moist Convection

Most meteorologists are acquainted with the notion of a weather hole—that is, a place that receives less exciting weather than does its surroundings. Exciting weather takes many forms, but when people use the term weather hole, they tend to mean a place that thunderstorms often barely miss, or near which approaching storms often dissipate. For this paper, that is the meaning we adopt. In our experience, many meteorologists and lay weather enthusiasts genuinely believe that they live in weather holes, and this belief, almost without fail, seems to stem from countless hours spent gazing at displays of radar reflectivity. We have generally pre- sumed that such people simply relish thunderstorms, are memorably disappointed whenever storms miss them, and erroneously conclude that their locations are subject to some kind of meteorologic disfavor. The recent availability of multiple years\u27 worth of national radar composites from the Weather Surveillance Radar-1988 Doppler (WSR-88D) network makes it possible to address objectively, if not definitively, whether meteorologists appear to live in weather holes and whether such an appearance is physical or artificial

Evaluation of Urban Canopy Models against Near-Surface Measurements in Houston during a Strong Frontal Passage

Urban canopy models (UCMs) in mesoscale numerical weather prediction models need evaluation to understand biases in urban environments under a range of conditions. The authors evaluate a new drag formula in the Weather Research and Forecasting (WRF) model’s multilayer UCM, the Building Effect Parameterization combined with the Building Energy Model (BEP+BEM), against both in-situ measurements in the urban environment as well as simulations with a simple bulk scheme and BEP+BEM using the old drag formula. The new drag formula varies with building packing density, while the old drag formula is constant. The case study is a strong cold frontal passage that occurred in Houston during the winter of 2017, causing high winds. It is found that both BEP+BEM simulations have lower peak wind speeds, consistent with near-surface measurements, while the bulk simulation has winds that are too strong. The constant-drag BEP+BEM simulation has a near-zero wind speed bias, while the new-drag simulation has a negative bias. Although the focus is on the impact of drag on the urban wind speeds, both BEP+BEM simulations have larger negative biases in the near-surface temperature than the bulk-scheme simulation. Reasons for the different performances are discussed

Evaluation of Boundary Layer and Urban Canopy Parameterizations for Simulating Wind in Miami during Hurricane Irma (2017)

The simulated winds within the urban canopy of landfalling tropical cyclones are sensitive to the representation of the planetary boundary and urban canopy layers in numerical weather prediction models. To assess the subgrid-scale parameterizations of these layers, mesoscale model simulations were executed and evaluated against near-surface observations as the outer wind field of Hurricane Irma (2017) interacted with the built-up region from downtown Miami northward to West Palm Beach. Four model simulations were examined, comprising two different planetary boundary layer (PBL) parameterizations (a local closure scheme with turbulent kinetic energy prediction and a nonlocal closure scheme) and two different urban canopy models (UCMs) [a zeroth-order bulk scheme and a multilayer building effect parameterization (BEP) that mimics the three-dimensionality of buildings]. Overall, the simulated urban canopy winds were weakly sensitive to the PBL scheme and strongly sensitive to the UCM. The bulk simulations compared most favorably to an analyzed wind swath in the urban environment, while the BEP simulations had larger negative biases in the same region. There is uncertainty in magnitude of the urban environment biases due to the lack of many urban sheltered measurements in the wind swath analysis. Biases in the rural environment were similar among the bulk and BEP simulations. An improved comparison with the analyzed wind swath in the urban region was obtained by reducing the drag coefficient in BEP in one of the PBL schemes. The usefulness of BEP was demonstrated in its ability to predict realistic heterogeneous near-surface velocity patterns in urban regions

The Influence of Terrain Smoothing on Simulated Convective Boundary-Layer Depths in Mountainous Terrain

Many applications rely on a correct estimation of the convective boundary layer (CBL) depth over mountainous terrain, but often these applications use numerical model simulations. Although models inevitably smooth terrain, the amount of smoothing depends on grid spacing. We investigate the behavior of the CBL in coarse- and fine-grid models applied to mountainous terrain by using output from an operational mesoscale modeling system and by performing quasi-idealized simulations. We investigate different areas in different climate zones using different CBL top derivation methods, grid spacing ratios, planetary boundary layer (PBL) schemes, and terrain smoothing. We find that when compared to fine-grid simulations, CBL depths are systematically larger in coarse domains over mountaintops, and to a lesser extent in valleys. On average, differences between coarse- and fine-domains over mountaintops could reach around 10%. In certain locations, differences could be as high as 25%. We attribute the result to terrain smoothing. Similarly, when using a coarse-grid CBL height (relative to mean sea level) interpolated using fine-grid terrain information, there is good agreement with fine-grid CBL depths over mountaintops and less agreement in valleys. Our results have implications for applications that use output from coarse model grids in mountainous terrain. These include inverse modeling studies (e.g., greenhouse gas budget estimations or integrated water vapor transport), PBL evaluation studies, climate research, air quality applications, planning and executing prescribed burns, and studies associated with precipitation over mountainous terrain

Evaluations of WRF Sensitivities in Surface Simulations with an Ensemble Prediction System

This paper investigates the sensitivities of the Weather Research and Forecasting (WRF) model simulations to different parameterization schemes (atmospheric boundary layer, microphysics, cumulus, longwave and shortwave radiations and other model configuration parameters) on a domain centered over the inter-mountain western United States (U.S.). Sensitivities are evaluated through a multi-model, multi-physics and multi-perturbation operational ensemble system based on the real-time four-dimensional data assimilation (RTFDDA) forecasting scheme, which was developed at the National Center for Atmospheric Research (NCAR) in the United States. The modeling system has three nested domains with horizontal grid intervals of 30 km, 10 km and 3.3 km. Each member of the ensemble system is treated as one of 48 sensitivity experiments. Validation with station observations is done with simulations on a 3.3-km domain from a cold period (January) and a warm period (July). Analyses and forecasts were run every 6 h during one week in each period. Performance metrics, calculated station-by-station and as a grid-wide average, are the bias, root mean square error (RMSE), mean absolute error (MAE), normalized standard deviation and the correlation between the observation and model. Across all members, the 2-m temperature has domain-average biases of −1.5–0.8 K; the 2-m specific humidity has biases from −0.5–−0.05 g/kg; and the 10-m wind speed and wind direction have biases from 0.2–1.18 m/s and −0.5–4 degrees, respectively. Surface temperature is most sensitive to the microphysics and atmospheric boundary layer schemes, which can also produce significant differences in surface wind speed and direction. All examined variables are sensitive to data assimilation