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Valentin Louf
and
Alain Protat

Abstract

We present an integrated framework that leverages multiple weather radar calibration and monitoring techniques to provide real-time diagnostics on reflectivity calibration, antenna pointing, and dual-polarization moments. This framework uses a volume-matching technique to track the absolute calibration of radar reflectivity with respect to the Global Precipitation Measurement (GPM) spaceborne radar, the relative calibration adjustment (RCA) technique to track relative changes in the radar calibration constant, the solar calibration technique to track daily change in solar power and antenna pointing error, and techniques that track properties of light-rain medium to monitor the differential reflectivity and dual-polarization moments. This framework allows for an evaluation of various calibration and monitoring techniques. For example, we found that a change in the RCA is highly correlated to a change in absolute calibration, with respect to GPM, if a change in antenna pointing can first be ruled out. It is currently monitoring 67+ radars from the Australian radar network. Because of the diverse and evolving nature of the Australian radar network, flexibility and modularity are at the core of the calibration framework. The framework can tailor its diagnostics to the specific characteristics of a radar (band, beamwidth, etc.). Because of its modularity, it can be expanded with new techniques to provide additional diagnostics (e.g., monitoring of radar sensitivity). The results are presented in an interactive dashboard at different level of details for a wide and diverse audience (radar engineers, researchers, forecasters, and management), and it is operational at the Australian Bureau of Meteorology.

Significance Statement

Weather radars, like all instruments, require maintenance and upgrades. Rainfall measurements are highly variable and sensitive to change, and this can lead to inconsistencies within a radar network. Calibration is the process to counteract those inconsistencies. Any calibration requires a fixed standard to which the changed/upgraded radar can be compared. The SCAR calibration framework presented herein makes use of several standards to retrieve a full set of diagnostics about the radar data. We apply these techniques over the entire Australian weather radar network and demonstrate that, by using this integrated approach, absolute calibration can be achieved to within 1 dBZ of reflectivity, antenna pointing can be monitored within 0.1°, and the various measurements of the radars can be quality controlled.

Free access
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.

Full access
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.

Full access
Alain Protat
,
Isztar Zawadzki
, and
Alain Caya

Abstract

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.

Full access
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.

Full access
Alain Protat
,
Valentin Louf
, and
Mark Curtis

Abstract

Doppler radars measure Doppler velocity within the [−VN , VN ] range, where VN is the Nyquist velocity. Doppler velocities outside this range are “folded” within this interval. All Doppler “unfolding” techniques use the folded velocities themselves. In this work, we investigate the potential of using velocities derived from optical flow techniques applied to the radar reflectivity field for that purpose. The analysis of wind speed errors using six months of multi-Doppler wind retrievals showed that 99.9% of all points are characterized by errors smaller than 26 m s−1 below 5-km height, corresponding to a failure rate of less than 0.1% if optical flow winds were used to unfold Doppler velocities for VN = 26 m s−1. These errors largely increase above 5-km height, indicating that vertical continuity tests should be included to reduce failure rates at higher elevations. Following these results, we have developed the Two-step Optical Flow Unfolding (TOFU) technique, with the specific objective to accurately unfold Doppler velocities with VN = 26 m s−1. The TOFU performance was assessed using challenging case studies, comparisons with an advanced Doppler unfolding technique using higher Nyquist velocities, and 6 months of high VN (47.2 m s−1) data artificially folded to 26 m s−1. TOFU failure rates were found to be very low. Three main situations contributed to these errors: high low-level wind shear, elevated cloud layers associated with high winds, and radar data artifacts. Our recommendation is to use these unfolded winds as the first step of advanced Doppler unfolding techniques.

Significance Statement

The potential of using optical flow winds operationally to accurately unfold Doppler velocities is demonstrated in this work. The operational significance is that the Nyquist velocity can confidently be reduced to 26 m s−1, allowing for extended first trip radar maximum range and reduced contamination from dual pulse repetition frequency artifacts.

Restricted access
Alain Protat
and
Christopher R. Williams

Abstract

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 (Vt Ze ) 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 Vt Ze relationship, but a more accurate microphysical retrieval is obtained using Doppler velocity measurements to better constrain the Vt Ze 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 Vt Ze 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–Ze H, 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.

Full access
Allyson Rugg
,
Julie Haggerty
, and
Alain Protat

Abstract

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.

Full access
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.

Open access
Gerald G. Mace
and
Alain Protat

Abstract

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.

Open access