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Christopher Lucas and Richard E. Orville

Abstract

A lightning detection network composed of three direction finders was installed in the western Pacific during TOGA COARE. The results are reported from one direction finder, at Kavieng, Papua New Guinea, for the months of January and February 1993, the latter half of the TOGA COARF 4-month period. Land and ocean sectors were defined. The land–ocean cloud-to-ground lightning ratio for 57 days of data is 8.7. The time between the two highest flash count days is 30–40 days, suggestive of the 30–60-day wave previously identified by Madden and Julian (1972). The highest lightning activity occurs around local midnight for both land and ocean sectors. The peak in activity of cloud-to-ground lightning over the ocean leads the peak in cold cloud area by 3–4 h. A small peak in lightning activity over the land sector occurs around 1500 LST, indicating the influence of the diurnal cycle of beating on convective activity around large tropical islands.

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Christopher Lucas and Edward J. Zipser

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This study provides quantitative estimates of the thermodynamic and kinematic structures of the troposphere during various convective regimes observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The data source is the upper air soundings from six stations in the intensive flux array. A correction algorithm has been applied to the humidity data to remove biases between the stations. The data are analyzed using the nonhierarchical clustering method known as k means. Eleven thermodynamic clusters and 20 kinematic clusters are selected.

The thermodynamic clusters are grouped into four general categories based on their midtropospheric equivalent potential temperature. Deep convective activity varies with the thermodynamic structure of the environment. When the “dry intrusion” group is observed, convection is suppressed. The “fair weather” category corresponds to undisturbed periods with light winds and small mesoscale convective systems (MCSs). The largest MCSs and the majority of the rainfall occur with the “active” and “convective recovery” categories.

The kinematic clusters are also divided into four general categories based on the strength, direction, and depth of the low-level zonal flow. The timing of the clusters is related to the intraseasonal oscillation (ISO). Dry phases of the ISO are characterized by the “low-level easterly” category. During transition periods between the easterly and westerly phases of the ISO, the “calm” category is often seen. The “moderate shear westerly” group is seen just before the strongest westerlies. The majority of the clusters fall into the “strong shear westerly” group, associated with the peak westerly phase of the ISO.

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Christopher Lucas, Peter T. May, and Robert A. Vincent

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An algorithm to detect frontal zones in time–height cross sections of horizontal wind from wind profiler measurements is described. The algorithm works by identifying regions with 1) a strong horizontal temperature gradient, estimated by using a quasigeostrophic thermal wind retrieval, 2) a strong temporal increase in the signal-to-noise ratio at a given range gate, and/or 3) a strong temporal shift in the horizontal winds at a given range gate. The type (e.g., cold or warm) of front is determined by examining the advection field and the characteristics of the boundary. Most weight is given to the horizontal temperature gradient component of the algorithm.

A springtime frontal system and an associated baroclinic wave over South Australia are examined using both routine synoptic observations and analyses as well as data from the profiler. Synoptic observations depict a prefrontal trough and two cold fronts at the surface and a deep trough in upper levels. The tropopause is identified at ∼6 km in one sounding. The algorithm successfully identifies the one main cold front and the lowered tropopause in the polar air. There are also hints of a prefrontal trough and a descending tropopause with the onset of the main cold front. After the passage of the upper trough, the ascending tropopause and the so-called jet front or trailing front are also identified by the algorithm. The latter represents the passage of the upper-level baroclinic wave and the reappearance of a strong jet stream.

Other regions are spuriously identified as fronts. These regions could be the reflection of some short-term meteorological phenomena, such as gravity waves; deviations from the assumed quasi-geostrophy; or simply reflections of noise in the analysis. An examination of the effect of random measurement uncertainties on the frontal analysis gives an estimate of error of around 2 K (100 km)−1 in the horizontal temperature gradient calculations for typical wind errors. The errors on the retrieved advection vary, depending on the wind speed, but are around 25 K day−1 for a ∼20 m s−1 wind speed. These values are typical of the noise in those fields, suggesting that the spuriously defined fronts likely reflect uncertainties in the data rather than actual meteorological phenomena.

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Margaret A. Lemone, Tae Y. Chang, and Christopher Lucas

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No abstract available.

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Christopher Lucas, Edward J. Zipser, and Margaret A. LeMone

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No abstract available.

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Christopher Lucas, Edward J. Zipser, and Margaret A. Lemone

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William E. Lewis, Christopher Rozoff, Stefano Alessandrini, and Luca Delle Monache

Abstract

The performance of the Hurricane Weather Research and Forecasting (HWRF) Model Rapid Intensification Analog Ensemble (RI-AnEn) is evaluated for real-time forecasts made during the National Oceanic and Atmospheric Administration (NOAA)’s 2018 Hurricane Forecast Improvement Program (HFIP) demonstration. Using a variety of assessment tools (Brier skill score, reliability diagrams, ROC curves, ROC skill scores), RI-AnEn is shown to perform competitively compared to both the deterministic HWRF and current operational probabilistic RI forecast aids. The assessment is extended to include forecasts from the 2017 HFIP demonstration and shows that RI-AnEn is the only model with significant RI forecast skill at all lead times in the Atlantic and eastern Pacific basins. Though RI-AnEn is overconfident in its RI forecasts, it is generally well calibrated for all lead times. Furthermore, significance testing indicates that for the 2017–18 Atlantic and eastern Pacific sample, RI-AnEn is more skillful than HWRF at all lead times and better than most of the other probabilistic guidance at 48 and 72 h. ROC curves reveal that RI-AnEn offers a good combination of sensitivity and specificity, performing comparably to SHIPS-RII at all lead times in both basins. With respect to specific high-impact cases from the 2018 Atlantic season, performance of RI-AnEn ranges from excellent (Hurricane Michael) to poor (Hurricane Florence). The multiyear assessment and results for two high-impact case studies from 2018 indicate that, while promising, RI-AnEn requires further work to refine its performance as well as to accurately situate its effectiveness relative to other RI forecasts aids.

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Christopher Lucas, Edward J. Zipser, and Margaret A. Lemone

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Time series of 1-Hz vertical velocity data collected during aircraft penetrations of oceanic cumulonimbus clouds over the western Pacific warm pool as part of the Equatorial Mesoscale Experiment (EMEX) are analyzed for updraft and downdraft events called cores. An updraft core is defined as occurring whenever the vertical velocity exceeds 1 m s−1 for at least 500 m. A downdraft core is defined analogously. Over 19 000 km of straight and level flight legs are used in the analysis. Five hundred eleven updraft cores and 253 downdraft cores are included in the dataset.

Core properties are summarized as distributions of average and maximum vertical velocity, diameter, and mass flux in four altitude intervals between 0.2 and 5.8 km. Distributions are approximately lognormal at all levels. Examination of the variation of the statistics with height suggests a maximum in vertical velocity between 2 and 3 km; slightly lower or equal vertical velocity is indicated at 5 km. Near the freezing level, virtual temperature deviations are found to be slightly positive for both updraft and downdraft cores. The excess in updraft cores is much smaller than that predicted by parcel theory.

Comparisons with other studies that use the same analysis technique reveal that EMEX cores have approximately the same strength as cores of other oceanic areas, despite warmer sea surface temperatures. Diameter and mass flux are greater than those in GATE but smaller than those in hurricane rainbands. Oceanic cores are much weaker and appear to be slightly smaller than those observed over land during the Thunderstorm Project.

The markedly weaker oceanic vertical velocities below 5.8 km (compared to the continental cores) cannot be attributed to smaller total convective available potential energy or to very high water loading. Rather, the authors suggest that water loading, although less than adiabatic, is more effective in reducing buoyancy of oceanic cores because of the smaller potential buoyancy below 5.8 km. Entrainment appears to be more effective in reducing buoyancy to well below adiabatic values in oceanic cores, a result consistent with the smaller oceanic core diameters in the lower cloud layer. It is speculated further that core diameters are related to boundary layer depth, which is clearly smaller over the oceans.

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Christopher Lucas, Edward J. Zipser, and Brad S. Ferrier

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Two-dimensional experiments using the Goddard Cumulus Ensemble model are performed in order to examine the influence of environmental profiles of wind and humidity on the dynamical and microphysical structure of mesoscale convective systems (MCSs) over the tropical oceans. The initial environments used in this study are derived from the results of a cluster analysis of the TOGA COARE sounding data. The model data are analyzed with methods and measurements similar to those used in observational studies.

Experiments to test the sensitivity of MCSs to the thermodynamic profile focus on the role of humidity in the free troposphere. In the experiments, a constant amount of relative humidity is added to every level above the boundary layer. As humidity is increased, model storms transition from weak, unsteady systems with little precipitation to strong, upshear-tilted systems with copious rainfall. This behavior is hypothesized to be the result of the entrainment of environmental air into the updraft cores.

Experiments to test the sensitivity of MCSs to the kinematic profile focus on the amount of vertical wind shear in the midlevels, between approximately 2 and 10 km. Five kinematic profiles are used. The dynamical and microphysical characteristics of the runs changed dramatically in different shear environments. Shear in the midlevels affects the convective systems by altering the perturbation pressure field. Stronger shear results in a broader and deeper mesolow below the updraft and a more intense dynamic high above the leading edge.

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Christopher Lucas, Andrew D. MacKinnon, Robert A. Vincent, and Peter T. May

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The retrieval of raindrop size distributions (DSDs) in precipitation using boundary layer wind profiler operating at VHF is described. To make the retrievals, a Fourier transform–based deconvolution technique, optimized to run with little human input, is used. The sensitivities of the technique and its overall accuracy are investigated using simulated spectra. The retrievals have an error that depends on the drop diameter, with relative errors varying between ∼10% and 35%. An overall average negative bias of about ∼20% is also found. The magnitude and direction of this bias depend on the spectral width of the input spectrum.

The radar and methodology are applied to a case study of a convective cell. Retrievals are made with ∼300 m resolution between 800 and 4600 m. The temporal resolution is 2 min. Comparisons with a rain gauge show that both the magnitude and timing of the precipitation are well captured by the radar. The relationship between the observed rain rate and exponential fits applied to the DSDs agrees very well with previously published studies. A careful analysis of the characteristics of the DSDs within the descending rainshafts provides direct observations of drop size sorting within the precipitation and the formation of hybrid DSDs formed by the overlapping of consecutive rainshafts.

This study highlights the potential of the boundary layer profiler in precipitation studies. Some drawbacks exist, such as the wide beam of the radar, which increases the spectral width of the radar and limits its use in windy conditions. However, when observations are available, they appear to be of high quality and fill a gap in observations unavailable to more conventional wind profilers. In the future, it is hoped that refinements in the technique will allow the temporal resolution of the radar to be increased and the quality of the retrievals to be improved.

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