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David J. Stensrud

Abstract

The ability of deep monsoon convection to influence the larger-scale circulation over North America is investigated for a 6-day-long case study during the 2006 North American monsoon. Results from Rossby wave ray tracing and numerical simulations using the Advanced Research Weather Research and Forecasting model indicate that North American monsoon convection provides a source region for stationary Rossby waves. Two wave trains are seen in the numerical model simulations, with behaviors that agree well with expectations from theory and ray tracing. The shorter and faster-moving wave train moves eastward from the source region in Mexico and reaches the western Atlantic within 4 days. The longer and slower-moving wave train travels northeastward and reaches the coastal New England region within 6 days. An upstream tail of anticyclonic vorticity extends westward from the source region into the central Pacific Ocean.

The monsoon convection appears to help cut off the low-level anticyclonic flow by developing low-level southerly flow in the Gulf of Mexico and northerly flow in the eastern Pacific, as suggested in earlier global model studies. However, the stationary Rossby wave trains further alter the location and intensity of deep convection in locations remote from the monsoon. These results suggest that unless a numerical model can correctly predict monsoon convection, the ability of the model to produce accurate forecasts of the large-scale pattern and associated convective activity beyond a few days is in question. This result may be important for global climate modeling, since an inaccurate prediction of monsoon convection would lead to an inaccurate Rossby wave response.

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David J. Stensrud

Abstract

Over a 2½-day period beginning 0000 UTC 11 May 1982, 15 mesoscale convective systems (MCSs) developed and moved eastward across the moist axis located over the southern plains of the United States. While the 6–18-h lifetimes of each of these individual MCSs are not sufficiently long to influence the large-scale environment greatly, it is possible that the cumulative effects of the entire group of MCSs can produce significant changes in the large-scale flow patterns. This hypothesis is investigated using output from two runs of a sophisticated mesoscale model. One run includes the effects of convection, and the other does not. Results indicate that in low levels, the inflow of warm, moist air into the convective region is increased when convection is allowed in the model, enhancing the likelihood that convection will continue and thereby acting as a positive feedback mechanism. In upper levels, the convection acts as a Rossby wave source region and produces significant upper-level perturbations that cover at least 50° longitude spread. Convective effects also influence cyclogenesis since the MCSs strengthen the low-level baroclinicity and modify the phase relationship between pressure and thermal waves in the midlevels. Thus, it is clear that the effects of a persistent, mesoscale region of convection on the large-scale environment are substantial.

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David J. Stensrud

Abstract

Elevated mixed layers (EMLs) are an important factor in the development of springtime thunderstorms over the United States. EMLs can be considered a subset of a larger class, called residual layers, since the mean state variables are the same, at least initially, as those of the boundary layers in which EMLs a formed. It is possible, however, for boundary or residual layers that are not necessarily well mixed to be advected off regions of elevated terrain and overrun boundary layers forming over lower terrain. These layers are called elevated residual layers (ERLs); ERLs may form frequently in regions near mountains where terrain gradients exist. A simple slab mixed-layer model is used to examine how idealized ERL potential temperature profiles influence surface boundary-layer development. In addition, several regionally generated ERLs were observed over Phoenix, Arizona, during the Southwest Area Monsoon Project. These ERLs appear to have produced a change from moistening to entrainment-drying surface boundary-layer regimes.

The thermodynamic structure of an ERL is determined by the processes that form the boundary layer, the timing and vertical extent of boundary-layer detachment from the elevated terrain relative to the diurnal heating cycle, and the vertical motion field (if any) accompanying the horizontal advection of the ERL away from the elevated terrain. Results suggest that the creation and evolution of ERLs may be important aspects of surface boundary-layer development in regions near and downstream of elevated terrain.

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David J. Stensrud

Abstract

Low-level jets (LLJs) occur frequently in many parts of the world. These low-level wind speed maxima are important for both the horizontal and vertical fluxes of temperature and moisture and have been found to be associated with the development and evolution of deep convection. Since deep convective activity produces a significant amount of upper-level cloudiness and is responsible for a large fraction of the warm season rainfall in the United States, the relationship between LLJs and deep convection suggests that LLJs are important contributors to regional climate. Results from a number of past studies are reviewed, and the potential for data from the Atmospheric Radiation Measurement program to augment our understanding of low-level jets is discussed.

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David J. Stensrud
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David J. Stensrud
and
J. Michael Fritsch

Abstract

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of five mesoscale convective systems (MCSs) developed within a weakly forced large-scale environment. Analyses indicate that the large-scale flow created a broad region of potential buoyant energy, but owing to a restraining inversion and weak large-scale upward motion, convection was initiated only where lifting associated with mesoscale features was able to eliminate the inversion. The series of MCSs developed sequentially and moved eastward across the moist axis. Two of these systems had a large component of motion against the mean tropospheric flow and propagated in a direction nearly opposite to that of the traveling upper-level disturbances. Each system produced an outflow of cold downdraft air that spread progressively farther south than that from the preceding system. This description of the development and evolution of convection is very different from traditional ones wherein convection develops and moves more or less in phase with traveling upper-level disturbances.

Simple analytic models are used to determine the likely mechanisms of upstream propagation. These results suggest that the combined effects of both density currents and internal gravity waves produce the upstream propagation of the region of convection. Density currents dominate the propagation of convection once it forms, while internal gravity waves may help initiate new convection upstream of the region of existing convection, thereby producing a jump in the region of convective activity.

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David J. Stensrud
and
J. Michael Fritsch

Abstract

A series of five mesoscale convective systems (MCSs) developed within a weakly forced large-scale environment on 11 and 12 May 1982. Two of these systems had a large component of motion against the midtroposphoric flow and propagated in a direction nearly opposite to that of the traveling upper-level disturbances. This description of the evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper-level disturbances. Observations indicate that the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional observing network, making the structure of the model initial condition a potentially crucial factor in the success or failure of any subsequent numerical simulation.

It is found that the initial conditions created using the conventional initialization procedure of The Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model do not include several of the mesoscale-sized features observed at 1200 UTC 11 May 1982, 9 h before the development of the first MCS. This is attributed to the lack of observed data with mesoscale resolution, and, therefore, likely is a deficiency in most initialization procedures in use today. Although it is true that new operational observing systems, such as the WSR-88D radar and the 404-MHz radar wind profilers, provide more detailed information, the data density on the mesoscale remains subcritical. A methodology to include mesoscale features, based upon using subjective interpretations of all the available observations, is developed. 11 is found that the mesoscale initial condition created using this subjective approach produces a more reasonable representation of the observed mesoscale features in comparison with the conventionally produced initial condition.

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David J. Stensrud
and
J. Michael Fritsch

Abstract

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of mesoscale convective systems developed within a weakly forced large-scale environment. Two of these systems had a large component of motion against the midtropospheric flow and propagated in a direction nearly opposite to that of the travelling upper-level disturbances. This evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper-level disturbances. It presents a tremendous challenge for three-dimensional numerical models, since the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional upper-air network and may not be well approximated in the model parameterization schemes.

Mesoscale model simulations are conducted to evaluate the ability of the model to reproduce this complex event and to examine the model sensitivities to differences in the convective trigger function and model initial condition. Results suggest deal mesoscale models may be capable of producing useful simulations of convective events associated with weak, large-scale forcing, including quantitative precipitation forecasts with the correct magnitude and approximate location of heavy rainfall. If the important mesoscale circulations are incorporated into the model initial condition and a sufficiently realistic trigger function is used. However, model sensitivities to both the initial condition and the convective trigger function are large. Results indicate that the effects of boundary layer forcing must be included in the trigger function in order to initiate convection at the proper time and location. Timing errors in the initial development of convection of greater than 4 h occur if an unpresentative trigger function is used. Mesoscale features in the model initial condition also play an important role in the development and evolution of convection. The locations of heavy rainfall are shifted by greater than 100 km, or disappear altogether, if particular mesoscale features are not included subjectively in the initial condition. These sensitivities suggest that using an ensemble forecast approach to mesoscale model output needs to be considered seriously as mesoscale models move into the operational forecasting environment.

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David J. Stensrud
and
Steven J. Weiss

Abstract

A six-member ensemble is developed in which the ensemble members only vary in their model physical process parameterization schemes. This approach is accomplished by mixing three different convective parameterization schemes with two different planetary boundary layer schemes within the nonhydrostatic Pennsylvania State University–National Center for Atmospheric Research Fifth-Generation Mesoscale Model (MM5). The initial and boundary conditions for each ensemble member are identical and are provided by the National Centers for Environmental Prediction Eta Model forecasts starting from 0000 UTC. Verification of the ensemble predictions against Eta Model analyses over 42 days indicates that, although this ensemble system is underdispersive and imperfect, the ensemble forecasts show some skill in predicting the probability of various severe-weather parameters exceeding selected threshold values. This model physics ensemble allows us to begin exploring the possible uses of ensemble forecasts for severe-weather events. Results from this six-member ensemble forecasting system of the 3 May 1999 tornado outbreak indicate that the ensemble provides a strong signal of two mesoscale-sized regions, one in Oklahoma and Kansas and the other in eastern Nebraska, that have the potential for supporting tornadic supercell thunderstorms. Several of the model forecasts also produce convection in these regions. Tornadic thunderstorm reports are found in both of these areas. This ensemble guidance does not provide any clues as to why the tornadoes in Oklahoma and Kansas were so severe, as compared with those in Nebraska, but it does provide hope that ensembles may be useful for short-range forecasting of severe weather.

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Larissa J. Reames
and
David J. Stensrud

Abstract

The world’s population is increasingly concentrated in large urban areas. Many observational and modeling studies have explored how these large, population-dense cities modify local and mesoscale atmospheric phenomena. These modeling studies often use an urban canopy model to parameterize urban surfaces. However, it is unclear whether this approach is appropriate for more suburban cities, such as those found in the Great Plains. Thus, the Weather Research and Forecasting Model was run for a week over Oklahoma City, Oklahoma, and results were compared with observations. Overall, four configurations were examined. Two simulations used the Noah LSM, one with all urban areas removed (CTRL), and the other with urban areas parameterized by a modified Noah land surface model with three urban categories (LSMMOD). Additional simulations utilized a single-layer urban canopy model (SLUCM) either with default urban fraction values (SLUCM1) or with urban fractions taken from the National Land Cover Database (SLUCM2). Results from the three urban runs compared favorably to high-density temperature observations of the urban heat island. The SLUCM1 run was the most realistic, although the urban fractions applied were the least representative of Oklahoma City. All urban runs also produced a drier and deeper planetary boundary layer over the city. The prediction of near-surface winds was most problematic, with the two SLUCM runs unable to correctly reproduce reduced wind speeds over the city. The modified Noah LSM provided best overall agreement with observations and represents a reasonable option for simulating the urban effects of more-suburban cities.

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