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John S. Kain, Michael E. Baldwin, and Steven J. Weiss

Abstract

Parameterized updraft mass flux, available as a unique predictive field from the Kain–Fritsch (KF) convective parameterization, is presented as a potentially valuable predictor of convective intensity. The KF scheme is described in some detail, focusing on a version that is currently being run semioperationally in an experimental version of the Eta Model. It is shown that updraft mass flux computed by this scheme is a function of the specific algorithm that it utilizes and is very sensitive to the thermodynamic characteristics of input soundings. These same characteristics appear to be related to the severity of convection, suggesting that updraft mass flux predicted by the KF scheme has value for predicting severe weather. This argument is supported by anecdotal evidence and a case study.

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Kimberly A. Hoogewind, Michael E. Baldwin, and Robert J. Trapp

Abstract

This study explores the potential impact anthropogenic climate change may have upon hazardous convective weather (HCW; i.e., tornadoes, large hail, and damaging wind gusts) in the United States. Utilizing the Weather Research and Forecasting (WRF) Model, high-resolution (4 km) dynamically downscaled simulations of the Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL CM3), are produced for a historical (1971–2000) and future (2071–2100) period. Synthetic HCW day climatologies are created using upward vertical velocity (UVV) exceeding 22 m s−1 as a proxy for HCW occurrence and subsequently compared to the environmental approach of estimating changes in daily frequency of convective environments favorable for HCW (NDSEV) from the driving climate model. Results from the WRF simulations demonstrate that the proxy for HCW becomes more frequent by the end of the twenty-first century, with the greatest absolute increases in daily frequency occurring during the spring and summer. Compared to NDSEV from GFDL CM3, both approaches suggest a longer HCW season, perhaps lengthening by more than a month. The change in environmental estimates are 2–4 times larger than that gauged from WRF; further analyses show that the conditional probability of HCW given NDSEV declines during summer for much of the central United States, a result that may be attributed to both an increase in the magnitude of convective inhibition (CIN) and decreased forcing for ascent, hindering convective initiation. Such an outcome supports the motivation for continued use of dynamical downscaling to overcome the limitations of the GCM-based environmental analysis.

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John S. Kain, Stephen M. Goss, and Michael E. Baldwin

Abstract

The process of atmospheric cooling due to melting precipitation is examined to evaluate its contribution to determining precipitation type. The “melting effect” is typically of second-order importance compared to other processes that influence the lower-tropospheric air temperature and hence the type of precipitation that reaches the ground. In some cases, however, cooling due to melting snowflakes can emerge as the dominant agent of temperature change, occasionally surprising forecasters (and the public) by inducing an unexpected changeover from rain to heavy snow. One such case occurred on 3–4 February 1998 in east-central Tennessee and surrounding areas.

Commonly applied considerations for predicting precipitation type had convinced forecasters that significant snowfall was not likely with this event. However, real-time observations and a postevent analysis by forecasters at the Storm Prediction Center led to the hypothesis that the melting effect must have provided the cooling necessary to allow widespread heavy snowfall. To test this hypothesis, the Pennsylvania State University–NCAR Mesoscale Model was used to generate a mesoscale-resolution, four-dimensional dataset for this event. Diagnostic analysis of the model output confirmed that cooling due to melting snowflakes was of a sufficient magnitude to account for the disparity between observed and forecasted lower-tropospheric temperatures in this case.

A simple formula is derived to provide a “rule of thumb” for anticipating the potential impact of the melting effect. In addition, guidelines are provided for identifying meteorological patterns that favor a predominance of the melting effect.

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Melissa S. Bukovsky, John S. Kain, and Michael E. Baldwin

Abstract

Bowing, propagating precipitation features that sometimes appear in NCEP's North American Mesoscale model (NAM; formerly called the Eta Model) forecasts are examined. These features are shown to be associated with an unusual convective heating profile generated by the Betts–Miller–Janjić convective parameterization in certain environments. A key component of this profile is a deep layer of cooling in the lower to middle troposphere. This strong cooling tendency induces circulations that favor expansion of parameterized convective activity into nearby grid columns, which can lead to growing, self-perpetuating mesoscale systems under certain conditions. The propagation characteristics of these systems are examined and three contributing mechanisms of propagation are identified. These include a mesoscale downdraft induced by the deep lower-to-middle tropospheric cooling, a convectively induced buoyancy bore, and a boundary layer cold pool that is indirectly produced by the convective scheme in this environment. Each of these mechanisms destabilizes the adjacent atmosphere and decreases convective inhibition in nearby grid columns, promoting new convective development, expansion, and propagation of the larger system. These systems appear to show a poor correspondence with observations of bow echoes on time and space scales that are relevant for regional weather prediction, but they may provide important clues about the propagation mechanisms of real convective systems.

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Eric D. Robinson, Robert J. Trapp, and Michael E. Baldwin

Abstract

Trends in severe thunderstorms and the associated phenomena of tornadoes, hail, and damaging winds have been difficult to determine because of the many uncertainties in the historical eyewitness-report-based record. The authors demonstrate how a synthetic record that is based on high-resolution numerical modeling may be immune to these uncertainties. Specifically, a synthetic record is produced through dynamical downscaling of global reanalysis data over the period of 1990–2009 for the months of April–June using the Weather Research and Forecasting model. An artificial neural network (ANN) is trained and then utilized to identify occurrences of severe thunderstorms in the model output. The model-downscaled precipitation was determined to have a high degree of correlation with precipitation observations. However, the model significantly overpredicted the amount of rainfall in many locations. The downscaling methodology and ANN generated a realistic temporal evolution of the geospatial severe-thunderstorm activity, with a geographical shift of the activity to the north and east as the warm season progresses. Regional time series of modeled severe-thunderstorm occurrences showed no significant trends over the 20-yr period of consideration, in contrast to trends seen in the observational record. Consistently, no significant trend was found over the same 20-yr period in the environmental conditions that support the development of severe thunderstorms.

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Kimberly L. Elmore, Michael E. Baldwin, and David M. Schultz

Abstract

The spatial structure of bias errors in numerical model output is valuable to both model developers and operational forecasters, especially if the field containing the structure itself has statistical significance in the face of naturally occurring spatial correlation. A semiparametric Monte Carlo method, along with a moving blocks bootstrap method is used to determine the field significance of spatial bias errors within spatially correlated error fields. This process can be completely automated, making it an attractive addition to the verification tools already in use. The process demonstrated here results in statistically significant spatial bias error fields at any arbitrary significance level.

To demonstrate the technique, 0000 and 1200 UTC runs of the operational Eta Model and the operational Eta Model using the Kain–Fritsch convective parameterization scheme are examined. The resulting fields for forecast errors for geopotential heights and winds at 850, 700, 500, and 250 hPa over a period of 14 months (26 January 2001–31 March 2002) are examined and compared using the verifying initial analysis. Specific examples are shown, and some plausible causes for the resulting significant bias errors are proposed.

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Qingyun Zhao, Thomas L. Black, and Michael E. Baldwin

Abstract

An explicit cloud prediction scheme has been developed and incorporated into the Eta Model at the National Centers for Environmental Prediction (NCEP) to improve the cloud and precipitation forecasts. In this scheme, the cloud liquid water and cloud ice are explicitly predicted by adding only one prognostic equation of cloud mixing ratio to the model. Precipitation of rain and snow in this scheme is diagnostically calculated from the predicted cloud fields. The model-predicted clouds are also used in the model’s radiation calculations. Results from the parallel tests performed at NCEP show improvements in precipitation forecasts when prognostic cloud water is included. Compared with the diagnostic clouds, the model-predicted clouds are more accurate in both amount and position. Improvements in specific humidity forecasts have also been found, especially near the surface and above the freezing level.

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Carl E. Hane, Michael E. Baldwin, Howard B. Bluestein, Todd M. Crawford, and Robert M. Rabin

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations.

The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels.

The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

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Carl E. Hane, Howard B. Bluestein, Todd M. Crawford, Michael E. Baldwin, and Robert M. Rabin

Abstract

Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations.

Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm initiation, extending southwestward into the dry air. Another prominent feature on this day was a mesoscale bulge along the dryline extending northeastward into southwest Kansas. Deep convection was initiated along this bulge, but was in general short-lived.

Potential causes of the lifting associated with the cloud line that was apparently key to the preferred location for storm development in northwest Oklahoma were investigated: (a) a mesoscale circulation resulting from horizontal differences in radiative (temperature) properties of the underlying surface and (b) upward motion induced by an upper-level mesoscale disturbance. Analysis of vegetative and surface temperature distributions from satellite observations suggests a potential (more research is needed) link between surface characteristics and the development of the dryline bulge and observed cloud line through horizontal differences in vertical momentum transport. A run of the currently operational eta model indicates some skill in predicting dryline location and motion and predicts upward motion in the northern part of the region that was generally more convectively active, but shows no indication of upper-level support in the vicinity of the observed cloud line.

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Carl E. Hane, Robert M. Rabin, Todd M. Crawford, Howard B. Bluestein, and Michael E. Baldwin

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas.

Aircraft measurements in the boundary layer reveal that both the east–west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures.

Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest.

Farther north there was a 70–100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night.

A severe thunderstorm complex that developed along the Kansas–Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.

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