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Tina J. Cartwright
and
Peter S. Ray

Abstract

Atmospheric warming from cloud heating has a major affect on worldwide atmospheric circulations and climate. Studies have shown that the dominant source for cloud heating is the phase change of water. The location and magnitude of cloud heating has a substantial impact on atmospheric circulations. Therefore, identifying the location of phase changes provides information necessary for accurate modeling of atmospheric circulations and climate.

Radar reflectivity is a signature predominantly produced from rain formed from condensation, the primary process that produces heating. Through the application of principal component analysis on a nonhydrostatic cloud model, heating, and derived reflectivity data, a technique to illustrate a future heating algorithm capable of estimating cloud heating from reflectivity data is examined. Formative, intensifying, and mature stages of a Convection and Precipitation Electrification Experiment squall-type convective system were used to demonstrate these results. The accuracy of the technique’s estimates for the mean convective and stratiform profiles to within 1.0 K h−1 on average throughout the vertical column shows the merit of this statistical technique. The use of this type of technique in conjunction with the network of NEXRAD and spaceborne radars could provide valuable data for applications ranging from cumulus parameterization to 4D data assimilation and model initialization.

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Peter S. Ray
and
Karen L. Sangren

Abstract

Observing programs utilizing Doppler radar must have them deployed in optimum locations to best satisfy experimental objectives and maximize economies. One wishes to determine the coordinate triples (xi , yi , zi ), where i equals the number of radars, which maximize the value of the data to be collected. The optimum location is governed by a value or objective function. Here, possible networks of two to nine radars are given for two different error specifications. The objective functions with both error distributions maximize the quantity (AREAL COVERAGE/ERROR). The procedure is to search the finite number of local maxima for the global maximum in the value of the objective function. This is done by employing a searching algorithm at each of a number of starting vectors which are close enough to the local maxima to converge to the desired local maxima. In all cases, the network obtained by considering all radars simultaneously is superior to that obtained by combining optimum smaller sub-networks. Our results suggest the expected benefits for networks with additional constraints, reflecting the more complex experimental objectives particular to some individual field program. For example, the number of radars needed and their optimal configuration can be determined for a field program requiring a specified areal coverage (probability that a desired event will occur) and resolution (to retrieve a specified scale of motion).

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Peter S. Ray
,
Alan Robinson
, and
Ying Lin

Abstract

During the Taiwan Area Mesoscale Experiment (TAMEX), three Doppler radars complemented enhanced surface and upper-air observations. The focus of the experiment was to better understand the interaction of the terrain with precipitation systems in the production of the important heavy rainfall. The intensive operational period (IOP) number 8 extended from 1400 IST (local standard time) 7 June 1987 until 0800 LST 9 June 1987. During this time, a mesoscale convective system (MCS) formed in the Straits of Taiwan and moved inland. It was interrogated by many observing instruments, including three Doppler radars, over a 6-h period. During this time the front moved through the radar network. The front was shallow and the precipitation widespread, both ahead of and behind the front. The front was only 1.6-km deep over a distance of 100 km.

Using velocity-azimuth display (VAD) data, a portion of the frontogenetic function was computed during the times the front was in the vicinity of the radar. The increase in both convergence and deformation contributed to large values of the frontogenetic function.

Dynamic retrieval was also attempted on the data during the time when the front was most favorably located for analysis. The results are very similar to what has been observed both for tropical squall lines and for midlatitude squall lines.

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Carl E. Hane
and
Peter S. Ray

Abstract

A method for retrieval of pressure and buoyancy distributions in deep convection is applied to Doppler radar data collected at two analysis times during the tornadic Del City (Oklahoma) thunderstorm of 20 May 1977. Change of a previous version of the technique, necessitated by application to real data, include procedures for handling irregularly-bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations. Internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields.

In the pretornadic stage the pressure distribution includes at each level a high–low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude. These results are in agreement with predictions of linear theory. Locations of vorticity maxima and areas of updraft development are also discussed in relation to pressure distributions. The buoyancy distribution includes a good correspondence between positive buoyancy and updraft areas. An analysis of the individual terms in the buoyancy equation reveals the importance of advective and vertical pressure gradient terms over water-related and turbulence terms.

In the tornadic stage the pressure field includes a pronounced minimum at low levels coincident with the mesocyclone. An analysis of the factors influencing the pressure distribution reveals that strong low-level vertical vorticity produces this minimum. Vorticity, vertical motion, and pressure relationships in the low-level mesocyclone region tend to agree quite well with results of recent fine-scale numerical simulations as well as with the observationally-based finding of others. The low-level buoyancy field, although noisier at this stage, tends to support the line of reasoning which stress the production of horizontal vorticity as a major factor in low-level mesocyclone development.

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Peter S. Ray
and
David P. Jorgensen

Abstract

Observations with airborne Doppler radar can expand the area of coverage and extend the time a moving weather system can remain under observation. Also, additional analysis methods are possible with the increase in independent estimates of the wind field that can be provided by an airborne sampling system. However, the advantages of airborne Doppler sensing are constrained by the geometry in which the data are collected, as well as errors introduced by uncertainties in the sampling platform location and orientation. Finally, a longer time required to sample a region than is typical for ground-based radar results in increased uncertainties due to the field's evolution and advection during the sampling interval. Uncertainties related to geometry are examined for flight patterns which are for aircraft alone and for those which also utilize data from one and two ground-based radars. These illustrate the distribution and relative magnitude of uncertainty expected for each type of flight pattern and data analysis method. Both the NOAA P−3, and the NCAR ELDORA scanning methodologies are examined.

To evaluate the different flight patterns, a relative quality index is used. It is defined as the reciprocal of the vertical velocity error variance integrated over the analysis domain. This normalized relative quality index is a mean value over the sampled volume. Flight patterns that utilize a single ground-based radar provide coverage over ∼ ten times the area in about one-half the time, and with relative quality about ten times better than that by aircraft alone.

Data collection, particularly aircraft data collection, often involves real-time decision making, and storms frequently are not in an ideal location relative to fixed ground-based radars. The best operational decisions require knowledge of eventual synthesis capabilities and the location of the volume to be interrogated relative to those facilities. These concepts are illustrated in a case example. Airborne Doppler and ground-based radar synthesis results are compared and discussed.

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J. Marshall Shepherd
,
Brad S. Ferrier
, and
Peter S. Ray

Abstract

Central Florida is the ideal test laboratory for studying convergence zone–induced convection. The region regularly experiences sea-breeze fronts and rainfall-induced outflow boundaries. The focus of this study is convection associated with the commonly occurring convergence zone established by the interaction of the sea-breeze front and an outflow boundary. Previous studies have investigated mechanisms primarily affecting storm initiation by such convergence zones. Few have focused on rainfall morphology, yet these storms contribute a significant amount of precipitation to the annual rainfall budget. Low-level convergence and midtropospheric moisture have been shown to be correlated with rainfall amounts in Florida. Using 2D and 3D numerical simulations, the roles of low-level convergence and midtropospheric moisture in rainfall evolution are examined.

The results indicate that area- and time-averaged, vertical moisture flux (VMF) at the sea-breeze front–outflow convergence zone is directly and linearly proportional to initial condensation rates. A similar relationship exists between VMF and initial rainfall. The VMF, which encompasses depth and magnitude of convergence, is better correlated to initial rainfall production than surface moisture convergence. This extends early observational studies that linked rainfall in Florida to surface moisture convergence. The amount and distribution of midtropospheric moisture affects how much rainfall associated with secondary cells develop. Rainfall amount and efficiency varied significantly over an observable range of relative humidities in the 850–500-mb layer even though rainfall evolution was similar during the initial or “first cell” period. Rainfall variability was attributed to drier midtropospheric environments inhibiting secondary cell development through entrainment effects. Observationally, a 850–500-mb moisture structure exhibits wider variability than lower-level moisture, which is virtually always present in Florida. A likely consequence of the variability in 850–500-mb moisture is a stronger statistical correlation to rainfall as noted in previous observational studies.

The VMF at convergence zones is critical in determining rainfall in the initial stage of development but plays a decreasing role in rainfall evolution as the system matures. The midtropospheric moisture (e.g., environment) plays an increasing role in rainfall evolution as the system matures. This suggests the need to improve measurements of depth and magnitude of convergence and midtropospheric moisture distribution. It also highlights that the influence of the environment needs to be better represented in convective parameterizations of larger-scale models to account for entrainment effects.

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Carl E. Hane
,
Cathy J. Kessinger
, and
Peter,S. Ray

Abstract

Mechanisms for maintenance of the strong convection along the leading edge of a broad squall line that occurred in Oklahoma on 19 May 1977 are investigated. The findings are based upon analysis of data from a surveillance radar, a surface mesonetwork, Doppler radars, proximity soundings and aircraft data, and upon the results of a two-dimensional, cloud-scale numerical simulation. The detailed results of the multiple Doppler analysis are contained in the Part I paper reporting results of research on this squall line.

It is found that at a preferred location along the squall line, an area of intense convection is maintained over a long time period. A meso-β scale organized structure, which includes an area of low pressure near the southeast edge of the intense convection and an associated area of convergence extending to the east, promotes the formation of small showers in short line segments. These showers, due to their differing motion from elements within the main line, merge with the line to the north of the mesolow, resulting in maintenance of the strong area of convection. The observed meso-β structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content.

At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head. These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields. Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.

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Brenda C. Johnson
,
Judith Stokes
, and
Peter S. Ray

Abstract

Optimum design of a Doppler radar system for operation in a severe storm environment will depend on the maximum unambiguous velocity. Radial velocities of severe storms are examined from four Doppler radars over several hours on 20 May 1977. The probability of a radial velocity occurrence for a given pulse repetition frequency-wavelength combination is presented.

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Conrad L. Ziegler
,
Peter S. Ray
, and
Nancy C. Knight

Abstract

Hail growth in an Oklahoma multicellular storm is studied using a numerical model of hailstone growth and decay and dual-Doppler derived wind fields. Hail was collected at the time of the Doppler radar data collection which provided input for computation of the modeled trajectories. A unique feature of this investigation is the subsequent comparison of model hail trajectories with deuterium derived trajectories obtained from the hail samples. Formation of large hail is found to be almost entirely due to injection of embryos into the major storm updraft from the upwind side, with subsequent growth occurring during repeated vertical excursions through the prime growth layer between 7 and 8 km. Primary embryo source regions are a feeder cell and the precipitation debris region between the feeder and main cells. Qualitative comparisons between observed and modeled hailstones falling near the collection site reveal strong similarities, particularly with respect to ambient temperature during ice formation and layer structure. Horizontal advection of hail across the updraft during growth is typical, so that particle recirculation in a singe updraft is unimportant for hail growth. Observed hail size distributions are related to the distributions of modeled hailstones at the ground. Either a modal or “leveling-off” tendency is evident in each of the hail samples, whose shapes agree qualitatively with the distribution of numerically simulated large hail falling in the vicinity of the storm core at the surface. The gamma function is found to generally provide a better fit to the sample distributions than the Marshall-Palmer function.

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Ying Lin
,
Peter S. Ray
, and
Kenneth W. Johnson

Abstract

A method is developed to initialize convective storm simulations with Doppler radar-derived fields. Input fields for initialization include velocity, rainwater derived from radar reflectivity, and pressure and temperature fields obtained through thermodynamic retrieval. A procedure has been developed to fill in missing wind data, followed by a variational adjustment to the filled wind field to minimize “shocks” that would otherwise cause the simulated fields to deteriorate rapidly.

A series of experiments using data from a simulated storm establishes the feasibility of the initialization method. Multiple-Doppler radar observations from the 20 May 1977 Del City tornadic storm are used for the initialization experiments. Simulation results are shown and compared to observations taken at a later time. The simulated storm shows good agreement with the subsequent observations, though the simulated storm appears to be evolving faster than observed. Possible reasons for the discrepancies are discussed.

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