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Lawrence D. Carey
and
Kurt M. Buffalo

Abstract

In this study, it is hypothesized that the mesoscale environment can indirectly control the cloud-to-ground (CG) lightning polarity of severe storms by directly affecting their structural, dynamical, and microphysical properties, which in turn directly control cloud electrification and ground flash polarity. A more specific hypothesis, which has been supported by past observational and laboratory charging studies, suggests that broad, strong updrafts and associated large liquid water contents in severe storms lead to the generation of an inverted charge structure and enhanced +CG lightning production. The corollary is that environmental conditions favoring these kinematic and microphysical characteristics should support severe storms generating an anomalously high (>25%) percentage of +CG lightning (i.e., positive storms) while environmental conditions relatively less favorable should sustain storms characterized by a typical (≤25%) percentage of +CG lightning (i.e., negative storms). Forty-eight inflow proximity soundings were analyzed to characterize the environment of nine distinct mesoscale regions of severe storms (4 positive and 5 negative) on 6 days during May–June 2002 over the central United States. This analysis clearly demonstrated significant and systematic differences in the mesoscale environments of positive and negative storms, which were consistent with the stated hypothesis. When compared to negative storms, positive storms occurred in environments associated with a drier low to midtroposphere, higher cloud-base height, smaller warm cloud depth, stronger conditional instability, larger 0–3 km AGL wind shear, stronger 0–2 km AGL storm relative wind speed, and larger buoyancy in the mixed-phase zone, at a statistically significant level. Differences in the warm cloud depth of positive and negative storms were by far the most dramatic, suggesting an important role for this parameter in controlling CG lightning polarity. In this study, strong correlations between the mesoscale environment and CG lightning polarity were demonstrated. However, causality could not be verified due to a lack of in situ observations to confirm the hypothesized microphysical, dynamical, and electrical responses to variations in environmental conditions that ultimately determined the dominant CG polarity. Future observational field programs and cloud modeling studies should focus on these critical intermediary processes.

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Lawrence D. Carey
and
Steven A. Rutledge

Abstract

One of the primary scientific objectives of the Maritime Continent Thunderstorm Experiment was to study cloud electrification processes in tropical island convection, in particular, the coupling between ice phase precipitation and lightning production. To accomplish this goal, a C-band polarimetric radar was deployed in the Tropics (11.6°S, 130.8°E) for the first time, accompanied by a suite of lightning measurements. Using observations of the propagation-corrected horizontal reflectivity and differential reflectivity, along with specific differential phase, rain and ice masses were estimated during the entire life cycle of an electrically active tropical convective complex (known locally as Hector) over the Tiwi Islands on 28 November 1995. Hector’s precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field, cloud-to-ground (CG) and total lightning flash rates, and ground strike locations.

During Hector’s developing stage, precipitating convective cells along island sea breezes were dominated by warm rain processes. No significant electric fields or lightning were associated with this stage of Hector, despite substantial rainfall rates. Aided by gust front forcing, a cumulus merger process resulted in larger, taller, and more intense convective complexes that were dominated by mixed-phase precipitation processes. During the mature phase of Hector, lightning and the surface electric field were strongly correlated to the mixed phase ice mass and rainfall. Merged convective complexes produced 97% of the rainfall and mixed-phase ice mass and 100% of the CG lightning. As Hector dissipated, lightning activity rapidly ceased.

As evidenced from the multiparameter radar observations, the multicell nature of Hector resulted in the continuous lofting of supercooled drops to temperatures between −10° and −20°C in discrete updraft cores during both the early and mature phases. The freezing of these drops provided instantaneous precipitation-sized ice particles that may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism.

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Jian-Jian Wang
and
Lawrence D. Carey

Abstract

A primary goal of the South China Sea Monsoon Experiment (SCSMEX), a major field campaign of the Tropical Rainfall Measuring Mission (TRMM), is to define the initiation, structure, evolution, and dynamics of precipitation processes associated with the onset of the South China Sea (SCS) summer monsoon. In this study, dual-Doppler and dual-polarimetric radar analysis techniques are used to investigate the development and structure of a squall-line system observed on 24 May 1998. The focus is the linkage between the airflow and the microphysical fields through the system.

The squall-line system, including three distinct lines, persisted from 1200 UTC 24 May to the following day. A detailed study was performed on the structure of the second and most intense line, lasting for over 10 h. Compared to tropical squall lines observed in other regions, this narrow squall-line system had some interesting features including 1) maximum reflectivity as high as 55 dBZ; 2) relatively little stratiform rainfall that preceded instead of trailed the convective line; and 3) a broad vertical velocity maximum in the rear part of the system, rather than a narrow ribbon of vertical velocity maximum near the leading edge.

Polarimetric radar–inferred microphysical (e.g., hydrometeor type, amount, and size) and rainfall properties are placed in the context of the mesoscale morphology and dual-Doppler-derived kinematics for this squall-line system. A comparison is made between results from this study for SCSMEX and the previous studies for the TRMM Large-Scale Biosphere–Atmosphere experiment (LBA). It was found that precipitation over the SCS monsoon region during the summer monsoon onset was similar to the precipitation over the Amazon monsoon region during the westerly regime of the TRMM–LBA, which has previously been found to be closer to typical conditions over tropical oceans. Both of these cases showed lower rain rates and rainwater contents, smaller raindrops, and significantly lower ice water contents between 5 and 8 km than the precipitation over the Amazon during the easterly regime of the TRMM–LBA with more tropical continental characteristics.

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Lawrence D. Burroughs
and
Samson Brand

Abstract

Twenty-five years of tropical cyclone data (1945–69) for the western North Pacific were evaluated to determine the, speed-of-movement characteristics of tropical storms and typhoons following recurvature. The results show that the acceleration of storms following recurvature is dependent on the meteorological characteristics of the storm, and the surrounding synoptic environment which is a function of the time of the year. Forecast equations derived by linear regression techniques are presented for the speed of movement of tropical cyclones 36 hr after recurvature.

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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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M. Lawrence Nicodemus
and
James D. McQuigg

Abstract

A simulation model is presented which hopefully is consistent with known physical and statistical properties of atmospheric events, and consistent with criteria that might be applied in the management of actual experiments in the atmosphere. The process being simulated is the possible modification of daytime surface temperatures during the summer in central Missoui through the generation of contrail cirrus clouds. Monte Carlo techniques are used in the model to allow for the likelihood of failure of the experiment on any particular day, and to allow for variations in the degree of success on days when the experiment is considered to not be a failure.

The model is applied to an observed time series (1946-1965) of surface and upper air observations from Columbia, Mo. Estimates of the results are based on analysis of the relationship between temperatures on the cirrus and cirrus-free days. If it can be assumed that it is possible to create enough contrial cirrus to reduce the per cent of possible sushine from 15-35%, it appears that it might be possible at reduce daily maximum temperatures by from 3-5F on about half of the days when soil moisture values are below "desirable" levels of when temperatures are expected to be above some "critical" level

This is a relatively inexpensive way to estimate the order of magnitude of the effect of weather modification, compared to the cost of conducting an actual experiment over a long period of time.

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P. D. Thompson
and
W. Lawrence Gates

Abstract

A two-parameter baroclinic model for vertically integrated flow is derived under the assumption of an isogonal geostrophic wind shear, and constitutes a generalization of the theory of equivalent baroclinic flow. The computational procedures used to apply both this theory and that for purely barotropic flow to the production of numerical forecasts are described in detail, and this is followed by a brief summary of the results of an extensive series of numerical integrations of the equations of both the barotropic and thermotropic models. In all, a total of 120 24-hour forecasts were made every 12 hr from a series of especially prepared maps throughout the entire month of January 1953, the largest collection of numerical forecasts assembled to the present time.

From a preliminary summary of the results supported by the discussion of several individual cases, it appears that the major characteristics of this series of numerical forecasts are: (1) the lack of a significant difference between the barotropic and thermotropic 500-millibar forecasts, the correlation between the predicted and observed 24-hr 500-mb height changes being 0.74 in each case, (2) the marked effects of a number of purely mathematical errors or errors of method, especially those introduced by the lateral boundary conditions and by the use of the finite-difference approximation, and (3) an important effect of the Rocky Mountain barrier on the forecasts. These results are interpreted as demonstrating the quasi-barotropy of the atmosphere at mid-tropospheric levels when considered on a day-to-day basis over a monthly period. The 1000-mb forecasts computed from the baroclinic theory, while displaying an average correlation between predicted and observed 24-hr height changes of only 0.62, show considerable synoptic “skill,” especially over the eastern United States. These forecasts are felt to demonstrate conclusively the applicability of numerical prediction techniques to operational forecasting. Further research on the effects of both the physical and mathematical approximations of the methods of numerical prediction is suggested.

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Lawrence D. Burroughs
and
Robert N. Larson

Abstract

No abstract available.

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Lawrence B. Dunn
and
John D. Horel

Abstract

The utility of numerical model guidance produced by the National Meteorological Center has been evaluated for the forecast of convection over central Arizona during the summer monsoon season. Model output from the Nested Grid Model (NGM) and Eta model has been compared to observations taken during the 1990 field experiment referred to as the Southwest Area Monsoon Project (SWAMP).

The NGM precipitation forecasts showed little skill for events in which heavy precipitation was observed over Phoenix, Arizona. Selected events during the SWAMP period were simulated using the Eta model. Qualitative comparisons of the Eta model's precipitation forecasts with lightning data and satellite imagery suggest that the model has little skill over Arizona during the warm season. Nocturnal heavy precipitation over the lower deserts of central Arizona is nearly always preceded by afternoon convection over the mountains to the north and east. The convection over the mountains was absent in the model.

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Lawrence B. Dunn
and
John D. Horel

Abstract

Output from simulations of the Eta model are compared to special observations collected during the 1990 Southwest Area Monsoon Project (SWAMP). The emphasis is on the model's prediction of the preconvection air mass over Phoenix, Arizona, and on the model's representation of the low-level jet and moisture surge observed over southwest Arizona.

At times the model fails to capture the rapid increase in low- and mid-level moisture that is observed in the hours prior to the onset of convection. Subsequent convection is not predicted by the Eta model. In one event the model very accurately predicts the evolution of the air mass over Phoenix during the period just prior to the outbreak of severe convection. However, no convection is predicted by the model. The model seems unable to generate convection over the high terrain or lower deserts of central Arizona regardless of whether the air mass is simulated correctly.

A low-level jet feature observed over southwest Arizona during SWAMP is not correctly simulated by the Eta model. The model produces a very strong sea-breeze circulation from the Gulf of California into western Arizona in each simulation. The moisture and stability profiles associated with the sea-breeze are inconsistent with observations over southwest Arizona, which leads to a misrepresentation of the low- and midlevel moisture field over the region. Poor initial conditions in the sea surface temperature field over the Gulf of California are, at least in part, responsible for the model error.

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