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Steven A. Rutledge
and
Walter A. Petersen

Abstract

This study presents further evidence in support of an in situ, noninductive charging mechanism as the process likely responsible for significant electrification of the trailing stratiform regions of mesoscale convective systems (MCSs). In contrast to previous studies of MCS electrification that have investigated observations of radar reflectivity and cloud-to-ground lightning in the horizontal (e.g., Orville et al.; Rutledge et al.), here the relationship between the location and occurrence of cloud-to-ground lightning in the stratiform regions of midlatitude and tropical MCSs and the vertical profile of radar reflectivity are examined. The vertical profile of radar reflectivity at elevations above the 0°C level is used as a proxy for the amount of mass present in the mixed-phase region of the stratiform clouds, which in turn is related to the generation of charge through a noninductive charging mechanism.

To further explore the relationship between radar reflectivity, mixed-phase microphysics, and in situ charging by means of a noninductive mechanism, we present calculations with a simple one-dimensional model used to diagnose the presence of supercooled liquid water between the 0° and −20°C levels in the stratiform region. We use the model to contrast two cases: 1) a case in which reflectivities greater than 15 dBZ existed above the 0°C level in the stratiform clouds, cloud-to-ground lightning occurred, and moderate amounts of supercooled liquid water were present in the stratiform region (as inferred from the model results); 2) a case where no lightning was observed in the stratiform region, reflectivities above the 0°C level were less than 15 dBZ, and very little supercooled water was present (as inferred from the model results). Based on observations in several MCSs, we show that the number of cloud-to-ground lightning flashes in the stratiform region is highly correlated with the vertical radar reflectivity profile.

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Lawrence D. Carey
and
Walter A. Petersen

Abstract

Estimating raindrop size has been a long-standing objective of polarimetric radar–based precipitation retrieval methods. The relationship between the differential reflectivity Z dr and the median volume diameter D 0 is typically derived empirically using raindrop size distribution observations from a disdrometer, a raindrop physical model, and a radar scattering model. Because disdrometers are known to undersample large raindrops, the maximum drop diameter D max is often an assumed parameter in the rain physical model. C-band Z dr is sensitive to resonance scattering at drop diameters larger than 5 mm, which falls in the region of uncertainty for D max. Prior studies have not accounted for resonance scattering at C band and D max uncertainty in assessing potential errors in drop size retrievals. As such, a series of experiments are conducted that evaluate the effect of D max parameterization on the retrieval error of D 0 from a fourth-order polynomial function of C-band Z dr by varying the assumed D max through the range of assumptions found in the literature. Normalized bias errors for estimating D 0 from C-band Z dr range from −8% to 15%, depending on the postulated error in D max. The absolute normalized bias error increases with C-band Z dr, can reach 10% for Z dr as low as 1–1.75 dB, and can increase from there to values as large as 15%–45% for larger Z dr, which is a larger potential bias error than is found at S and X band. Uncertainty in D max assumptions and the associated potential D 0 retrieval errors should be noted and accounted for in future C-band polarimetric radar studies.

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Walter A. Petersen
and
Steven A. Rutledge

Abstract

Observation of the vertical profile of precipitation over the global Tropics is a key objective of the Tropical Rainfall Measuring Mission (TRMM) because this information is central to obtaining vertical profiles of latent heating. This study combines both TRMM precipitation radar (PR) and Lightning Imaging Sensor (LIS) data to examine “wet-season” vertical structures of tropical precipitation across a broad spectrum of locations in the global Tropics. TRMM-PR reflectivity data (2A25 algorithm) were utilized to produce seasonal mean three-dimensional relative frequency histograms and precipitation ice water contents over grid boxes of approximately 5°–10° in latitude and longitude. The reflectivity histograms and ice water contents were then combined with LIS lightning flash densities and 2A25 mean rainfall rates to examine regional relationships between precipitation vertical structure, precipitation processes, and lightning production.

Analysis of the reflectivity vertical structure histograms and lightning flash density data reveals that 1) relative to tropical continental locations, wet-season isolated tropical oceanic locations exhibit relatively little spatial (and in some instances seasonal) variability in vertical structure across the global Tropics; 2) coastal locations and areas located within 500–1000 km of a continent exhibit considerable seasonal and spatial variability in mean vertical structure, often resembling “continental” profiles or falling intermediate to that of tropical continental and isolated oceanic regimes; and 3) interior tropical continental locations exhibit marked variability in vertical structure both spatially and seasonally, exhibiting a continuum of characteristics ranging from a near-isolated oceanic profile observed over the central Amazon and India to a more robust continental profile observed over regions such as the Congo and Florida. Examination of regional and seasonal mean conditional instability for a small but representative subset of the geographic locations suggests that tropospheric thermodynamic structure likely plays a significant role in the regional characteristics of precipitation vertical structure and associated lightning flash density.

In general, the largest systematic variability in precipitation vertical structure observed between all of the locations examined occurred above the freezing level. It is important that subfreezing temperature variability in the vertical reflectivity structures was well reflected in the seasonal mean lightning flash densities and ice water contents diagnosed for each location. In turn, systematically larger rainfall rates were observed on a pixel-by-pixel basis in locations with larger precipitation ice water content and lightning flash density. These results delineate, in a regional sense, the relative importance of mixed-phase precipitation production across the global Tropics.

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Andrew L. Molthan
and
Walter A. Petersen

Abstract

The Canadian CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Validation Project (C3VP) was designed to acquire aircraft, surface, and satellite observations of particle size distributions during cold season precipitation events for the purposes of validating and improving upon satellite-based retrievals of precipitation and the representation of cloud and precipitation processes within numerical weather prediction schemes. During an intensive observation period on 22 January 2007, an instrumented aircraft measured ice crystal size distributions, ice and liquid water contents, and atmospheric state parameters within a broad shield of precipitation generated by a passing midlatitude cyclone. The 94-GHz CloudSat radar acquired vertical profiles of radar reflectivity within light to moderate snowfall, coincident with C3VP surface and aircraft instrumentation. Satellite-based retrievals of cold season precipitation require relationships between remotely sensed quantities, such as radar reflectivity or brightness temperature, and the ice water content present within the sampled profile.

In this study, three methods for simulating CloudSat radar reflectivity are investigated by comparing Mie spheres, single dendrites, and fractal aggregates represented within scattering databases or parameterizations. It is demonstrated that calculations of radar backscatter from nonspherical crystal shapes are required to represent the vertical trend in CloudSat radar reflectivity for this particular event, as Mie resonance effects reduce the radar backscatter from precipitation-sized particles larger than 1 mm. Remaining differences between reflectivity from nonspherical shapes and observations are attributed to uncertainty in the mass–diameter relationships for observed crystals and disparities between naturally occurring crystals and shapes assumed in the development of ice crystal scattering databases and parameterizations.

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Kenneth D. Leppert II
and
Walter A. Petersen

Abstract

It has been hypothesized that intense convective-scale “hot” towers play a role in tropical cyclogenesis via dynamic and thermodynamic feedbacks on the larger-scale circulation. In this study the authors investigate the role that widespread and/or intense lightning-producing convection (i.e., electrically hot towers) present in African easterly waves (AEWs) may play in tropical cyclogenesis over the east Atlantic Ocean.

The 700-hPa meridional wind from the NCEP–NCAR reanalysis dataset was analyzed to divide the waves into northerly, southerly, trough, and ridge phases. The AEWs were subsequently divided into waves that developed into tropical storms (i.e., developing) and those that did not develop into tropical storms (i.e., nondeveloping). Finally, composites were created using various NCEP variables, lightning data gathered with the Zeus network and worldwide lightning location network (WWLLN), and brightness temperature data extracted from the NASA global-merged infrared brightness temperature dataset.

Results indicate that in all regions examined the developing waves seem to be associated with more widespread and/or intense lightning-producing convection. This increased convection associated with the developing waves might be related to the increased midlevel moisture, low-level vorticity, low-level convergence, upper-level divergence, and increased upward vertical motion found to be associated with the developing waves. In addition, the phasing of the convection with the AEWs as they move from East Africa to the central Atlantic shows some variability, which may have implications for tropical cyclogenesis.

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Jackson Tan
,
Walter A. Petersen
, and
Ali Tokay

Abstract

The comparison of satellite and high-quality, ground-based estimates of precipitation is an important means to assess the confidence in satellite-based algorithms and to provide a benchmark for their continued development and future improvement. To these ends, it is beneficial to identify sources of estimation uncertainty, thereby facilitating a precise understanding of the origins of the problem. This is especially true for new datasets such as the Integrated Multisatellite Retrievals for GPM (IMERG) product, which provides global precipitation gridded at a high resolution using measurements from different sources and techniques. Here, IMERG is evaluated against a dense network of gauges in the mid-Atlantic region of the United States. A novel approach is presented, leveraging ancillary variables in IMERG to attribute the errors to the individual instruments or techniques within the algorithm. As a whole, IMERG exhibits some misses and false alarms for rain detection, while its rain-rate estimates tend to overestimate drizzle and underestimate heavy rain with considerable random error. Tracing the errors to their sources, the most reliable IMERG estimates come from passive microwave satellites, which in turn exhibit a hierarchy of performance. The morphing technique has comparable proficiency with the less skillful satellites, but infrared estimations perform poorly. The approach here demonstrated that, underlying the overall reasonable performance of IMERG, different sources have different reliability, thus enabling both IMERG users and developers to better recognize the uncertainty in the estimate. Future validation efforts are urged to adopt such a categorization to bridge between gridded rainfall and instantaneous satellite estimates.

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Kenneth D. Leppert II
,
Walter A. Petersen
, and
Daniel J. Cecil

Abstract

In this study, the authors investigated the characteristics of tropical easterly wave convection and the possible implications of convective structure on tropical cyclogenesis and intensification over the Atlantic Ocean and the east Pacific Ocean. Easterly waves were partitioned into northerly, southerly, trough, and ridge phases based on the 700-hPa meridional wind from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. Waves were subsequently divided according to whether they did or did not develop tropical cyclones (i.e., developing and nondeveloping, respectively), and developing waves were further subdivided according to development location. Finally, composites as a function of wave phase and category were created using data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, Precipitation Radar (PR), and Lightning Imaging Sensor as well as infrared (IR) brightness temperature data from the NASA global-merged IR brightness temperature dataset.

Results suggest that the convective characteristics that best distinguish developing from nondeveloping waves vary according to where developing waves spawn tropical cyclones. For waves that develop a cyclone in the Atlantic basin, coverage by IR brightness temperatures ≤240 and ≤210 K provide the best distinction between developing and nondeveloping waves. In contrast, several variables provide a significant distinction between nondeveloping waves and waves that develop cyclones over the east Pacific as these waves near their genesis location including IR threshold coverage, lightning flash rates, and low-level (<4.5 km) PR reflectivity. Results of this study may be used to help develop thresholds to better distinguish developing from nondeveloping waves and serve as another aid for tropical cyclogenesis forecasting.

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Kenneth D. Leppert II
,
Daniel J. Cecil
, and
Walter A. Petersen

Abstract

In this study, a wave-following Lagrangian framework was used to examine the evolution of tropical easterly wave structure, circulation, and convection in the days leading up to and including tropical cyclogenesis in the Atlantic and east Pacific basins. After easterly waves were separated into northerly, southerly, trough, and ridge phases using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis 700-hPa meridional wind, waves that developed a tropical cyclone [developing waves (DWs)] and waves that never developed a cyclone [nondeveloping waves (NDWs)] were identified. Day zero (D0) was defined as the day on which a tropical depression was identified for DWs or the day the waves achieved maximum 850-hPa vorticity for NDWs. Both waves types were then traced from five days prior to D0 (D − 5) through one day after D0. Results suggest that as genesis is approached for DWs, the coverage by convection and cold cloudiness (e.g., fractional coverage by infrared brightness temperatures ≤240 K) increases, while convective intensity (e.g., lightning flash rate) decreases. Therefore, the coverage by convection appears to be more important than the intensity of convection for tropical cyclogenesis. In contrast, convective coverage and intensity both increase from D − 5 to D0 for NDWs. Compared to NDWs, DWs are associated with significantly greater coverage by cold cloudiness, large-scale moisture throughout a deep layer, and large-scale, upper-level (~200 hPa) divergence, especially within the trough and southerly phases, suggesting that these parameters are most important for cyclogenesis and for distinguishing DWs from NDWs.

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Jackson Tan
,
Walter A. Petersen
,
Pierre-Emmanuel Kirstetter
, and
Yudong Tian

Abstract

The Integrated Multisatellite Retrievals for GPM (IMERG), a global high-resolution gridded precipitation dataset, will enable a wide range of applications, ranging from studies on precipitation characteristics to applications in hydrology to evaluation of weather and climate models. These applications focus on different spatial and temporal scales and thus average the precipitation estimates to coarser resolutions. Such a modification of scale will impact the reliability of IMERG. In this study, the performance of the Final Run of IMERG is evaluated against ground-based measurements as a function of increasing spatial resolution (from 0.1° to 2.5°) and accumulation periods (from 0.5 to 24 h) over a region in the southeastern United States. For ground reference, a product derived from the Multi-Radar/Multi-Sensor suite, a radar- and gauge-based operational precipitation dataset, is used. The TRMM Multisatellite Precipitation Analysis (TMPA) is also included as a benchmark. In general, both IMERG and TMPA improve when scaled up to larger areas and longer time periods, with better identification of rain occurrences and consistent improvements in systematic and random errors of rain rates. Between the two satellite estimates, IMERG is slightly better than TMPA most of the time. These results will inform users on the reliability of IMERG over the scales relevant to their studies.

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Di Wu
,
Christa Peters-Lidard
,
Wei-Kuo Tao
, and
Walter Petersen

Abstract

The Iowa Flood Studies (IFloodS) campaign was conducted in eastern Iowa as a pre-GPM-launch campaign from 1 May to 15 June 2013. During the campaign period, real-time forecasts were conducted utilizing the NASA-Unified Weather Research and Forecasting (NU-WRF) Model to support the daily weather briefing. In this study, two sets of the NU-WRF rainfall forecasts are conducted with different soil initializations, one from the spatially interpolated North American Mesoscale Forecast System (NAM) and the other produced by the Land Information System (LIS) using daily analysis of bias-corrected stage IV data. Both forecasts are then compared with NAM, stage IV, and Multi-Radar Multi-Sensor (MRMS) quantitative precipitation estimation (QPE) to understand the impact of land surface initialization on the predicted precipitation. In general, both NU-WRF runs are able to reproduce individual peaks of precipitation at the right time. NU-WRF is also able to replicate a better rainfall spatial distribution compared with NAM. Further sensitivity tests show that the high-resolution runs (1 and 3 km) are able to better capture the precipitation event compared to its coarser-resolution counterpart (9 km). Finally, the two sets of NU-WRF simulations produce very close rainfall characteristics in bias, spatial and temporal correlation scores, and probability density function. The land surface initialization does not show a significant impact on short-term rainfall forecast, which is largely because of high soil moisture during the field campaign period.

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