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Alain Protat and Isztar Zawadzki

Abstract

Recently, Protat and Zawadzki described an analysis method to retrieve the three wind components and their temporal derivatives from measurements collected by a bistatic multiple-Doppler radar network deployed around Montreal for nowcasting and research purposes. In the present paper, an extension of this method to retrieve the corresponding pressure and potential temperature perturbations is presented. The method consists of adding the three projections of the momentum equations as weak constraints to the minimization procedure, as is classically done. An evaluation of the performance of this basic constraining model indicates that, after minimization, the residuals of the horizontal momentum equations are of the same order of magnitude as the dominant terms of these equations. It is then shown that including the vorticity equation as an additional constraint substantially improves the perturbation pressure and temperature solutions, leading to negligible residuals of the horizontal momentum equations. This is due to the fact that the vorticity equation is equivalent to the condition that pressure derives from a potential (the second-order horizontal cross-derivatives of pressure are equal), which ensures that the problem has a mathematical solution.

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Alain Protat and Isztar Zawadzki

Abstract

A variational method for the retrieval of the 3D wind field from bistatic multiple-Doppler radar network data is developed, and its performance is evaluated. This bistatic network consists of one S-band weather radar and two passive low-gain receivers at remote sites. To allow for measurement error, the method uses the Doppler velocities of the three receivers as weak constraints and uses the continuity equation as a strong constraint in a cost function in which the two horizontal wind components are the control variables. Improvements are brought to the classical upward integration of the continuity equation, using a weighted combination of upward and downward integrations and its adjoint. A unique characteristic of a bistatic network is that all Doppler velocity measurements from individual resolution volumes are collected simultaneously, which minimizes the errors on the vertical wind component arising from the local evolution of the airflow. However, the time required to sample a complete weather volume with a Doppler radar (typically 5 min) represents another source of error that must be accounted for. Consequently, linear time interpolation of the measurements to a single reference time is used.

Finally, evaluation of the volume scanning strategy for aviation weather services shows that two retrieval modes can be processed in parallel, using the McGill University radar scanning strategy: a “fast” mode that provides the 3D wind field below 3-km height every 2.5 min and a “standard” mode that provides the 3D wind field for the full volume every 5 min.

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Alain Protat, Isztar Zawadzki, and Alain Caya

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In this paper, the authors examine the kinematic and thermodynamic characteristics of a shallow hailstorm sampled by the McGill bistatic multiple-Doppler radar network on 26 May 1997. This storm consists of two main shallow convective cells (depth less than 5 km) aligned along a SW–NE convective line propagating to the southeast. The authors also analyze the interactions between the two cells during the life cycle of the convective line. In particular it is shown that dynamic interactions play a major role in the intensification of the second cell. This storm is found to evolve in a manner that shares some characteristics with both multicell and supercell storms. A rotating updraft associated with a mesocyclone develops in the mature stage of the storm, which is characteristic of a supercell. However, the lack of a “vault” structure on the precipitation field, the relatively fast evolution of the cells, and other characteristics detailed henceforth seem to indicate that this storm only shares a few of the typical characteristics of supercells. Some morphological and thermodynamic similarities are found between this storm and recent numerical simulations of shallow supercell storms. While the first cell starts dissipating, a cold downward rear inflow is developing, which resembles the “rear-flank” downdraft documented in several numerical and observational studies of tornadic storms. This downdraft acts to intensify the updraft associated with the second cell and produces a precipitation overhang within which hail eventually forms. When this pocket of hail falls to the ground a bit later, it accelerates the low-level rear inflow that progressively cuts off the inflow ahead of the storm, leading to the progressive dissipation of the second cell.

The physical processes involved in the evolution of rotation at low levels to midlevels within this storm are evaluated using the vorticity equation. It is shown that the time tendency of the positive and negative vertical vorticity anomalies associated with the two cells are mainly driven by tilting of horizontal vorticity. Strong negative vertical vorticity associated with the intensification of the rear-flank downdraft in the later stage of the storm is also produced by tilting, which is consistent with previous studies of tornadic storms.

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Gerald G. Mace and Alain Protat

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The properties of clouds derived from measurements collected using a suite of remote sensors on board the Australian R/V Investigator during a 5-week voyage into the Southern Ocean during March and April 2016 are examined. Based on the findings presented in a companion paper (Part I), we focus our attention on a subset of marine boundary layer (MBL) clouds that form a substantial portion of the cloud-coverage fraction. We find that the MBL clouds that dominate the coverage fraction tend to occur in decoupled boundary layers near the base of marine inversions. The thermodynamic conditions under which these clouds are found are reminiscent of marine stratocumulus studied extensively in the subtropical eastern ocean basins except that here they are often supercooled with a rare presence of the ice phase, quite tenuous in terms of their physical properties, rarely drizzling, and tend to occur in migratory high pressure systems in cold-air advection. We develop a simple cloud property retrieval algorithm that uses as input the lidar-attenuated backscatter, the W-band radar reflectivity, and the 31-GHz brightness temperature. We find that the stratocumulus clouds examined have water paths in the 15–25 g m−2 range, effective radii near 8 μm, and number concentrations in the 20 cm−3 range in the Southern Ocean with optical depths in the range of 3–4. We speculate that addressing the high bias in absorbed shortwave radiation in climate models will require understanding the processes that form and maintain these marine stratocumulus clouds in southern mid- and high latitudes.

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Gerald G. Mace and Alain Protat

Abstract

The properties of clouds derived using a suite of remote sensors on board the Australian research vessel (R/V) Investigator during the 5-week Clouds, Aerosols, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean (CAPRICORN) voyage south of Australia during March and April 2016 are examined and compared to similar measurements collected by CloudSat and CALIPSO (CC) and from data collected at Graciosa Island, Azores (GRW). In addition, we use depolarization lidar data to examine the thermodynamic phase partitioning as a function of temperature and compare those statistics to similar information reported from the CALIPSO lidar in low-Earth orbit. We find that cloud cover during CAPRICORN was 76%, dominated by clouds based in the marine boundary layer. This was lower than comparable measurements collected by CC during these months, although the CC dataset observed significantly more high clouds. In the surface-based data, approximately 2/3 (1/2) of all low-level layers observed had a reflectivity below −20 dBZ in the CAPRICORN data (GRW) with 30% (20%) of the layers observed only by the lidar. The phase partitioning in layers based in the lower 4 km of the atmosphere was similar in the two surface-based datasets, indicating a greater occurrence of the ice phase in subfreezing low clouds than what is reported from analysis of CALIPSO data.

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Robert A. Warren and Alain Protat

Abstract

Interpolation of ground-based radar measurements is required when mapping data from their native spherical coordinates to a Cartesian grid. For reflectivity the question arises as to whether this processing should be performed in units of Z (mm6 m−3) or dBZ. This study addresses this question using one year of data from three radars, operating in diverse climates across Australia. For each radar, a subset of 800 volume scans is processed to identify “triads”—groups of three consecutive gates with valid data—in each of the three coordinate directions: range, azimuth, and elevation. For every triad, the reflectivity at the central gate is estimated by linearly interpolating between the outer two gates in both Z and dBZ. The resulting values are then compared with the true reflectivity at the central gate to quantify the interpolation errors. For all three sites and in all three coordinate directions, we find that interpolation in Z is more accurate on average, especially in regions of high reflectivity and strong reflectivity gradient (i.e., convective cores). However, interpolation in dBZ is better in regions of low and monotonically increasing/decreasing reflectivity. It is therefore recommended that reflectivities be converted from dBZ to Z prior to interpolation except when identifying echo-top height or other low-reflectivity boundaries.

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Alain Protat and Christopher R. Williams

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Doppler radar measurements at different frequencies (50 and 2835 MHz) are used to characterize the terminal fall speed of hydrometeors and the vertical air motion in tropical ice clouds and to evaluate statistical methods for retrieving these two parameters using a single vertically pointing cloud radar. For the observed vertical air motions, it is found that the mean vertical air velocity in ice clouds is small on average, as is assumed in terminal fall speed retrieval methods. The mean vertical air motions are slightly negative (downdraft) between the melting layer (5-km height) and 6.3-km height, and positive (updraft) above this altitude, with two peaks of 6 and 7 cm s−1 at 7.7- and 9.7-km height. For the retrieved hydrometeor terminal fall speeds, it is found that the variability of terminal fall speeds within narrow reflectivity ranges is typically within the acceptable uncertainties for using terminal fall speeds in ice cloud microphysical retrievals. This study also evaluates the performance of previously published statistical methods of separating terminal fall speed and vertical air velocity from vertically pointing Doppler radar measurements using the 50-/2835-MHz radar retrievals as a reference. It is found that the variability of the terminal fall speed–radar reflectivity relationship (VtZe) is large in ice clouds and cannot be parameterized accurately with a single relationship. A well-defined linear relationship is found between the two coefficients of a power-law VtZe relationship, but a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the VtZe relationship for each cloud. When comparing the different statistical methods to the reference, the distribution of terminal fall speed residual is wide, with most residuals being in the ±30–40 cm s−1 range about the mean. The typical mean residual ranged from 15 to 20 cm s−1, with different methods having mean residuals of <10 cm s−1 at some heights, but not at the same heights for all methods. The so-called VtZe technique was the most accurate above 9-km height, and the running-mean technique outperformed the other techniques below 9-km height. Sensitivity tests of the running-mean technique indicate that the 20-min average is the best trade-off for the type of ice clouds considered in this analysis. A new technique is proposed that incorporates simple averages of Doppler velocity for each (Ze, H) couple in a given cloud. This technique, referred to as DOP–ZeH, was found to outperform the three other methods at most heights, with a mean terminal fall residual of <10 cm s−1 at all heights. This error magnitude is compatible with the use of such retrieved terminal fall speeds for the retrieval of microphysical properties.

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Scott Collis, Alain Protat, and Kao-Shen Chung

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This article investigates the source and impact of artifacts produced by ordered linear interpolation techniques on variationally retrieved updraft intensities. Qualitative reasoning for the generation of periodic perturbations in gridded products is presented, and a simple analytical investigation into the impact of gridding artifacts on updraft retrieval is carried out. By projecting a nonconvergent flow typical of Darwin, Australia, onto the viewing geometry of a scanning radar, a numerical assessment of the impact of gridding artifacts is carried out. A simple enhancement to ordered linear interpolation, mixed-order linear interpolation, is proposed to reduce gridding artifacts. Radial velocity grids produced using both techniques are used to investigate the generation of spurious updrafts, with the simple ordered linear interpolation technique producing erroneous updrafts on the order of 2 m s−1. To investigate the impact on vertical velocities retrieved from a real weather event, radar-derived measurements taken during the active monsoon phase of Tropical Warm Pool International Cloud Experiment are gridded using both techniques, and vertical velocities are retrieved and contrasted.

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Allyson Rugg, Julie Haggerty, and Alain Protat

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Conditions of high ice water content (HIWC; defined herein as at least 1.0 g m−3) are often found in the anvils of convective systems and can cause engine damage and/or failure in aircraft. We use ice water content (IWC) retrievals from satellite-borne radar and lidar (CloudSat and CALIOP) to provide the first analysis of global HIWC frequency using 11 years of data (2007–17). Results show that HIWC is generally present in 1%–2% of CloudSat and CALIOP IWC retrievals between flight level 270 (FL270; 27 000 ft or 8.230 km) and FL420 (42 000 ft or 12.801 km) in areas with frequent convection. Similar rates of HIWC are found over midlatitude oceans at relatively low altitudes (below FL270). Possible nonconvective mechanisms for the formation of this low-level HIWC are discussed, as are the uncertainties suggesting that the results at these low altitudes are an overestimation of the true threat of HIWC to aircraft engines. The satellite IWC retrievals are also used to validate an HIWC diagnostic tool that provides storm-scale statistics on HIWC over the contiguous United States (CONUS) during the summer convective season (May–August from 2012 to 2019). Results over the CONUS suggest that HIWC over the Great Plains is highest in June, when a point in the region is under HIWC conditions for approximately 25 h of 30 days on average. The mean area-equivalent diameters of HIWC conditions in some areas of the Great Plains exceed 350 km, and the conditions can persist for 4–5 h.

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Guillaume Penide, Alain Protat, Vickal V. Kumar, and Peter T. May

Abstract

C-band polarimetric radar measurements spanning two wet seasons are used to perform a critical evaluation of two algorithms for the classification of stratiform and convective precipitation. The first approach is based on the horizontal texture of the radar reflectivity field (two classes: stratiform, convective), and the second approach is based on the properties of the drop size distribution (DSD) parameters as derived from a set of polarimetric variables (three classes: stratiform, mixed, convective). To investigate how well those two methods compare quantitatively, probability density functions of reflectivity, rain rate, 5-dBZ echo top height, and DSD parameters (namely, the median volume diameter and the “generalized” intercept parameter) are built. The study found that while the two methods agree well on the identification of stratiform precipitation, large differences are obtained for convective rainfall. The texture-based approach seems to classify too many points as being of convective nature compared to the DSD-based method. Among the points that are classified as convective by the texture-based approach, 25% correspond to low concentration of relatively small particles associated with rain rates below 10 mm h−1. This large proportion of unrealistically low convective rain rates is not produced by the DSD-based approach, which only classifies 4% of the convective points with rain rates below 10 mm h−1. These points were found to be mainly isolated points embedded within stratiform precipitation and associated with low cloud-top height, suggesting a misclassification of the texture-based approach. Thus, to improve the statistics of the convective class, three modified equations of the peakedness criterion used in the radar-based algorithm are proposed to decrease the number of misclassified points.

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