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Daniel J. Cecil

Abstract

The Tropical Rainfall Measuring Mission (TRMM) satellite has been used to infer distributions of intense thunderstorms. Besides the lightning measurements from TRMM, the radar reflectivities and passive microwave brightness temperatures have been used as proxies for convective vigor. This is based on large graupel or hail lofted by strong updrafts being the cause of high–radar reflectivity values aloft and extremely low brightness temperatures. This paper seeks to empirically confirm that extremely low brightness temperatures are often accompanied by large hail at the surface. The three frequencies examined (85, 37, and 19 GHz) all show an increasing likelihood of hail reports with decreasing brightness temperature. Quantification is limited by the sparsity of hail reports. Hail reports are common when brightness temperatures are below 70 K at 85 GHz, 180 K at 37 GHz, or 230 K at 19 GHz.

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Daniel J. Cecil

Abstract

Tropical Rainfall Measuring Mission (TRMM) Microwave Imager and precipitation radar measurements are examined for strong convective systems. Storms having similar values of minimum 37-GHz polarization-corrected temperature (PCT) are grouped together, and their vertical profiles of maximum radar reflectivity are composited. Lower 37-GHz PCT corresponds to stronger radar profiles (high reflectivity through a deep layer), but characteristic profiles for a given 37-GHz PCT are different for deep tropical ocean, deep tropical land, subtropical ocean, and subtropical land regions. Tropical oceanic storms have a sharper decrease of reflectivity just above the freezing level than storms from other regions with the same brightness temperature. Storms from subtropical land regions have the slowest decrease of reflectivity with height and the greatest mixed-phase-layer ice water content (IWC). Linear fits of 37-GHz PCT versus IWC for each region are used to scale the brightness temperatures. Counts of storms with these scaled brightness temperatures below certain thresholds suggest that not as many of the strongest storms occur in central Africa as in subtropical parts of South America, the United States, and central Asia.

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Daniel J. Cecil and Themis Chronis

Abstract

Coefficients are derived for computing the polarization-corrected temperature (PCT) for 10-, 19-, 37- and 89-GHz (and similar) frequencies, with applicability to satellites in the Global Precipitation Measurement mission constellation and their predecessors. PCTs for 10- and 19-GHz frequencies have been nonexistent or seldom used in the past; developing those is the main goal of this study. For 37 and 89 GHz, other formulations of PCT have already become well established. We consider those frequencies here in order to test whether the large sample sizes that are readily available now would point to different formulations of PCT. The purpose of the PCT is to reduce the effects of surface emissivity differences in a scene and draw attention to ice scattering signals related to precipitation. In particular, our intention is to develop a PCT formula that minimizes the differences between land and water surfaces, so that signatures resulting from deep convection are not easily confused with water surfaces. The new formulations of PCT for 10- and 19-GHz measurements hold promise for identifying and investigating intense convection. Four examples are shown from relevant cases. The PCT for each frequency is effective at drawing attention to the most intense convection, and removing ambiguous signals that are related to underlying land or water surfaces. For 37 and 89 GHz, the older formulations of PCT from the literature yield generally similar values as ours, with the differences mainly being a few kelvins over oceans. An optimal formulation of PCT can depend on location and season; results are presented here separated by latitude and month.

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Daniel J. Cecil and Edward J. Zipser

Abstract

Part I of this two-part paper treats Tropical Rainfall Measuring Mission (TRMM) radar, passive microwave, and lightning observations in hurricanes individually. This paper (Part II) examines relationships between these parameters (and implications of the relationships). Quantitative relationships between lightning occurrence and 85-GHz brightness temperature, 37-GHz brightness temperature, and radar reflectivity in the mixed phase region are established separately for hurricane eyewall regions, inner rainband regions, and outer rainband regions; other tropical oceanic regions; and tropical continental regions. When any of the brightness temperature or radar parameters are held constant as controls, lightning is more frequent in hurricane outer rainbands than elsewhere over tropical oceans, and more frequent over continents than even in the outer rainbands. Reflectivity profiles associated with specific brightness temperatures are presented, demonstrating a link between high-altitude ice phase precipitation and 85-GHz scattering and a link between lower-altitude precipitation and 37-GHz scattering. Based on the combination of radar, passive microwave, and lightning observations, it is proposed that supercooled cloud water occurs preferentially in outer rainbands compared to other tropical oceanic precipitation. The suspected microphysical differences produce only subtle differences in the remote sensing parameters other than lightning.

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Daniel J. Cecil and Edward J. Zipser

Abstract

A key component in the maintenance and intensification of tropical cyclones is the transverse circulation, which transports mass and momentum and provides latent heat release via inner core convective updrafts. This study examines these updrafts indirectly, using satellite-borne observations of the scattering of upwelling microwave radiation by precipitation-sized ice particles and satellite-borne observations of lightning. The observations are then compared to tropical cyclone intensity (defined here as maximum sustained wind speed) and the resulting relationships are assessed. Substantial updrafts produce large ice particles aloft, which in turn produce microwave ice-scattering signatures. The large ice, together with supercooled liquid water also generated by substantial updrafts, is a necessary ingredient in charge separation, which leads to lightning. Various parameters derived from the inner core ice-scattering signature are computed for regions encircling hurricanes and typhoons, and observations of lightning activity or inactivity are analyzed.

High correlations with future tropical cyclone intensity result from the ice-scattering signature parameters most closely associated with the areal extent of at least moderate precipitation rates. As expected, the relationship reveals increasing intensity with increasing ice-scattering signature. Indicators of more intense convection yield less information concerning tropical cyclone intensity. Correlations tend to be of the same sign for both present cyclone intensity at the time of the satellite overpass and subsequent intensity change. Correlations are higher for future cyclone intensity than for either of these. The lightning observations are much more limited than the microwave observations, because the short amount of time in which lightning can be detected may not adequately represent a particular storm’s electrical activity. The inner core lightning observations show no clear relationship to tropical cyclone intensification. However, the lightning observations do suggest an increased likelihood of inner core lightning in weak tropical storms and strong hurricanes/typhoons. In the examination of case studies, the paradoxical situation of much greater lightning frequency in rainbands than in eyewalls is noted.

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Thomas A. Jones and Daniel J. Cecil

Abstract

Three hurricanes, Claudette (2003), Isabel (2003), and Dora (1999), were selected to examine the Statistical Hurricane Intensity Prediction Scheme with Microwave Imagery (SHIPS-MI) forecast accuracy for three particular storm types. This research was conducted using model analyses and tropical cyclone best-track data, with forecasts generated from a dependent sample. The model analyses and best-track data are assumed to be a “perfect” representation of the actual event (e.g., perfect prog assumption). Analysis of intensity change forecasts indicated that SHIPS-MI performed best, compared to operational SHIPS output, for tropical cyclones that were intensifying from tropical storm to hurricane intensity. Passive microwave imagery, which is sensitive to the intensity and coverage of precipitation, improved intensity forecasts during these periods with a positive intensity change contribution resulting from above normal inner-core precipitation. Forecast improvement was greatest for 12–36-h forecasts, where the microwave contribution to SHIPS-MI was greatest. Once a storm reached an intensity close to its maximum potential intensity, as in the case of Isabel and Dora, both SHIPS and SHIPS-MI incorrectly forecast substantial weakening despite the positive contribution from microwave data. At least in Dora’s case, SHIPS-MI forecasts were slightly stronger than those of SHIPS. Other important contributions to SHIPS-MI forecasts were examined to determine their importance relative to the microwave inputs. Inputs related to sea surface temperature (SST) and persistence–climatology proved to be very important to intensity change forecasts, as expected. These predictors were the primary factor leading to the persistent weakening forecasts made by both models for Isabel and Dora. For Atlantic storms (Claudette and Isabel), the contribution from shear also proved important at characterizing the conduciveness of the environment toward intensification. However, the shear contribution was often small as a result of multiple offsetting shear-related predictors. Finally, it was observed that atmospheric parameters not included in SHIPS, such as eddy momentum flux, could substantially affect the intensity, leading to large forecast errors. This was especially true for the Claudette intensity change forecasts throughout its life cycle.

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Kenneth D. Leppert II and Daniel J. Cecil

Abstract

Global Precipitation Measurement (GPM) Microwave Imager (GMI) brightness temperatures (BTs) were simulated over a case of severe convection in Texas using ground-based S-band radar and the Atmospheric Radiative Transfer Simulator. The median particle diameter D o of a normalized gamma distribution was varied for different hydrometeor types under the constraint of fixed radar reflectivity to better understand how simulated GMI BTs respond to changing particle size distribution parameters. In addition, simulations were conducted to assess how low BTs may be expected to reach from realistic (although extreme) particle sizes or concentrations. Results indicate that increasing D o for cloud ice, graupel, and/or hail leads to warmer BTs (i.e., weaker scattering signature) at various frequencies. Channels at 166.0 and 183.31 ± 7 GHz are most sensitive to changing D o of cloud ice, channels at ≥89.0 GHz are most sensitive to changing D o of graupel, and at 18.7 and 36.5 GHz they show the greatest sensitivity to hail D o. Simulations contrasting BTs above high concentrations of small (0.5-cm diameter) and low concentrations of large (20-cm diameter) hailstones distributed evenly across a satellite pixel showed much greater scattering using the higher concentration of smaller hailstones with BTs as low as ~110, ~33, ~22, ~46, ~100, and ~106 K at 10.65, 18.7, 36.5, 89.0, 166.0, and 183.31 ± 7 GHz, respectively. These results suggest that number concentration is more important for scattering than particle size given a constant S-band radar reflectivity.

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Lori A. Schultz and Daniel J. Cecil

Abstract

An expanded “climatology” of U.S. tropical cyclone (TC) tornadoes covering the period 1950–2007 is presented. A major climatology published in 1991 included data on 626 TC tornadoes. Since then, almost 1200 more TC tornado records have been identified, with almost half of that number from the 2004–05 seasons alone. This work reexamines some findings from previous studies, using a substantially larger database. The new analyses strongly support distinctions between inner- and outer-region tornadoes, which were suggested in previous studies. Outer-region tornadoes (beyond 200 km from the TC center) have a stronger diurnal signal, commonly occurring during the afternoon. Inner-region tornadoes typically occur within ∼12 h of TC landfall, with no strong preference for a particular time of day. They are disproportionately less damaging tornadoes, with more rated F0 than in the outer-region sample. In more general terms, the TC tornado database includes a smaller percentage of significant (≥F2) tornadoes (14%) than does the overall U.S. tornado database (22%). Most TC tornadoes (60%) occur within 100 km of the coast; this includes core-region tornadoes near the time of landfall as well as tornadoes from rainbands coming ashore far from the circulation center. The F0-rated tornadoes are slightly more common near the coast but compose a smaller percentage of the tornadoes inland. The threat often persists for 2–3 days after landfall and extends ∼400 km inland and ∼500 km from the TC center, although there is much case-to-case variability. This puts locations at risk that might otherwise avoid damage from the TC.

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Kenneth D. Leppert II and Daniel J. Cecil

Abstract

Passive microwave brightness temperatures (BTs) collected above severe thunderstorms using the Advanced Microwave Precipitation Radiometer and Conical Scanning Millimeter-Wave Imaging Radiometer during the Midlatitude Continental Convective Clouds Experiment are compared with a hydrometeor identification applied to dual-polarimetric Weather Surveillance Radar-1988 Doppler radar data collected at Vance Air Force Base, Oklahoma (KVNX). The goal of this work is to determine the signatures of various hydrometeor species in terms of BTs measured at frequencies used by the Global Precipitation Measurement mission Microwave Imager. Results indicate that hail is associated with an ice-scattering signature at all frequencies examined, including 10.7 GHz. However, it appears that frequencies ≤ 37.1 GHz are most useful for identifying hail. Low-level (below 2.5 km) hail becomes probable for a BT below 240 K at 19.4 GHz, 170 K at 37.1 GHz, 90 K at 85.5 GHz, 80 K at 89.0 GHz, 100 K at 165.5 GHz, and 100 K at 183.3 ± 7 GHz. Graupel may be distinguished from hail and profiles without any hydrometeor species by its strong scattering signature at higher frequencies (e.g., 165.5 GHz) and its relative lack of scattering at frequencies ≤ 19.4 GHz. There is a clearer distinction between profiles that contain liquid precipitation and profiles without any hydrometeors when the liquid is associated above with hail and/or graupel (i.e., a hydrometeor category with a strong scattering signature) than when the liquid is associated with smaller ice. Near-surface precipitation is much more likely for a 19.4-GHz BT < 250 K, 37.1-GHz BT < 240 K, 89.0-GHz BT < 220 K, and 165.5-GHz BT < 140 K.

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Matthew T. Wingo and Daniel J. Cecil

Abstract

The response of the precipitation field for tropical cyclones in relation to the surrounding environmental vertical wind shear has been investigated using ∼20 000 snapshots of passive-microwave satellite rain rates. Composites of mean rain rates, 95th percentile rain rates, and rain coverage were constructed to compare how the spatial distribution of the precipitation was organized under varying environmental shear. Results indicated that precipitation is displaced downshear and to the left (right for Southern Hemisphere) of the shear vector. The amplitude of this displacement increases with stronger shear. The majority of the asymmetry found in the mean rain rates is accounted for by the asymmetry in the occurrence of heavy rain. Although rain is common in all quadrants of the sheared tropical cyclones, heavy rain (≥8 mm h−1 at the ∼25-km scale) is comparatively rare in the upshear-right quadrant. It is shown that the effect that shear has on the rain field is nearly instantaneous. Strong westerly shear formed slightly more asymmetric patterns than strong easterly shear.

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