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Joseph A. Grim and James O. Pinto

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This study demonstrates a method of temporally and spatially scaling precipitation rates at low probability of precipitation-rate exceedance levels (e.g., 0.1%) from coarser-resolution global datasets to near-instantaneous localized rain gauge precipitation rates. In particular, the 8-km-, 1-h-resolution Climate Prediction Center Morphing (CMORPH) dataset was scaled to 1-min localized rates using the Automated Surface Observing Station (ASOS) rain gauge data. Maps of these scaled precipitation rates show overall patterns and magnitudes that are nearly identical to the lower-spatial-resolution rain gauge maps yet retain the much higher resolution of the original remotely sensed global dataset, which is particularly important over regions of complex geography and sparse surface observing stations. To scale the CMORPH data, temporal and spatial conversion factor arrays were calculated by comparing precipitation rates at different temporal (ASOS 1-min and 1-h) and spatial (ASOS 1-h and CMORPH 1-h) resolutions. These temporal and spatial conversion factors were found to vary by probability level, season, and climatological region. Meteorological implications of these variations are discussed.

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Joseph A. Grim, Neil F. Laird, and David A. R. Kristovich
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Joseph A. Grim, Jason C. Knievel, and Erik T. Crosman

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This study describes a stepwise methodology used to provide daily high-spatial-resolution water surface temperatures from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data for use nearly in real time for the Great Salt Lake (GSL). Land surface temperature (LST) is obtained each day and then goes through the following series of steps: land masking, quality control based on other concurrent datasets, bias correction, quality control based on LSTs from recent overpasses, temporal compositing, spatial hole filling, and spatial smoothing. Although the techniques described herein were calibrated for use on the GSL, they can also be applied to any other inland body of water large enough to be resolved by MODIS, as long as several months of in situ water temperature observations are available for calibration. For each of the buoy verification datasets, these techniques resulted in mean absolute errors for the final MODIS product that were at least 62% more accurate than those from the operational Real-Time Global analysis. The MODIS product provides realistic cross-lake temperature gradients that are representative of those directly observed from individual MODIS overpasses and is also able to replicate the temporal oscillations seen in the buoy datasets over periods of a few days or more. The increased accuracy, representative temperature gradients, and ability to resolve temperature changes over periods down to a few days can be vital for providing proper surface boundary conditions for input into numerical weather models.

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James O. Pinto, Joseph A. Grim, and Matthias Steiner

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An object-based verification technique that keys off the radar-retrieved vertically integrated liquid (VIL) is used to evaluate how well the High-Resolution Rapid Refresh (HRRR) predicted mesoscale convective systems (MCSs) in 2012 and 2013. It is found that the modeled radar VIL values are roughly 50% lower than observed. This mean bias is accounted for by reducing the radar VIL threshold used to identify MCSs in the HRRR. This allows for a more fair evaluation of the model’s skill at predicting MCSs. Using an optimized VIL threshold for each summer, it is found that the HRRR reproduces the first (i.e., counts) and second moments (i.e., size distribution) of the observed MCS size distribution averaged over the eastern United States, as well as their aspect ratio, orientation, and diurnal variations. Despite threshold optimization, the HRRR tended to predict too many (few) MCSs at lead times less (greater) than 4 h because of lead time–dependent biases in the modeled radar VIL. The HRRR predicted too many MCSs over the Great Plains and too few MCSs over the southeastern United States during the day. These biases are related to the model’s tendency to initiate too many MCSs over the Great Plains and too few MCSs over the southeastern United States. Additional low biases found over the Mississippi River valley region at night revealed a tendency for the HRRR to dissipate MCSs too quickly. The skill of the HRRR at predicting specific MCS events increased between 2012 and 2013, coinciding with changes in both the model physics and in the methods used to assimilate the three-dimensional radar reflectivity.

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Mei Han, Robert M. Rauber, Mohan K. Ramamurthy, Brian F. Jewett, and Joseph A. Grim

Abstract

The dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm-frontal regions of two extratropical winter cyclones are examined using numerical simulations. In each cyclone, the warm, moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front. One branch of the flow turned anticyclonically eastward, corresponding to the warm conveyor belt, while the second branch turned cyclonically westward becoming the trowal airstream. The dynamic forcing of vertical motion within the two airstreams was investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), both from an analysis of the Sawyer–Eliassen (SE) equation in two dimensions and from complete model solutions.

Shearing deformation, associated with the jet stream and the low-level cyclonic flow, dominated confluent deformation near the trowal in both cases. The shearing deformation was accompanied by cold advection associated with an intrusion of cold, dry air aloft. The configuration of isentropes and the wind field led to frontogenesis on the equatorward side of the trowal and frontolysis farther south on the poleward side of the jet stream. The SE solution showed a circulation centered on the frontogenesis–frontolysis couplet, with air rising in the trowal and sinking within the dry air mass on the trowal’s equatorward side. The rising branch of the circulation was responsible for the wide swath of snowfall coincident with the trowal. From the vicinity of the bifurcation axis eastward along the warm-frontal zone, confluent deformation dominated within the troposphere. Frontogenesis in this region produced a direct circulation whose rising branch accounted for the production of precipitation over the warm-frontal zone. Diabatic processes associated with latent heating and cooling produced frontogenesis–frontolysis couplets and significantly modified the transverse frontal circulations. The ascending motion was amplified by a factor of 2 or greater compared with the ascending motion solely due to horizontal deformation. The width of the ascending branch was also narrowed compared with that solely from deformation. Vertical tilting, a result of the secondary circulation generated by horizontal deformation, produced frontogenesis–frontolysis couplets that acted to oppose and reduce the magnitude of the secondary circulation. A conceptual model of the effect of these processes on the production and organization of snowfall in the two cyclones is presented.

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Joseph A. Grim, Robert M. Rauber, Mohan K. Ramamurthy, Brian F. Jewett, and Mei Han

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This paper compares the structure of the trough of warm air aloft (trowal)–warm-frontal region of two continental wintertime cyclones. The cyclones were observed over the central Great Lakes region during the Lake-Induced Convection Experiment/Snowband Dynamics Project field campaign. The cyclones had different origins, with the first forming east of the Colorado Rockies and the second forming over the Gulf of Mexico. They were associated with different upper-level flow regimes, one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet. Both storms produced heavy swaths of snow across the states of Illinois, Wisconsin, and Michigan. High-resolution observations of frontal structure were made during flights of the National Center for Atmospheric Research Electra aircraft using dropsondes and the Electra Doppler Radar tail radar system. The high-resolution observations suggest a different arrangement of air masses in the trowal region compared with the classical occlusion model, where the trowal axis forms at the intersection of a warm front and a cold front that has overtaken and subsequently ascended the warm front. In both cyclones dry air intruded over the warm front, isolating the warm, moist airflow within the trowal airstream. Very sharp moisture gradients were present at the leading edge of the dry air in both cyclones. In each case, relative humidity differences of over 50% were observed over distances of 10–20 km. The thermal gradient near the leading edge of the dry air in one cyclone was diffuse, so that the moist–dry boundary could best be characterized as an upper-level humidity front. In the other cyclone, the thermal gradient was sharper and aligned with the moisture boundary and was best characterized as a cold front aloft. The analyses suggest that the classical conceptual model of the trowal, at least in some cyclones such as the two illustrated here, needs to be revised to include the possibility that the warm moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front. Since the two cyclones discussed here had different origins, tracks, and flow regimes, the similarity of their structure suggests that these features may be common.

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Joseph A. Grim, Greg M. McFarquhar, Robert M. Rauber, Andrea M. Smith, and Brian F. Jewett

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This study employed a nondynamic microphysical column model to evaluate the degree to which the microphysical processes of sublimation, melting, and evaporation alone can explain the evolution of the relative humidity (RH) and latent cooling profiles within the trailing stratiform region of mesoscale convective systems observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). Simulations revealed that observations of a sharp change in the profile of RH, from saturated air with respect to ice above the melting layer to subsaturated air with respect to water below, developed in response to the rapid increase in hydrometeor fall speeds from 1–2 m s−1 for ice to 2–11 m s−1 for rain. However, at certain times and locations, such as the first spiral descent on 29 June 2003 within the notch of lower reflectivity, the air may remain subsaturated for temperatures (T) < 0°C. Sufficiently strong downdrafts above the melting level, such as the 1–3 m s−1 downdrafts observed in the notch of lower reflectivity on 29 June, could enable this state of sustained subsaturation. Sensitivity tests, where the hydrometeor size distributions and upstream RH profiles were varied within the range of BAMEX observations, revealed that the sharp contrast in the RH field across the melting layer always developed. The simulations also revealed that latent cooling from sublimation and melting resulted in the strongest cooling at altitudes within and above the melting layer for locations where hydrometeors did not reach the ground, such as within the rear anvil region, and where sustained subsaturated air is present for T < 0°C, such as is observed within downdrafts. Within the enhanced stratiform rain region, the air is typically at or near saturation for T < 0°C, whereas it is typically subsaturated for T > 0°C; thus, evaporation and melting result in the primary cooling in this region. The implications of these results for the descent of the rear inflow jet across the trailing stratiform region are discussed.

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Joseph A. Grim, Robert M. Rauber, Greg M. McFarquhar, Brian F. Jewett, and David P. Jorgensen

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This study examines the development, structure, and forcing of the rear inflow jet (RIJ) through the life cycle of a small, short-lived squall line over north-central Kansas on 29 June 2003. The analyses were developed from airborne quad-Doppler tail radar data from the NOAA and NRL P-3 aircraft, obtained over a 2-h period encompassing the formation, development, and decay of the squall line during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The strengthening of the system-relative rear inflow to 17 m s−1 was concurrent with the formation of a bow echo, an increased dynamic pressure gradient beneath the rearward-tilted updraft, and two counterrotating vortices at either end of the bow. The later weakening of the RIJ to 8 m s−1 was concurrent with the weakening of the bow, a decreased dynamic pressure gradient at midlevels behind the bow, and the weakening and spreading of the vortices. In a modeling study, Weisman quantified the forcing mechanisms responsible for the development of an RIJ. This present study is the first to quantitatively analyze these mechanisms using observational data. The forcing for the horizontal rear inflow was analyzed at different stages of system evolution by evaluating the contributions of four forcing mechanisms: 1) the horizontal pressure gradient resulting from the vertical buoyancy distribution (δPB), 2) the dynamic pressure gradient induced by the circulation between the vortices (δPV), 3) the dynamic irrotational pressure gradient (δPI), and 4) the background synoptic-scale dynamic pressure gradient (δPS). During the formative stage of the bow, δPI was the strongest forcing mechanism, contributing 50% to the rear inflow. However, during the mature and weakening stages, δPI switched signs and opposed the rear inflow while the combination of δPB and δPV accounted for at least 70% of the rear inflow. The δPS forced 4%–25% of the rear inflow throughout the system evolution.

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Joseph A. Grim, James O. Pinto, Thomas Blitz, Kenneth Stone, and David C. Dowell

Abstract

The severity, duration, and spatial extent of thunderstorm impacts are related to convective storm mode. This study assesses the skill of the High-Resolution Rapid Refresh Ensemble (HRRR-E) and its deterministic counterpart (HRRRv4) at predicting convective mode and storm macrophysical properties using 35 convective events observed during the 2020 warm season across the eastern United States. Seven cases were selected from each of five subjectively determined convective organization modes: tropical cyclones, mesoscale convective systems (MCSs), quasi-linear convective systems, clusters, and cellular convection. These storm events were assessed using an object-based approach to identify convective storms and determine their individual size. Averaged across all 35 cases, both the HRRR-E and HRRRv4 predicted storm areas were generally larger than observed, with this bias being a function of storm lifetime and convective mode. Both modeling systems also underpredicted the rapid increase in storm counts during the initiation period, particularly for the smaller-scale storm modes. Interestingly, performance of the HRRRv4 differed from that of the HRRR-E, with the HRRRv4 generally having a larger bias in total storm area than the HRRR-E due to HRRRv4 predicting up to 66% more storm objects than the HRRR-E. The HRRR-E accurately predicted the convective mode 65% of the time, with complete misses being very rare (<5% of the time overall). However, an evaluation of rank histograms across all 35 cases revealed that the HRRR-E tended to be underdispersive when predicting storm size for all but the MCS mode.

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Greg M. McFarquhar, Michael S. Timlin, Robert M. Rauber, Brian F. Jewett, Joseph A. Grim, and David P. Jorgensen
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