Radar Wind Observations Research Articles

Application of Radar Wind Observations for Low-Level NWP Wind Forecast Validation

AbstractThe Finnish Meteorological Institute has produced a new numerical weather prediction model–based wind atlas of Finland. The wind atlas provides information on local wind conditions in terms of annual and monthly wind speed and direction averages. In the context of the wind atlas project, low-level Applications of Research to Operations at Mesoscale (AROME) model wind forecasts have been validated against radar radial wind observations and, as a comparison, against conventional radiosonde observations to confirm the realism of the wind forecasts. The results indicate that the systematic and random errors in the AROME wind forecasts are relatively small and are of the same order of magnitude independent of the validating observation type. The validation benefits from the high spatial and temporal resolution of the radar observations. There are over 4000 times as many radar observations as radiosonde observations available for the considered validation period of July 2008–May 2009.

Assimilation of Indian Doppler Weather Radar observations for simulation of mesoscale features of a land-falling cyclone

In this paper, impact of Indian Doppler Weather Radar (DWR) data, i.e., reflectivity (Z), radial velocity (Vr) data individually and in combination has been examined for simulation of mesoscale features of a land-falling cyclone with Advance Regional Prediction System (ARPS) Model at 9-km horizontal resolution. The radial velocity and reflectivity observations from DWR station, Chennai (lat. 13.0°N and long. 80.0°E), are assimilated using the ARPS Data Assimilation System (ADAS) and cloud analysis scheme of the model. The case selected for this study is the Bay of Bengal tropical cyclone NISHA of 27–28 November 2008. The study shows that the ARPS model with the assimilation of radial wind and reflectivity observations of DWR, Chennai, could simulate mesoscale characteristics, such as number of cells, spiral rain band structure, location of the center and strengthening of the lower tropospheric winds associated with the land-falling cyclone NISHA. The evolution of 850 hPa wind field super-imposed vorticity reveals that the forecast is improved in terms of the magnitude and direction of lower tropospheric wind, time, and location of cyclone in the experiment when both radial wind and reflectivity observations are used. With the assimilation of both radial wind and reflectivity observations, model could reproduce the rainfall pattern in a more realistic way. The results of this study are found to be very promising toward improving the short-range mesoscale forecasts.

3D-Var Assimilation of Insect-Derived Doppler Radar Radial Winds in Convective Cases Using a High-Resolution Model

AbstractThe assimilation of Doppler radar radial winds for high-resolution NWP may improve short-term forecasts of convective weather. Using insects as the radar target, it is possible to provide wind observations during convective development. This study aims to explore the potential of these new observations, with three case studies. Radial winds from insects detected by four operational weather radars were assimilated using three-dimensional variational data assimilation (3D-Var) into a 1.5-km resolution version of the Met Office Unified Model, using a southern U.K. domain and no convective parameterization. The effect on the analyzed wind was small, with changes in direction and speed up to 45° and 2 m s−1, respectively. The forecast precipitation was perturbed in space and time but not substantially modified. Radial wind observations from insects show the potential to provide small corrections to the location and timing of showers, but not to completely relocate convergence lines. Overall, quantitative analysis indicated the observation impact in the three case studies was small and neutral. However, the small sample size and possible ground clutter contamination issues preclude unequivocal impact estimation. The study shows the potential positive impact of insect winds; future operational systems using dual-polarization radars that are better able to discriminate between insects and clutter returns should provide a much greater impact on forecasts.

Effect of Doppler Radial Velocity Data Assimilation on the Simulation of a Typhoon Approaching Taiwan: A Case Study of Typhoon Aere (2004)

Compared to conventional data, radar observations have an advantage of high spatial and temporal resolutions, and Doppler radars are capable of capturing detailed characteristics of flow fields, including typhoon circulation. In this study, the possible improvement of short-term typhoon predictions near Taiwan, particularly with regard to related rainfall forecasts over the mountainous island, using Doppler radial wind observations is explored. The case of Typhoon Aere (2004) was chosen for study, and a series of experiments were carried out using the Penn State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model Version 5 (MM5) with its three-dimensional variational (3D-VAR) data assimilation system. The results show that once the Doppler radial velocities were assimilated into the model, the typhoon's circulation intensified within one hour. However, when Typhoon Aere approached from the east and only the western half of its core area could be observed by the radar, the assimilation caused the typhoon to deflect southward due to the incomplete and uneven data coverage. In another experiment in which Doppler radar data assimilation did not start until Typhoon Aere moved closer, such that its entire core region could be observed. A similar track deflection was avoided. Overall, the assimilation of Doppler radial velocity data reduced the intensity error (in wind speed) by about 25%. Furthermore, the improvements in location, intensity, and circulation structure of Typhoon Aere lead to better rainfall prediction over the island of Taiwan.

Towards the operational use of Doppler radar radial winds in HIRLAM

This article discusses the pre-operational development for data assimilation of radial wind observations in the High Resolution Limited Area Model (HIRLAM) at the Finnish Meteorological Institute (FMI). The HIRLAM variational data assimilation system includes all the needed tools for exploitation of radial wind observations. A measurement task designed especially for the radial wind data assimilation purposes has been implemented to the FMI radar network. Observation quality monitoring indicates that the main error sources for the radial wind observations are ground clutter and velocity ambiguity. The HIRLAM quality control procedures are able to detect and reject most of these erroneous observations. Impact studies show encouraging results. Surface verification indicates that the use of radar wind observations has a positive impact on 10 m wind forecasts. Precipitation forecasts are also slightly improved. Upper air verification shows positive impact on wind and temperature forecasts at the 925–700 hPa levels.

Use of a Vertical Vorticity Equation in Variational Dual-Doppler Wind Analysis

Abstract The utility of the anelastic vertical vorticity equation in a weak-constraint (least squares error) variational dual-Doppler wind analysis procedure is explored. The analysis winds are obtained by minimizing a cost function accounting for the discrepancies between observed and analyzed radial winds, errors in the mass conservation equation, errors in the anelastic vertical vorticity equation, and spatial smoothness constraints. By using Taylor’s frozen-turbulence hypothesis to shift analysis winds to observation points, discrepancies between radially projected analysis winds and radial wind observations can be calculated at the actual times and locations the data are acquired. The frozen-turbulence hypothesis is also used to evaluate the local derivative term in the vorticity equation. Tests of the analysis procedure are performed with analytical pseudo-observations of an array of translating and temporally decaying counterrotating updrafts and downdrafts generated from a Beltrami flow solution of the Navier–Stokes equations. The experiments explore the value added to the analysis by the vorticity equation constraint in the common scenario of substantial missing low-level data (radial wind observations at heights beneath 1.5 km are withheld from the analysis). Experiments focus on the sensitivity of the most sensitive analysis variable—the vertical velocity component—to values of the weighting coefficients, volume scan period, number of volume scans, and errors in the estimated frozen-turbulence pattern-translation components. Although the vorticity equation constraint is found to add value to many of these analyses, the analysis can become significantly degraded if estimates of the pattern-translation components are largely in error or if the frozen-turbulence hypothesis itself breaks down. However, tests also suggest that these negative impacts can be mitigated if data are available in a rapid-scan mode.

Radar observations of the diurnal tide in the tropical mesosphere-lower thermosphere region: Longitudinal variabilities

Abstract Significant attention is being paid in recent times by several observational and modeling studies to quantify the spatial and temporal variabilities of diurnal tide in the mesosphere and lower thermosphere (MLT) region. These variabilities are ascribed to spatial and temporal variations in the tidal forcing or interactions between the propagating tides and background wind, planetary waves or gravity waves. The present work makes use of simultaneous ground-based radar wind observations of different durations from five equatorial/low latitude sites in the Indian, Indonesian and Pacific sectors: Tirunelveli (8.7°N, 77.8°E), Jakarta (6.4°S, 106.7°E), Pontianak (0.03°N, 109°E), Kauai (22°N, 160°W) and Christmas Island (2°N, 157°W). This study delineates the longitudinal differences in the tidal characteristics in (i) interannual time scales over Tirunelveli and Kauai during 1993–2002, (ii) seasonal time scales over Christmas Island, Jakarta and Tirunelveli for the years 1993–1997 and (iii) shorter than seasonal time scales over Christmas Island, Pontianak and Tirunelveli during 1996–1997. An important observational feature noticed in this work is the differing behavior of the long-term tidal fields over Tirunelveli and Kauai. The monthly tidal amplitudes over Tirunelveli reveal a strong QBO signature whereas a similar, strong QBO signal could not be traced in the long-term observations from Kauai.

Doppler radar radial winds in HIRLAM. Part II: optimizing the super-observation processing

Doppler radar radial wind observations are modelled in numerical weather prediction (NWP) within observation errors which consist of instrumental, modelling and representativeness errors. The systematic and random modelling errors can be reduced through a careful design of the observation operator (Part I). The impact of the random instrumental and representativeness errors can be decreased by optimizing the processing of the so-called super-observations (spatial averages of raw measurements; Part II).The super-observation processing is experimentally optimized in this article by determining the optimal resolution for the super-observations for differentNWPmodel resolutions. A 1-month experiment with the HIRLAM data assimilation and forecasting system is used for radial wind data monitoring and for generating observation minus background (OmB) differences. The OmB statistics indicate that the super-observation processing reduces the standard deviation of the radial wind speedOmBdifference, while themean vectorwindOmBdifference tends to increase. The optimal parameter settings correspond at a measurement range of 50 km (100 km) to an averaging area of 1.7 km2 (7.3 km2).In conclusion, an accurate and computationally feasible observation operator for the Doppler radar radial wind observations is developed (Part I) and a super-observation processing system is optimized (Part II).

Comparison of wind measurements in the troposphere and mesosphere by VHF/MF radars and in-situ techniques

Abstract. Radar wind observations at frequencies between 1.98 and 53.5 MHz obtained at polar latitudes were compared to in-situ wind measurements by radiosondes at tropospheric altitudes and to winds from falling spheres at mesospheric altitudes. Comparisons are shown for several campaigns of radiosonde and falling sphere observations. The radar wind directions agree well to the radiosonde and falling sphere observations and are highly correlated. The winds estimated from radar measurements are less than the radiosonde data by about 15% for spaced antenna observations and by about 10% for the Doppler beam swinging experiment. At mesospheric altitudes the spaced antenna winds obtained from the wide-beam Andenes MF radar are underestimated in the order of 35% and winds from the narrow-beam Saura MF radar are underestimated by about 20% compared to falling sphere winds at altitudes between 70 and 80 km. Furthermore, the relation between wind measurements using narrow-beam and wide-beam antenna arrangements for the MF radars is discussed and VHF radar observations are compared to the wide-beam MF radar.

Simulation of microphysical structure associated with tropical cloud clusters using mesoscale model and comparison with TRMM observations

An attempt has been made in the present study to examine the microphysical structure of a non‐squall Tropical Cloud Cluster (TCC). Three‐dimensional model simulations of cloud microphysical structure associated with a non‐squall TCC occurred on 26 October 2005 over the South Bay of Bengal have been carried out. The initial conditions for the model simulations were improved by incorporating upper air radiosonde observations and Indian Mesosphere Stratosphere Troposphere (MST) radar wind observations through analysis nudging. The horizontal and vertical distribution of the cloud hydrometeor fields observed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) are compared to those simulated by a mesoscale model using a sophisticated microphysical scheme. Substantial differences are noticed in the amounts of cloud microphysical parameters, with simulated values of hydrometeors being higher than TMI retrievals. Spatial distribution of Cloud Liquid Water (CLW) and Rain Water (RNW) from TMI and model simulations correspond well with each other. The cloud microphysical structure during the initial and mature phases of the storm is also investigated. Comparisons of horizontal and vertical reflectivity structure from the TRMM‐Precipitation Radar (PR) and those simulated by the model show reflectivity cores of values greater than 30 dBZ. The TRMM‐PR echo tops are 3–4 km higher than the simulated echo tops. The 24 hr accumulated precipitation from model simulations are then verified with the combined rainfall product from the TRMM observations.

Doppler radar radial wind data in NWP model validation

AbstractValidation of mesoscale numerical weather prediction models calls for high‐resolution observations. Conventional data sources, such as radiosonde data, provide a reliable benchmark but their spatial resolution is insufficient for model validation in the kilometric scale. Doppler radar radial wind observations are used in this study as an alternative data source for model validation. Data assimilation system in general, and observation modelling in particular, are used as a toolbox for validation.Two versions of the High Resolution Limited Area Model (HIRLAM), which differ only in the formulation of the surface stress direction, are validated over a period of 1 month. The observation minus background (OmB) statistics advocate the merits of using high‐resolution radar data as model validation material. It is demonstrated that subtle differences in model versions due to different parameterization details can be distinguished with high statistical confidence. Copyright © 2008 Royal Meteorological Society

Bias estimation of Doppler‐radar radial‐wind observations

AbstractDoppler radars provide measurements of the radar radial wind component at high spatial and temporal resolution. The variational data‐assimilation framework enables direct use of these measurements in numerical weather prediction models. Bias estimation of Doppler‐radar radial winds requires special attention because of the azimuthal scanning strategy. Calculation of the bias statistic over all azimuthal directions results in a near‐zero value even in the presence of systematic differences between the measured and modelled wind speed and direction. This paper describes a method for bias estimation of Doppler‐radar radial‐wind observations. Intercomparison between the estimated biases of Doppler‐radar and radiosonde wind observations reveals that the wind speed and direction biases are of comparable magnitude. Copyright © 2007 Royal Meteorological Society

Latitudinal and longitudinal variability of mesospheric winds and temperatures during stratospheric warming events

Continuous MF and meteor radar observations allow detailed studies of winds in the mesosphere and lower thermosphere (MLT) as well as temperatures around the mesopause. This height region is characterized by a strong variability in winter due to enhanced planetary wave activity and related stratospheric warming events, which are distinct coupling processes between lower, middle and upper atmosphere. Here the variability of mesospheric winds and temperatures is discussed in relation with major and minor stratospheric warmings as observed during winter 2005/06 in comparison with results during winter 1998/99. Our studies are based on MF radar wind measurements at Andenes (69°N, 16°E), Poker Flat (65°N, 147°W) and Juliusruh (55°N, 13°E) as well as on meteor radar observations of winds and temperatures at Resolute Bay (75°N, 95°W), Andenes (69°N, 16°E) and Kühlungsborn (54°N, 12°E). Additionally, energy dissipation rates have been estimated from spectral width measurements using a 3 MHz Doppler radar near Andenes. Particular attention is directed to the changes of winds, turbulence and the gravity wave activity in the mesosphere in relation to the planetary wave activity in the stratosphere. Observations indicate an enhancement of planetary wave 1 activity in the mesosphere at high latitudes during major stratospheric warmings. Daily mean temperatures derived from meteor decay times indicate that strong warming events are connected with a cooling of the 90 km region by about 10–20 K. The onset of these cooling processes and the reversals of the mesospheric circulation to easterly winds occur some days before the changes of the zonal circulation in the stratosphere start indicating a downward propagation of the circulation disturbances from the MLT region to the stratosphere and troposphere during the stratospheric warming events. The short-term reversal of the mesospheric winds is followed by a period of strong westerly winds connected with enhanced turbulence rates and an increase of gravity wave activity in the altitude range 70–85 km.

Single-Doppler Retrieval of the Three-Dimensional Wind in a Deep Convective System Based on an Optimal Moving Frame of Reference

A single-Doppler radar velocity retrieval method, based on an optimally determined moving frame of reference, is developed. This method enables the recovery of the complete three-dimensional wind field using reflectivity, and radial wind observations, detected by a single Doppler radar over multiple volume scans. This method first computes a set of optimal moving speeds (U, V, W), which can be functions of the height. It is considered optimal in the sense that when a reference frame is traveling at this speed, as much of the radar reflectivity, interpolated onto this frame can be conserved as possible. The perturbation velocities (u′, v′, w′) are then solved simultaneously, with respect to the moving speeds, by minimizing a cost function through the three-dimensional variational (3D-Var) approach. This cost function consists of a set of weak constraints, mainly the equation for radar reflectivity conservation, the continuity equation, the geometric relationship between the radar-observed radial wind and the Cartesian winds, and a smoothness filter. Finally, by adding the optimal moving speed to the perturbation velocities, a complete three-dimensional wind field is obtained.The performance of this method is tested under different scenarios, including cases with deep convections, embedded in an environment without mean wind, with constant mean wind, with vertical wind shear of the horizontal mean wind, and with differences in the radar observational time at each grid point within one volume scan are considered.This method is demonstrated to be feasible by applying it to recover the three-dimensional wind structure of a subtropical squall line from Doppler radar data, collected during the 1987 Taiwan Area Mesoscale Experiment (TAMEX).

A Report on Radar Observations of 5-8-day Waves in the Equatorial MLT Region

The MF and Meteor radar observations of horizontal winds over Koto Tabang (0.2°S, 100.3°E), Pontianak (0°, 109.3°E), and Pameungpeuk (7.5°S, 107.5°E) are utilized to study the characteristics of 5-8-day waves in the mesosphere and lower thermosphere (MLT) region over Indonesia. The wave activity is larger in zonal wind than in meridional wind and in the year 2003, it maximizes before and after northern hemispheric spring equinox. In the year 2004, the wave activity in zonal wind is larger during June-August 2004. During this period, the phase of the wave over Pontianak leads that over Koto Tabang, indicating that the wave is westward propagating with zonal wave number 1. The wave period ranges from 6.3 to 7.0 days over the equatorial sites Koto Tabang and Pontianak, whereas over the southern hemispheric site, Pameungpeuk, it is in the range 6.2-6.4 days. This shows that the dominant period of the wave changes with latitude. The amplitude and phase structures of the wave with altitude are similar among the three sites considered. The wave dominates the MLT dynamics over Pameungpeuk even during second half of CPEA (Coupling Processes in the Equatorial Atmosphere) campaign period (10 April-9 May 2004) with similar dominant wave period near 6.4-days. The wave amplitude is larger during last quarter of the campaign period.

Estimating vertical velocity and radial flow from Doppler radar observations of tropical cyclones

AbstractThe mesoscale vorticity method (MVM) is used in conjunction with the ground‐based velocity track display (GBVTD) to derive the inner‐core vertical velocity from Doppler radar observations of tropical cyclone (TC) Danny (1997). MVM derives the vertical velocity from vorticity variations in space and in time based on the mesoscale vorticity equation. The use of MVM and GBVTD allows us to derive good correlations among the eye‐wall maximum wind, bow‐shaped updraught and echo east of the eye‐wall in Danny. Furthermore, we demonstrate the dynamically consistent radial flow can be derived from the vertical velocity obtained from MVM using the wind decomposition technique that solves the Poisson equations over a limited‐area domain. With the wind decomposition, we combine the rotational wind which is obtained from Doppler radar wind observations and the divergent wind which is inferred dynamically from the rotational wind to form the balanced horizontal wind in TC inner cores, where rotational wind dominates the divergent wind. In this study, we show a realistic horizontal and vertical structure of the vertical velocity and the induced radial flow in Danny's inner core. In the horizontal, the main eye‐wall updraught draws in significant surrounding air, converging at the strongest echo where the maximum updraught is located. In the vertical, the main updraught tilts vertically outwards, corresponding very well with the outward‐tilting eye‐wall. The maximum updraught is located at the inner edge of the eye‐wall clouds, while downward motions are found at the outer edge. This study demonstrates that the mesoscale vorticity method can use high‐temporal‐resolution data observed by Doppler radars to derive realistic vertical velocity and the radial flow of TCs. The vorticity temporal variations crucial to the accuracy of the vorticity method have to be derived from a high‐temporal‐frequency observing system such as state‐of‐the‐art Doppler radars. Copyright © 2006 Royal Meteorological Society

Global variability of mesospheric temperature: Planetary‐scale perturbations at equatorial and tropical latitudes

The study examines the source of a cold temperature perturbation observed in the Wind Imaging Interferometer (WINDII) daytime daily zonal mean temperatures at 87 km height during March/April 1993, also seen to some extent in 1995 and 1997 over the latitude range 25°S to 25°N. The perturbation appears centered at 10°S as a departure of 10K to 35K below the semiannual climatological mean, which peaks at equinox. After accounting for tidal contributions to the observations, by adopting tidal parameters determined from the Microwave Limb Sounder (MLS) temperature observations the cold temperature signature still remained in the residual data. The perturbation is a global phenomenon immediately following March equinox and lasts over a period of 2 weeks. The period is marked by the presence of mesospheric temperature inversions over the altitude range of 70–90 km. Harmonic analysis reveals the presence of planetary waves of wave number 1 with periods of 3 days, 4–5 days and 7 days. A cross section of the temperature monthly zonal mean data and month of the year over the period from January 1992 to December 1995 revealed a distinct MSAO, modulated by a QBO with strong cold anomalies in March/April 1993, 1995 and 1997 in excellent agreement with the MSAO easterly phase of correlative MF radar wind observations at Tirunelveli (8.7°N, 77.8°E). It is shown that the observed temperature anomalies can be produced by the residual circulation associated with the wave driving of the MSAO itself.

Mesoscale Observations of the Genesis of Hurricane Dolly (1996)

Abstract Recent numerical studies of tropical cyclone genesis suggest a new paradigm for how the surface vortex is established based on a highly nonaxisymmetric mechanism involving the interaction of low-level cyclonic circulations generated by deep cumulonimbus convection. A reexamination of mesoscale observations during the genesis of Hurricane Guillermo (1991) confirms the presence of multiple cyclonic circulations. More recently, airborne Doppler radar wind observations during the genesis of Atlantic Hurricane Dolly (1996) also reveal multiple lower-to-middle-tropospheric mesoscale cyclonic circulations during sequential 15–20-min compositing periods. A particularly well-organized, but initially weak (mean tangential wind of 7 m s−1), low-level cyclonic vortex embedded within the pre-Dolly tropical disturbance is observed coincident with deep, vertically penetrating cumulonimbus convection. The earliest observations of this vortex show the peak circulation near 2-km height with a mean diameter of 30–40 km. The circulation undergoes a slight intensification over a 2-h period, with the maximum tangential winds ultimately peaking below 1-km height. Approximately 18 h after these observations Dolly is classified as a hurricane by the National Hurricane Center. A synthesis of observations during the early development of Dolly supports a stochastic view of tropical cyclone genesis in which multiple lower-to-middle-tropospheric mesoscale cyclonic circulations are involved in building the surface cyclonic circulation. It is suggested that, in particular, the interaction of low-level circulations generated by a series of deep cumulonimbus convective events, like the one documented here, within an environment of elevated cyclonic vorticity was instrumental to the formation of the Dolly surface vortex.

A two-step variational method for three-dimensional wind retrieval from single Doppler radar

A two-step variational method is developed to retrieve the three-dimension wind field from single-Doppler radar observation for operational applications. The first step produces a proxy background wind field, and the second step retrieves the detailed wind structures in data dense areas on a high-resolution grid. The background field is called proxy because it is obtained in the first step by a spectral analysis of the same radial-wind observations that will be used in the second step and thus the background is not independent of the observations. In the second step, the wind retrieval is constrained by the proxy background and the advection equation of liquid water content in the cost function. Tested with a dataset collected during Meiyu front heavy rainfall on 22 July 2002 in China, the two-step approach is shown to yield the best performance in comparison with a single-step approach that uses either only the proxy background field or the retrieval without the proxy background.

Doppler Radar Wind Data Assimilation with HIRLAM 3DVAR

Abstract A Doppler radar wind data assimilation system has been developed for the three-dimensional variational data assimilation (3DVAR) scheme of the High Resolution Limited Area Model (HIRLAM). Radar wind observations can be input for the multivariate HIRLAM 3DVAR either as radial wind superobservations (SOs) or as vertical profiles of horizontal wind obtained with the velocity–azimuth display (VAD) technique. The radar wind data handling system, including data processing, quality control, and observation operators for the 3DVAR, are described and evaluated. Background error standard deviation (σb) in observation space for wind and radial wind have been estimated by the so-called randomization method. The derived values of σb are used in the quality control of observations and also in the assignment of radar wind observation error standard deviations (σo). Parallel data assimilation and forecast experiments confirm reasonably tuned error statistics and indicate a small positive impact of radar wind dat...