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Ralph A. Petersen and Louis W. Uccellini

Abstract

An explicit technique for computing atmospheric trajectories, based on Greenspan's discrete model formulation, is presented as an alternative to the commonly used implicit scheme. The method provides an economical means of objectively obtaining computer-generated trajectories and accounts for the variable accelerations and local ψtendencies along the entire trajectory path. The initial results presented show that the explicit computations are stable and very nearly energy-conservative. An application of the discrete model approach to a real data base and comparisons with trajectories determined by the implicit method yield favorable results, illustrating the utility of the explicit technique as a diagnostic tool.

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V. N. Bringi, L. Tolstoy, M. Thurai, and W. A. Petersen

Abstract

Polarimetric radar data obtained at high spatial and temporal resolutions offer a distinct advantage in estimating the spatial correlation function of drop size distribution (DSD) parameters and rain rate compared with a fixed gauge–disdrometer network. On two days during the 2011 Midlatitude Continental Convective Clouds Experiment (MC3E) campaign in Oklahoma, NASA’s S-band polarimetric radar (NPOL) performed repeated PPI scans every 40 s over six 2D video disdrometer (2DVD) sites, located 20–30 km from the radar. The two cases were 1) a rapidly evolving multicell rain event (with large drops) and 2) a long-duration stratiform rain event. From the time series at each polar pixel, the Pearson correlation coefficient is computed as a function of distance along each radial in the PPI scan. Azimuthal dependence is found, especially for the highly convective event. A pseudo-1D spatial correlation is computed that is fitted to a modified-exponential function with two parameters (decorrelation distance R 0 and shape F). The first event showed significantly higher spatial variability in rain rate (shorter decorrelation distance R 0 = 3.4 km) compared with the second event with R 0 = 10.2 km. Further, for the second event, the spatial correlation of the DSD parameters and rain rate from radar showed good agreement with 2DVD-based spatial correlations over distances ranging from 1.5 to 7 km. The NPOL also performed repeated RHI scans every 40 s along one azimuth centered over the 2DVD network. Vertical correlations of the DSD parameters as well as the rainwater content were determined below the melting level, with the first event showing more variability compared with the second event.

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Anthony Mostek, Louis W. Uccellini, Ralph A. Petersen, and Dennis Chesters

Abstract

Retrievals from the VISSR Atmospheric Sounder (VAS) an combined with conventional data to assess the impact of geosynchronous satellite soundings upon the analysis of a pre-convective environment over the central United States on 13 July 1981. VAS retrievals of temperature, dewpoint, equivalent potential temperature, precipitable water, and lifted index are derived with 30 km resolution at 3 hour intervals. When VAS fields are combined with analyses from conventional data sources regions with convective instability are more clearly delineated prior to the rapid development of the thunderstorms. The retrievals differentiate isolated areas in which most air extends throughout the lower troposphere (and are therefore more conducive for the development of deep convective storms) from those regions where moisture is confined to a thin layer near the earth's surface (where convection is less likely to occur). The analyses of the VAS retrievals identify significant spatial gradients and temporal changes in the thermal and moisture fields, especially in the regions between radiosonde observations. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clew skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by small clouds which are unresolved in the infrared imagery. These analyses, however, demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements that are useful for the quantitative analysis of a cloud-free pre-convective environment.

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R. Cifelli, S. W. Nesbitt, S. A. Rutledge, W. A. Petersen, and S. Yuter

Abstract

Ship-based radar data are used to compare the structure of precipitation features in two regions of the east Pacific where recent field campaigns were conducted: the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC-2001; 10°N, 95°W) in September 2001 and the Tropical Eastern Pacific Process Study (TEPPS; 8°N, 125°W) in August 1997. Corresponding July–September 1998–2004 Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) data are also used to provide context for the field campaign data. An objective technique is developed to identify precipitation features in the ship and TRMM PR data and to develop statistics on horizontal and vertical structure and precipitation characteristics. Precipitation features were segregated into mesoscale convective system (MCS) and sub-MCS categories, based on a contiguous area threshold of 1000 km2 (these features were required to have at least one convective pixel), as well as an “other” (NC) category. Comparison of the satellite and field campaign data showed that the two datasets were in good agreement for both regions with respect to MCS features. Specifically, both the satellite and ship radar data showed that approximately 80% of the rainfall volume in both regions was contributed by MCS features, similar to results from other observational datasets. EPIC and TEPPS MCSs had similar area distributions but EPIC MCSs tended to be more vertically developed and rain heavier than their TEPPS counterparts. In contrast to MCSs, smaller features (NCs and sub-MCSs) sampled by the ship radar in both regions showed important differences compared with the PR climatology. In the EPIC field campaign, a large number of small (<100 km2), shallow (radar echo tops below the melting level) NCs and sub-MCSs were sampled. A persistent dry layer above 800 mb during undisturbed periods in EPIC may have been responsible for the high occurrence of these features. Also, during the TEPPS campaign, sub-MCSs were larger and deeper with respect to the TRMM climatology, which may have been due to the higher than average SSTs during 1997–98 when TEPPS was conducted. Despite these differences, it was found that for sizes greater than about 100 km2, EPIC precipitation features had 30-dBZ echos at higher altitudes and also had higher rain rates than similar sized TEPPS features. These results suggest that ice processes play a more important role in rainfall production in EPIC compared with TEPPS.

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R. Cifelli, S. W. Nesbitt, S. A. Rutledge, W. A. Petersen, and S. Yuter

Abstract

This study examines the diurnal cycle of precipitation features in two regions of the tropical east Pacific where field campaigns [the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) and the Tropical Eastern Pacific Process Study (TEPPS)] were recently conducted. EPIC (10°N, 95°W) was undertaken in September 2001 and TEPPS (8°N, 125°W) was carried out in August 1997. Both studies employed C-band radar observations on board the NOAA ship Ronald H. Brown (RHB) and periodic upper-air sounding launches to observe conditions in the surrounding environment. Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Geostationary Operational Environmental Satellite (GOES) IR data are used to place the RHB data in a climatological context and Tropical Atmosphere Ocean (TAO) buoy data are used to evaluate changes in boundary layer fluxes in context with the observed diurnal cycle of radar observations of precipitation features.

Precipitation features are defined as contiguous regions of radar echo and are subdivided into mesoscale convective system (MCS) and sub-MCS categories. Results show that MCSs observed in EPIC and TEPPS have distinct diurnal signatures. Both regions show an increase in intensity starting in the afternoon hours, with the timing of maximum rain intensity preceding maxima in rain area and accumulation. In the TEPPS region, MCS rain rates peak in the evening and rain area and accumulation in the late night–early morning hours. In contrast, EPIC MCS rain rates peak in the late night–early morning, and rain area and accumulation are at a maximum near local sunrise. The EPIC observations are in agreement with previous satellite studies over the Americas, which show a phase lag response in the adjacent oceanic regions to afternoon–evening convection over the Central American landmass. Sub-MCS features in both regions have a broad peak extending through the evening to late night–early morning hours, similar to that for MCSs. During sub-MCS-only periods, the rainfall patterns of these features are closely linked to diurnal changes in SST and the resulting boundary layer flux variability.

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Robert M. Aune, James J. Tuccillo, Louis W. Uccellini, and Ralph A. Petersen

Abstract

Numerical experiments are conducted to assess the impact of incorporating temperature data from the VISSR Atmospheric Sounder (VAS) into a regional-scale numerical model using an assimilation technique developed by Gal-Chen. The technique uses a three-dimensional variational approach to combine the VAS observations with model temperature fields during the numerical integration. A nudging technique is also tested, whereby the model temperature field is constrained toward the VAS “updated” values during the assimilation cycle. Results of the experiments indicate that the Gal-Chen assimilation technique successfully combines actual VAS temperature observations with the dynamically balanced model fields without destabilizing the model during the assimilation cycle. Furthermore, increasing the temporal frequency of VAS temperature insertions during the assimilation cycle enhances the impact on the model forecast through successively longer forecast periods. These results support the conclusions of earlier experiments with simulated geostationary satellite data that show an increasing positive impact on numerical simulations as the insertion rate of the satellite-based temperature information is increased. Incorporating the nudging technique further enhances the impact of the VAS temperature data, permitting the model wind field to adjust to the updated temperature fields and increasing the impact on the VAS data through a longer portion of the model simulation.

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M. Thurai, V. N. Bringi, W. A. Petersen, and P. N. Gatlin

Abstract

Two rain events are analyzed using two collocated 2D-video disdrometers (2DVD) and a C-band polarimetric radar at 15-km distance. Both events had moderate-to-intense rainfall rates, but the second event had an embedded convective line. For the first event, the fall speed distribution for a given drop diameter interval showed a narrow and symmetric distribution with a mode at the expected value; the second event produced a wider distribution with a significant skewness toward lower fall speeds. The “slower” drops in the second event were detected while the convective line was directly over the 2DVD site. Drop shape information from the two 2DVD instruments showed that, during the passage of the convection line, around 30%–40% of the drops did not have an axis of rotational symmetry, whereas for event 1, it was only 5%. The implications are that for event 1 the dominant mode of drop oscillation is the axisymmetric mode, and that within the convective line of event 2 other fundamental modes were frequent. The radar data for the second event were analyzed in terms of the self-consistency among the radar-measured quantities. The K dp/Z h versus Z dr variations within the line convection were not consistent with the corresponding variation determined from the scattering calculations using the measured 1-min drop size distributions and using the “reference” drop shapes. Also found were low ρhv regions within the line convection that were considerably lower than the scattering calculations. These findings are consistent with the asymmetric oscillation modes inferred from the 2DVD measurements for event 2 (probably collision induced) within the convective line.

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Ralph A. Petersen, Louis W. UccellinI, Anthony Mostek, and Dennis A. Keyser

Abstract

Infrared and visible imagery from VAS are used to delineate mid- and lower-tropospheric moisture fields for a variety of severe storm cases in the southern and central United States. The ability of sequences of images to isolate areas of large negative vertical moisture gradients and apparent convective instability prior to the onset of convective storms is assessed. Midlevel dryness is diagnosed directly from the VAS 6.7 channel observations, while low-level water vapor is either inferred from the presence of clouds in visible and infrared imagery or, in cloud-free areas, calculated from VAS "split window" channels. A variety ofimage combination procedures are used to deduce the stability fields which are then compared with the available radiosonde data. The results for several severe storm cases indicate that VAS can detect mid- and low-level mesoscale water vapor fields as distinct radiometric signals. The VAS imagery shows a strong tendency for thunderstorms to develop along the edges of bands of midlevel dryness as they overtake either pre-existing or developing low-level moisture maxima. Image sequences depict the speed with which deep moist and dry layers can develop and move, often at scales not resolvable using conventional radiosonde data. The images thus demonstrate the ability of VAS radiance data to detect differential moisture advectionsin rapidly changing pre-convective environments.

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Andrew L. Molthan, Walter A. Petersen, Stephen W. Nesbitt, and David Hudak

Abstract

The Canadian CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Validation Project (C3VP) was a field campaign designed to obtain aircraft, surface, and radar observations of clouds and precipitation in support of improving the simulation of snowfall and cold season precipitation, their microphysical processes represented within forecast models, and radiative properties relevant to remotely sensed retrievals. During the campaign, a midlatitude cyclone tracked along the U.S.–Canadian border on 22 January 2007, producing an extensive area of snowfall. Observations of ice crystals from this event are used to evaluate the assumptions and physical relationships for the snow category within the Goddard six-class, single-moment microphysics scheme, as implemented within the Weather Research and Forecasting (WRF) model.

The WRF model forecast generally reproduced the precipitation and cloud structures sampled by radars and aircraft, permitting a comparison between C3VP observations and model snowfall characteristics. Key snowfall assumptions in the Goddard scheme are an exponential size distribution with fixed intercept and effective bulk density, and the relationship between crystal diameter and terminal velocity. Fixed values for the size distribution intercept and density did not represent the vertical variability of naturally occurring populations of aggregates, and the current diameter and fall speed relationship underestimated terminal velocities for all sizes of crystals.

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