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Dennis J. Boccippio

Abstract

Scaling relations connecting storm electrical generator power (and hence lightning flash rate) to charge transport velocity and storm geometry were originally posed by Vonnegut in the 1960s. These were later simplified to yield simple parameterizations for lightning based upon cloud-top height, with separate parameterizations derived over land and ocean. It is demonstrated that the most recent ocean parameterization 1) yields predictions of storm updraft velocity, which appear inconsistent with observation, and 2) is formally inconsistent with Vonnegut's original theory. Revised formulations consistent with Vonnegut's original framework are presented. These demonstrate that Vonnegut's theory is, to first order, consistent with recent satellite observations. The implications of assuming that flash rate is set by the electrical generator power, rather than the electrical generator current, are examined. The two approaches yield significantly different predictions about the dependence of charge transfer per flash on storm dimensions, which should be empirically and numerically testable. The two approaches also differ significantly in their physical explanations of regional variability in lightning observations.

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Dennis J. Boccippio

Abstract

A diagnostic analysis of the VVP (volume velocity processing) retrieval method is presented, with emphasis on understanding the technique as a linear, multivariate regression. Similarities and differences to the velocity–azimuth display and extended velocity–azimuth display retrieval techniques are discussed, using this framework. Conventional regression diagnostics are then employed to quantitatively determine situations in which the VVP technique is likely to fail. An algorithm for preparation and analysis of a robust VVP retrieval is developed and applied to synthetic and actual datasets with high temporal and spatial resolution.

A fundamental (but quantifiable) limitation to some forms of VVP analysis is inadequate sampling dispersion in the n space of the multivariate regression, manifest as a collinearity between the basis functions of some fitted parameters. Such collinearity may be present either in the definition of these basis functions or in their realization in a given sampling configuration. This nonorthogonality may cause numerical instability, variance inflation (decrease in robustness), and increased sensitivity to bias from neglected wind components. It is shown that these effects prevent the application of VVP to small azimuthal sectors of data. The behavior of the VVP regression is further diagnosed over a wide range of sampling constraints, and reasonable sector limits are established.

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Dennis J. Boccippio, William J. Koshak, and Richard J. Blakeslee

Abstract

Laboratory calibration and observed background radiance data are used to determine the effective sensitivities of the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS), as functions of local hour and pixel location within the instrument arrays. The effective LIS thresholds, expressed as radiances emitted normal to cloud top, are 4.0 ± 0.7 and 7.6 ± 3.3 μJ sr−1 m−2 for night and local noon; the OTD thresholds are 11.7 ± 2.2 and 16.8 ± 4.6 μJ sr−1 m−2. LIS and OTD minimum signal-to-noise ratios occur from 0800 to 1600 local time, and attain values of 10 ± 2 and 20 ± 3, respectively. False alarm rate due to instrument noise yields ∼5 false triggers per month for LIS, and is negligible for OTD. Flash detection efficiency, based on prior optical pulse sensor measurements, is predicted to be 93 ± 4% and 73 ± 11% for LIS night and noon; 56 ± 7% and 44 ± 9% for OTD night and noon, corresponding to a 12%–20% diurnal variability and LIS:OTD ratio of 1.7. Use of the weighted daily mean detection efficiency (i.e., not controlling for local hour) corresponds to σ = 8%–9% uncertainty. These are likely overestimates of actual flash detection efficiency due to differences in pixel ground field of view across the instrument arrays that are not accounted for in the validation optical pulse sensor data.

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Dennis J. Boccippio, Walter A. Petersen, and Daniel J. Cecil

Abstract

A taxonomy of tropical convective and stratiform vertical structures is constructed through cluster analysis of 3 yr of Tropical Rainfall Measuring Mission (TRMM) “warm-season” (surface temperature greater than 10°C) precipitation radar (PR) vertical profiles, their surface rainfall, and associated radar-based classifiers (convective/stratiform and brightband existence). Twenty-five archetypal profile types are identified, including nine convective types, eight stratiform types, two mixed types, and six anvil/fragment types (nonprecipitating anvils and sheared deep convective profiles). These profile types are then hierarchically clustered into 10 similar families, which can be further combined, providing an objective and physical reduction of the highly multivariate PR data space that retains vertical structure information. The taxonomy allows for description of any storm or local convective spectrum by the profile types or families. The analysis provides a quasi-independent corroboration of the TRMM 2A23 convective/stratiform classification. The global frequency of occurrence and contribution to rainfall for the profile types are presented, demonstrating primary rainfall contribution by midlevel glaciated convection (27%) and similar depth decaying/stratiform stages (28%–31%). Profiles of these types exhibit similar 37- and 85-GHz passive microwave brightness temperatures but differ greatly in their frequency of occurrence and mean rain rates, underscoring the importance to passive microwave rain retrieval of convective/stratiform discrimination by other means, such as polarization or texture techniques, or incorporation of lightning observations. Close correspondence is found between deep convective profile frequency and annualized lightning production, and pixel-level lightning occurrence likelihood directly tracks the estimated mean ice water path within profile types.

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Dennis J. Boccippio, Steven J. Goodman, and Stan Heckman

Abstract

Observations from the National Aeronautics and Space Administration Optical Transient Detector (OTD) and Tropical Rainfall Measuring Mission (TRMM)-based Lightning Imaging Sensor (LIS) are analyzed for variability between land and ocean, various geographic regions, and different (objectively defined) convective “regimes.” The bulk of the order-of-magnitude differences between land and ocean regional flash rates are accounted for by differences in storm spacing (density) and/or frequency of occurrence, rather than differences in storm instantaneous flash rates, which only vary by a factor of 2 on average. Regional variability in cell density and cell flash rates closely tracks differences in 85-GHz microwave brightness temperatures. Monotonic relationships are found with the gross moist stability of the tropical atmosphere, a large-scale “adjusted state” parameter. This result strongly suggests that it will be possible, using TRMM observations, to objectively test numerical or theoretical predictions of how mesoscale convective organization interacts with the larger-scale environment. Further parameters are suggested for a complete objective definition of tropical convective regimes.

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Robert Cifelli, Steven A. Rutledge, Dennis J. Boccippio, and Thomas Matejka

Abstract

Vertical motion profiles can be diagnosed with the mass continuity equation using horizontal divergence fields derived from various single-Doppler radar techniques such as EVAD (extended velocity-azimuth display), CEVAD (concurrent extended velocity-azimuth display), and VVP (volume velocity processing). These methods allow for the retrieval of mesoscale air motions in precipitating regions when the wind field is relatively homogeneous. In contrast, VHF wind profiler data can provide a direct measurement of vertical motion, albeit across a much smaller domain compared to the single-Doppler radar techniques. In this study, we compare horizontal divergence and vertical motion patterns derived from the various single-Doppler methods with those obtained from VHF profiler data.

The diagnosed profiles of horizontal divergence and vertical velocity from the single-Doppler (scanning radar) techniques are in qualitative agreement in the lower troposphere but often exhibit large variability at higher levels. Because of less stringent radar echo requirements, the VVP technique often analyzed data above the top of the EVAD-CEVAD analysis domain, resulting in a deeper layer of upper-level divergence. The CEVAD technique often produced a deeper and larger region of upward motion despite similar profiles of divergence, probably due to the CEVAD top boundary condition specification of particle terminal fall speed as opposed to the vertical air motion, as well as to the adjustment procedure employed during the regression solution.

The wind profiler data showed much larger vertical gradients and magnitudes of divergence and vertical velocity when averaged over the same time interval required to collect data for a single-Doppler retrieval. However, when all the available data were composited, the high-frequency variability in the wind profiler retrievals was reduced resulting in relatively good agreement between all analysis methods. The wind profiler usually sampled vertical motion (divergence) several kilometers above the single-Doppler retrievals, which the authors attribute to the stringent precipitation echo coverage requirements imposed by the scanning radar analysis techniques, thus limiting their vertical extent new echo top.

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Daniel J. Cecil, Steven J. Goodman, Dennis J. Boccippio, Edward J. Zipser, and Stephen W. Nesbitt

Abstract

During its first three years, the Tropical Rainfall Measuring Mission (TRMM) satellite observed nearly six million precipitation features. The population of precipitation features is sorted by lightning flash rate, minimum brightness temperature, maximum radar reflectivity, areal extent, and volumetric rainfall. For each of these characteristics, essentially describing the convective intensity or the size of the features, the population is broken into categories consisting of the top 0.001%, top 0.01%, top 0.1%, top 1%, top 2.4%, and remaining 97.6%. The set of “weakest/smallest” features composes 97.6% of the population because that fraction does not have detected lightning, with a minimum detectable flash rate of 0.7 flashes (fl) min−1. The greatest observed flash rate is 1351 fl min−1; the lowest brightness temperatures are 42 K (85 GHz) and 69 K (37 GHz). The largest precipitation feature covers 335 000 km2, and the greatest rainfall from an individual precipitation feature exceeds 2 × 1012 kg h−1 of water. There is considerable overlap between the greatest storms according to different measures of convective intensity. The largest storms are mostly independent of the most intense storms. The set of storms producing the most rainfall is a convolution of the largest and the most intense storms.

This analysis is a composite of the global Tropics and subtropics. Significant variability is known to exist between locations, seasons, and meteorological regimes. Such variability will be examined in Part II. In Part I, only a crude land–ocean separation is made. The known differences in bulk lightning flash rates over land and ocean result from at least two differences in the precipitation feature population: the frequency of occurrence of intense storms and the magnitude of those intense storms that do occur. Even when restricted to storms with the same brightness temperature, same size, or same radar reflectivity aloft, the storms over water are considerably less likely to produce lightning than are comparable storms over land.

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Dennis J. Boccippio, Kenneth L. Cummins, Hugh J. Christian, and Steven J. Goodman

Abstract

Four years of observations from the NASA Optical Transient Detector and Global Atmospherics National Lightning Detection Network are combined to determine the geographic distribution of the climatological intracloud–cloud-to-ground (CG) lightning ratio, termed Z, over the continental United States. The value of Z over this region is 2.64–2.94, with a standard deviation of 1.1–1.3 and anomalies as low as 1.0 or less over the Rocky and Appalachian Mountains and as high as 8–9 in the central-upper Great Plains. There is some indication that Z covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance. The dynamic range of local variability is comparable to the range of values cited by previous studies for latitudinal variation from the deep Tropics to midlatitudes. Local high Z anomalies in the Great Plains are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of large positive CGs characteristic of organized or severe storms. This suggests that storm type, morphology, and level of organization may dominate over environmental cofactors in the local determination of this ratio.

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Walter A. Petersen, Robert Cifelli, Dennis J. Boccippio, Steven A. Rutledge, and Chris Fairall

Abstract

During September–October 2001, the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC-2001) intertropical convergence zone (ITCZ) field campaign focused on studies of deep convection in the warm-pool region of the eastern Pacific. This study combines C-band Doppler radar, sounding, and surface heat flux data collected aboard the R/V Ronald H. Brown during EPIC to describe the kinematic and thermodynamic states of the ITCZ environment, together with tendencies in convective structure, lightning, rainfall, and surface heat fluxes as a function of 3–5-day easterly wave phase.

Three easterly waves were observed at the location of the R/V Brown during EPIC-2001. Wind and thermodynamic data reveal that the wave trough axes exhibited positively correlated u and υ winds, a slight westward phase tilt with height, and relatively strong (weak) northeasterly tropospheric shear following the trough (ridge) axis. Temperature and humidity perturbations exhibited mid- to upper-level cooling (warming) and drying (moistening) in the northerly (trough and southerly) phase. At low levels, warming (cooling) and moistening (drying) occurred in the northerly (southerly) phase.

Composited radar, sounding, lightning, and surface heat flux observations suggest the following systematic behavior as a function of wave phase: zero to one-quarter wavelength ahead of (behind) the wave trough in northerly (southerly) flow, larger (smaller) convective available potential energy (CAPE), lower (higher) convective inhibition (CIN), weaker (stronger) tropospheric shear, larger (smaller) convective rain fractions, higher (lower) conditional mean rain rates, higher (lower) lightning flash densities, and more (less) robust convective vertical structure occurred. Latent and sensible heat fluxes reached a minimum in the northerly phase and then increased through the trough, reaching a peak during the ridge phase (leading the peak in CAPE). Larger areas of light convective and stratiform rain and slightly larger (10%) area-averaged rain rates occurred in the vicinity of, and just behind, the trough axes in southerly and ridge flow. Importantly, the transition in convective structure observed across the trougth axis when considered with the relatively small change in area mean rain rates suggests the presence of a transition in the vertical structure of diabatic heating across the easterly waves examined. The inferred transition in heating structure is supported by radar-diagnosed divergence profiles that exhibit convective (stratiform) characteristics ahead of (behind) the trough.

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