To better connect radar observations to microphysical processes, the authors analyze concurrent polarimetric radar observations at vertical incidence and roughly side incidence during the Front Range Orographic Storms (FROST) project. Data from three events show signatures of riming, aggregation, and dendritic growth. Riming and the growth of graupel are suggested by negative differential reflectivity ZDR and vertically pointing Doppler velocity magnitude |VR| > 2.0 m s−1; aggregation is indicated by maxima in the downward-relative gradient of radar reflectivity at horizontal polarization ZH below the −15°C isotherm and positive downward-relative gradients in |VR| when averaged over time. A signature of positive downward-relative gradients in ZH, negative downward-relative gradients in |VR|, and maxima in ZDR is observed near −15°C during all three events. This signature may be indicative of dendritic growth; preexisting, thick platelike crystals fall faster and grow slower than dendrites, allowin...

Connecting Microphysical Processes in Colorado Winter Storms with Vertical Profiles of Radar Observations

A 4D Barnes objective analysis scheme for wind profiler data is developed in order to improve upon previously developed 2D analysis schemes. A significant shortcoming of the 2D schemes is their sensitivity to gaps in the profiler time‐height series; they may produce unrealistic gradient information if large data-void regions are present. The 4D analysis scheme described herein, however, provides an effective means for dealing with data voids within profiler time‐height series. The 4D analysis scheme differs from the 2D techniques in that data from neighboring profiler stations affect the time‐height wind analysis at each site. This allows the analysis scheme to produce smooth, spatially and temporally consistent time‐height wind analyses for each station of the Wind Profiler Network (WPN), even if large data gaps are present. Gridded time‐height wind fields at each profiler site resulting from the 4D analysis scheme are provided as input to a line integral-equivalent technique that is applied over many WPN triangles for the purpose of diagnosing the kinematic and thermodynamic structure of subsynoptic-scale weather systems. Time‐height series of trianglederived variables have been proven elsewhere to be an effective method for diagnosing subsynoptic-scale temporal structure of weather systems; mapping the irregularly spaced triangle information onto a regular, quasihorizontal grid provides a complementary perspective of their spatial structure and evolution.

http://www.flame.org/~cdoswell/publications/spenceretal_99.pdf

A Four-Dimensional Objective Analysis Scheme and Multitriangle Technique for Wind Profiler Data

Mesoscale height and temperature fields can be extracted from the observed wind field by making use of the full divergence equation. Mass changes associated with irrotational ageostrophic motions are retained for a nearly complete description of the height field. Above the boundary layer, in the absence of friction, the divergence equation includes terms composed of the components of the wind and a Laplacian of the geopotential height field. Once the mass field is determined, the thermal structure is obtained through application of the hypsometric equation. In this paper an error analysis of this divergence method is undertaken to estimate the potential magnitude of errors associated with random errors in the wind data. Previous applications of the divergence method have been refined in the following ways. (i) The domain over which the method is applied is expanded to encompass the entire STORM-FEST domain. (ii) Wind data from 23 profiler and 38 rawinsonde sites are combined in the analysis. (iii) Observed profiler and rawinsonde data are interpolated to grid points through a modified objective analysis, and (iv) the variation in elevation of the profiler sites is taken into account. The results of the application of the divergence method to the combined wind data from profiler and rawinsonde sites show good agreement between the retrieved heights and temperatures and the observed values at rawinsonde sites. Standard deviations of the difference between the retrieved and observed data lie well within the precision of the rawinsonde instruments. The difference field shows features whose magnitude is significantly larger than the errors predicted by the error analysis, and these features are systematic rather than random in nature, suggesting that the retrieved fields are able to resolve mesoscale signatures not fully captured by the rawinsonde data alone. The divergence method is also applied solely to the profiler data to demonstrate the potential of the divergence method to provide mass and thermal fields on a routine basis at synoptic times when operational rawinsonde data are not available. A comparison of the heights derived from the profiler winds with those independently measured by rawinsondes indicates that valuable information on the evolution of atmospheric height and temperature fields can be retrieved between conventional rawinsonde release times through application of the divergence method. The implications of the results for applications of the method in weather analysis and in numerical weather prediction are discussed.

/pdf/extraction-of-geopotential-height-and-temperature-structure-47qbps09ru.pdf

Extraction of Geopotential Height and Temperature Structure from Profiler and Rawinsonde Winds

Abstract Observations of thermodynamic and kinematic parameters associated with derivatives of the thermodynamics and wind fields, namely, advection, vorticity, divergence, and deformation, can be obtained by applying Green’s theorem to a network of observing sites. The five nodes that comprise the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) profiling network, spaced 50–80 km apart, are used to obtain measurements of these parameters over a finite region. To demonstrate the applicability of this technique at this location, it is first applied to gridded model output from the High-Resolution Rapid Refresh (HRRR) numerical weather prediction model, using profiles from the locations of ARM network sites, so that values calculated from this method can be directly compared to finite difference calculations. Good agreement is found between both approaches as well as between the model and values calculated from the observations. Uncertainties for the observations are obtained via a Monte Carlo process in which the profiles are randomly perturbed in accordance with their known error characteristics. The existing size of the ARM network is well suited to capturing these parameters, with strong correlations to model values and smaller uncertainties than a more closely spaced network, yet it is small enough that it avoids the tendency for advection to go to zero over a large area. 

/pdf/observing-profiles-of-derived-kinematic-field-quantities-2gja16n1.pdf

Observing Profiles of Derived Kinematic Field Quantities Using a Network of Profiling Sites

Previous studies have shown that ageostrophic circulations play an important role in frontogenesis phenomena. They result from the maintenance of the balanced state of the atmosphere which tends to be broken down by the large-scale forcing of the atmosphere. Their existence explains how a frontal discontinuity can be produced in a few days. From an experimental and forecasting point of view it would be interesting to obtain these circulations in real time. Unfortunately, the ageostrophic wind is not a measurable quantity and its retrieval from experimental data is difficult, particularly at the mesoscale, since instrumental noise leads to a need for significant data correction.



In this context, this paper provides answers to some of the main problems related to the retrieval of ageostrophic winds in frontal systems. It focuses on the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the future implementation of regular meteorological wind-profiler networks with a sampling time of one hour.



This paper proposes to overcome numerous practical problems using an approach based on the variational Analyse du Vent AGeostrophique (AVAG) analysis. Tests of such an approach are done on real data extracted from the Mesoscale Frontal Dynamic Project/FRONTS 87 data base for a typical two-dimensional frontal case (the IOP 7 case). They confirm that the AVAG analysis is able to provide a non-noisy and consistent ageostrophic circulation. Comparisons with classical methods demonstrate the robustness of AVAG and its capability to obtain physically meaningful ageostrophic wind fields. This study also estimates the best and most suitable experimental sampling for practical applications. Sensitivity tests on the quality of the retrieved wind field with respect to (i) the sampling time used, (ii) the availability of additional thermodynamic data and (iii) the geometrical pattern of the network are presented. These tests provide evidence of the importance of accounting for thermodynamical- and dynamical-field along-front variations.

Retrieval of ageostrophic wind from a radiosounding network and a single ST radar

Horizontal divergence, relative vorticity, kinematic vertical velocity, and geostrophic and ageostrophic winds are computed from Colorado profiler network data to investigate an upslope snowstorm in northeastern Colorado. Horizontal divergence and relative vorticity are computed using the Gauss and Stokes theorems, respectively. Kinematic vertical velocities are obtained from the surface to 9 km by vertically integrating the continuity equation. The geostrophic and ageostrophic winds are computed by applying a finite differencing technique to evaluate the derivatives in the horizontal equations of motion. Comparison of the synoptic-scale data with the profiler network data reveals that the two datasets are generally consistent. Also, the profiler-derived quantities exhibit coherent vertical and temporal patterns consistent with conceptual and theoretical flow fields of various meteorological phenomena. It is suggested that the profiler-derived quantities are of potential use to weather forecasters in that they enable the dynamic and kinematic interpretation of weather system structure to be made and thus have nowcasting and short-term forecasting value.

A Case Study Using Kinematic Quantities Derived from a Triangle of VHF Doppler Wind Profilers

Mesoscale phenomena such as thunderstorm and sea breeze frontal circulations are being investigated using a 50 MHz Doppler wind profiler at the Kennedy Space Center. The profiler installation will begin October 1, 1988 and will be completed by February 17, 1989. The focus of current research and plans for next year include: examination of vertical velocities associated with local thunderstorm activity and sea breeze frontal circulations and compare the vertical velocities to conceptual mesoscale models; implementation of space-time conversion analysis techniques to blend profiler data with National Meteorological Center's model output and other wind data such as jimsphere, windsonde and rawinsonde for mesoscale analysis; development of suggestions for use of wind profiler data in mesoscale analysis and forecasting at Kennedy Space Center; and problems detection in the quality of the profiler data during this research project. Researchers will work closely with MSFC to identify and solve the data quality problems.

/pdf/low-level-remote-sensing-the-doppler-radar-wind-profiler-4g3jcnblww.pdf

Catherine A. Carlson

Papers

A Case Study Using Kinematic Quantities Derived from a Triangle of VHF Doppler Wind Profilers

Low level remote sensing: The Doppler Radar wind profiler