Doppler Radar Data Research Articles

Improving Wind and Rain Simulations for Tropical Cyclones with the Assimilation of Doppler Radar Data

More accurate prediction of the strong winds and heavy rain associated with tropical cyclones using numerical weather prediction (NWP) models would be helpful in the provision of weather services for the public. In this paper, the impact of assimilating radar data in the simulation of Typhoon Neoguri and Severe Tropical Storm Kammuri in 2008 is studied using Weather Research and Forecasting (WRF) version 2.2 and WRF VAR version 2.1. Only the data from the radar at Tate's Cairn in Hong Kong are considered. Four experiments are conducted, namely, (a) simulation without radar data, (b) simulation with radar data assimilated at the initial time, (c) cycling simulation with the assimilation of radar data (Doppler velocity and reflectivity) directly assimilated, and (d) cycling simulation with the assimilation of 2D wind field retrieved from the Doppler velocity data from the radar. By comparing with actual observations of the surface wind distribution in Hong Kong and the actual radar reflectivity data, it turns out that both (c) and (d) outperform (a) and (b), and (c) and (d) show comparable skills. As a result, cycling simulation with the assimilation of weather radar data (even for a single radar) could improve the prediction of winds and rain bands associated with tropical cyclones.

Terrain Effects on an Afternoon Heavy Rainfall Event, Observed over Northern Taiwan on 20 June 2000 during Monsoon Break

An afternoon heavy rainfall event in northern Taiwan, observed on June 20, 2000 during the monsoon break, is investigated using surface observation and Doppler radar data and a nonhydrostatic model with a horizontal grid spacing of 1.33 km. Heavy rainfall was brought majorly by two precipitation systems, namely A and B. System A was initiated and developed in Taipei Basin, associated with a local wind convergence line. System B was formed on the western slopes south of Taipei Basin, extended northward of the Taipei Basin, and lasted for 4 h. The formation and maintenance mechanisms of the two systems are examined using sensitivity experiments in which the terrain of Central Mountain Range (CMR) or Yangmin Mountain (YM) located north of the Taipei Basin is removed or reduced. In addition, the effect of cold outflow from System A on System B is examined.The wind convergence line responsible for forming and maintaining System A is brought from the interaction between the prevailing southwesterly winds and the easterly/northeasterly flow caused by the terrain effect of CMR. Furthermore, the northeasterly onshore flow and the low-level northwesterly/northerly flow resulting from the prevailing southwesterly winds turning southward over southern YM, enhance System A inside Taipei Basin. System B is produced over relatively high sloped areas (elevations exceeding 200 m) in southwestern Taipei Basin by the prevailing southwesterly winds. Over relatively low lands (elevations less than 200 m), the cold outflow from System A inside Taipei Basin converges with the prevailing southwesterly winds and helps the maintenance of System B.

A 3.5‐Dimensional Variational Method for Doppler Radar Data Assimilation and Its Application to Phased‐Array Radar Observations

A 3.5‐dimensional variational method is developed for Doppler radar data assimilation. In this method, incremental analyses are performed in three steps to update the model state upon the background state provided by the model prediction. First, radar radial‐velocity observations from three consecutive volume scans are analyzed on the model grid. The analyzed radial‐velocity fields are then used in step 2 to produce incremental analyses for the vector velocity fields at two time levels between the three volume scans. The analyzed vector velocity fields are used in step 3 to produce incremental analyses for the thermodynamic fields at the central time level accompanied by the adjustments in water vapor and hydrometeor mixing ratios based on radar reflectivity observations. The finite element B‐spline representations and recursive filter are used to reduce the dimension of the analysis space and enhance the computational efficiency. The method is applied to a squall line case observed by the phased‐array radar with rapid volume scans at the National Weather Radar Testbed and is shown to be effective in assimilating the phased‐array radar observations and improve the prediction of the subsequent evolution of the squall line.

Observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system

AbstractA series of mesoscale convective systems (MCSs) was observed in the UK during one of the Intensive Observation Periods (IOP 3) in the Convective Storm Initiation Project (CSIP). A detailed case‐study of one of the MCSs, involving some novel interpretation of single Doppler radar data, reveals the evolution of stacked slantwise circulations associated with elevated upright convection. A feature of this occasion was the existence near the surface of a strong undercurrent of cool air flowing against the direction of travel of the storm. The air feeding both the upright and slantwise ascent originated not from near the surface but, rather, from layers between 1 and 3 km. The upright convection was intense at first and was accompanied by a region of dry subsidence surrounding it, together with the alternating layers of slantwise ascent and descent within the storm's precipitation area. When the elevated upright convection weakened, vigorous slantwise motions continued for some time, but their slope decreased from the initial 1 in 4 to 1 in 9. The lowermost layer of slantwise descent corresponded to a moderately intense rear‐inflow jet. The rear‐inflow jet did not penetrate the cool undercurrent and reach the surface; instead, beneath where it impacted the warm air capping the undercurrent, the undercurrent took on the structure of a gravity wave without stagnation. The raised head of the undercurrent lifted overlying air of high wet‐bulb potential temperature sufficiently to overcome the convective inhibition. Copyright © 2010 Royal Meteorological Society

Estimation of vertical wind velocity using doppler radar data with vorticity constraints

The vorticity equation and the mass continuity equation are used as constraints to aid in calculation of the vertical component of a wind field from horizontal wind components. Typically the horizontal wind components result from estimations obtained from radar data. Using a Hilbert space minimization formulation, we characterize the estimated vertical velocity as a solution of an elliptic boundary value problem whose coefficients are functions of the horizontal wind components. Differentiable dependence of the vertical component of the wind field with respect to the horizontal fields is established. Finite dimensional approximating problems are obtained. A numerical study is presented using a sample problem based on the Beltrami flow to compare accuracy of the estimated vertical wind component with the exact. Also, sensitivity to perturbations in horizontal wind components are observed.

Impact of Airborne Doppler Radar Data Assimilation on the Numerical Simulation of Intensity Changes of Hurricane Dennis near a Landfall

Abstract Accurate forecasting of a hurricane’s intensity changes near its landfall is of great importance in making an effective hurricane warning. This study uses airborne Doppler radar data collected during the NASA Tropical Cloud Systems and Processes (TCSP) field experiment in July 2005 to examine the impact of airborne radar observations on the short-range numerical simulation of hurricane track and intensity changes. A series of numerical experiments is conducted for Hurricane Dennis (2005) to study its intensity changes near a landfall. Both radar reflectivity and radial velocity–derived wind fields are assimilated into the Weather Research and Forecasting (WRF) model with its three-dimensional variational data assimilation (3DVAR) system. Numerical results indicate that the radar data assimilation has greatly improved the simulated structure and intensity changes of Hurricane Dennis. Specifically, the assimilation of radar reflectivity data shows a notable influence on the thermal and hydrometeor structures of the initial vortex and the precipitation structure in the subsequent forecasts, although its impact on the intensity and track forecasts is relatively small. In contrast, assimilation of radar wind data results in moderate improvement in the storm-track forecast and significant improvement in the intensity and precipitation forecasts of Hurricane Dennis. The hurricane landfall, intensification, and weakening during the simulation period are well captured by assimilating both radar reflectivity and wind data.

Convective-Scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005)

Abstract Airborne Doppler radar data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) document downdrafts in the principal rainband of Hurricane Katrina (2005). Inner-edge downdrafts (IEDs) originating at 6–8-km altitude created a sharp reflectivity gradient along the inner boundary of the rainband. Low-level downdrafts (LLDs) evidently driven by precipitation drag originated at 2–4 km within the heavy rain cells of each convective element. The IED and LLD were spatially separated by but closely associated with the updrafts within the rainband. The IED was forced aloft by pressure perturbations formed in response to the adjacent buoyant updrafts. Once descending, the air attained negative buoyancy via evaporative cooling from the rainband precipitation. A convective-scale tangential wind maximum tended to occur in the radial inflow at lower levels in association with the IED, which enhanced the inward flux of angular momentum at lower levels. Convergence at the base of the downdrafts on the upwind end of the principal rainband contributed to the principal rainband growing in length. New updraft elements triggered by this convergence led to the formation of new IED and LLD pockets, which were subsequently advected downwind around the storm by the vortex winds while additional new cells continued to form on the upwind end of the band. These processes sustained the principal rainband and helped to make it effectively stationary relative to the storm center, thus maintaining its impact on the hurricane dynamics over an extended period.

Experiments of Hurricane Initialization with Airborne Doppler Radar Data for the Advanced Research Hurricane WRF (AHW) Model

Abstract Initialization of the hurricane vortex in weather prediction models is vital to intensity forecasts out to at least 48 h. Airborne Doppler radar (ADR) data have sufficiently high horizontal and vertical resolution to resolve the hurricane vortex and its imbedded structures but have not been extensively used in hurricane initialization. Using the Weather Research and Forecasting (WRF) three-dimensional variational data assimilation (3DVAR) system, the ADR data are assimilated to recover the hurricane vortex dynamic and thermodynamic structures at the WRF model initial time. The impact of the ADR data on three hurricanes, Jeanne (2004), Katrina (2005) and Rita (2005), are examined during their rapid intensification and subsequent weakening periods before landfall. With the ADR wind data assimilated, the three-dimensional winds in the hurricane vortex become stronger and the maximum 10-m winds agree better with independent estimates from best-track data than without ADR data assimilation. Through the multivariate incremental structure in WRF 3DVAR analysis, the central sea level pressures (CSLPs) for the three hurricanes are lower in response to the stronger vortex at initialization. The size and inner-core structure of each vortex are adjusted closer to observations of these attributes. Addition of reflectivity data in assimilation produces cloud water and rainwater analyses in the initial vortex. The temperature and moisture are also better represented in the hurricane initialization. Forty-eight-hour forecasts are conducted to evaluate the impact of ADR data using the Advanced Research Hurricane WRF (AHW), a derivative of the Advanced Research WRF (ARW) model. Assimilation of ADR data improves the hurricane-intensity forecasts. Vortex asymmetries, size, and rainbands are also simulated better. Hurricane initialization with ADR data is quite promising toward reducing intensity forecast errors at modest computational expense.

Injection Heights of Biomass Burning Debris Estimated From WSR-88D Radar Observations

Understanding the vertical distribution of aerosols is critical to accurately determining their effects on air quality. Since current tools for obtaining this information have limited spatial and temporal coverage, we explore the use of Doppler radar data for obtaining the injection heights of biomass burning debris (BBD) produced from large fires in southern Georgia during Spring 2007. Due to their submicrometer sizes, the smoke aerosols are not detected by the radar. Therefore, we use BBD as a possible surrogate for aerosol height since smoke aerosols are often collocated with the debris. Using 32 h of Weather Surveillance Radar-1988 Doppler (WSR-88D) radar data from Jacksonville, FL, between May 23 and 25, 2007, the injection heights of BBD (D ~ 1mm) are calculated. Our analysis indicates that the maximum injection height is ~5 km for the strongest fire, with a mean injection height of 3 plusmn 1.0 km. Maximum injection heights are present between 1800 and 0000 UTC, during the late afternoon periods when both the intensity of the fire (based on radar information) and the convective mixing are greatest. The injection heights estimated from this approach represent the first step at providing inputs for future air-quality forecasting applications within numerical simulations, particularly ones that require diurnal information.

The Formation and Early Evolution of the Greensburg, Kansas, Tornadic Supercell on 4 May 2007

Abstract During the evening of 4 May 2007, a large, powerful tornado devastated Greensburg, Kansas. The synoptic and mesoscale environments of the parent supercell that spawned this and other tornadoes are described from operational data. The formation and early evolution of this long-track supercell, within the context of its larger-scale environment, are documented on the basis of Weather Surveillance Radar-1988 Doppler (WSR-88D) data and mobile Doppler radar data. The storm produced tornadoes cyclically for about 30 min before producing a large, long-lived tornado. It is shown that in order to have forecasted the severe weather locations and times accurately, it would have been necessary to have predicted 1) the localized formation of an isolated convective storm near/east of a dryline, 2) the subsequent splitting and resplitting of the storm several times, 3) the growth of a new storm along the right-rear flank of an existing storm, and 4) the transition from the cyclic production of small tornadoes to the production of one, large, long-track tornado. It is therefore suggested that both extreme sensitivity to initial conditions associated with storm formation and the uncertainty of storm behavior made it unusually difficult to forecast this event accurately.

Application of Canadian weather radar data to plant disease management schemes in southern Ontario

Management schemes based on the mean temperature and duration of wetness periods (dew or rain) have been developed to schedule fungicide applications rationally in a number of high‐value crops. Due to the spatial variability of convective rain events during the growing season in southern Ontario, data from an existing rain‐gauge network is inadequate for delivering advice to the farming community. Therefore, the use of both conventional and Doppler weather radar data to supplement rainfall data was investigated. Hourly measurements of leaf wetness duration and tipping bucket rainfall data were compared to data from the King City and Exeter radars in two disease warning schemes. When used in conjunction with the Penman‐Monteith model to estimate disease indices, the King City radar data were in better agreement with the leaf wetness sensor than the Exeter radar. There was no marked difference between conventional and Doppler data. When both conventional and Doppler radar data were used in combination, the success rate in detecting rainfall occurrence was 80–85%, with most errors due to false positives. Rainfall duration estimates from radar were sufficiently accurate in two disease management schemes that cumulate information from several successive wet periods.

An Example of the Use of Mobile, Doppler Radar Data for Tornado Verification

Abstract On 16 May 2003, two ground-based, mobile, Doppler radars scanned a potentially tornadic supercell in the Texas Panhandle intermittently from ∼0200 to 0330 UTC. The storm likely was tornadic, but because it was dark, visual confirmation of any tornadoes was not possible. A damage survey was completed after the storm moved through the area. The final conclusion of the damage survey prior to this analysis was that there were two tornadoes near Shamrock, Texas: one that formed prior to 0300 UTC and one that formed at or after 0300 UTC. High-resolution, mobile, Doppler radar data of the supercell were compared with the damage survey information at different times. The location of the first tornado damage path was not consistent with the locations of the low-level circulations in the supercell identified through the mobile, Doppler radar data. The damage within the first path, which consisted mostly of downed trees, may have been caused by straight-line winds in a squall line that moved through the area after the passage of the supercell. The mobile, Doppler radar data did not provide any supporting evidence for the first tornado, but the data did support the existence of the second tornado in Wheeler County on the evening of 15 May 2003. Ground-based, mobile, Doppler radar data may be used as an important tool to help to confirm (or deny) tornado damage reports in situations in which a damage survey cannot be completed or in which the survey does not provide clear evidence as to what phenomenon caused the damage.

Assimilation of coastal Doppler radar data with the ARPS 3DVAR and cloud analysis for the prediction of Hurricane Ike (2008)

The impact of radar data on the analysis and prediction of the structure, intensity, and track of landfalling Hurricane Ike (2008), at a cloud‐resolving resolution, is examined. Radial velocity (Vr) and reflectivity (Z) data from coastal radars are assimilated over a 6‐h period before Ike landfall, using the ARPS 3DVAR and cloud analysis package through 30‐min assimilation cycles. Eighteen‐hour predictions were made. All 4 experiments that assimilate radar data produce better structure, intensity and precipitation forecasts than that from operational GFS analysis. The improvement to the track forecast lasts for the entire 18 hours while that to intensity prediction lasts about 12 hours. The Vr data help improve the track forecast more while reflectivity data help improve intensity forecast most. Best results are obtained when both Z and Vr data are assimilated.

Bow Echo Mesovortices. Part II: Their Genesis

Abstract This two-part study examines the damaging potential and genesis of low-level, meso-γ-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). In Part II of this study, mesovortex genesis was investigated for vortices formed at different stages of convective system evolution. During the early “cellular” stage, cyclonic mesovortices were observed. The cyclonic mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. As the convective system evolved into a bow echo, cyclonic–anticyclonic mesovortex pairs were also observed. The vortex couplet was produced by a local updraft maximum that tilted baroclinically generated vortex lines upward into arches. The local updraft maximum was created by a convective-scale downdraft that produced an outward bulge in the gust front position. Cyclonic-only mesovortices were predominantly observed as the convective system evolved into the mature bow echo stage. Similar to the early cellular stage, these mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. The downdraft parcels descended within the rear-inflow jet. The generality of the mesovortex genesis mechanisms was assessed by examining the structure of observed mesovortices in Doppler radar data. The mesovortex genesis mechanisms were also compared to others reported in the literature and the genesis of low-level mesocyclones in supercell thunderstorms.

Statistic characteristics of severe convective storm during Warm-Season in the Beijing-Tianjin region and its vicinity

This study analyzed the climatological characteristics of severe convective storms in the Beijing and Tianjin region and its vicinity based on the Doppler radar data of Tanggu during May–August of 2003–2007. The climatological characteristics, e.g. storm area, volume, top height, max reflectivity, life time and motion, are analyzed. The results include: 75% of all storms in the Beijing-Tianjin region last no more than 30 minutes, and most storms have a volume less than 400 km3; most storms move from southwest to northeast while the speed is between 10–30 km/h; the mean storm top height is about 6 km, but some strong convective storms can have a top height larger than 15 km; finally, storm area and volume have a similar geographical distribution character showing increasing trends from west to east. Compared with the statistic results based on the conventional surface meteorological observations, the results based on the radar data can present not only 3D spatial statistic results of convective storms (e.g., volume and top height), but also the quantitative climatological characteristics, such as the lifetime and speed distributions. These statistical results are useful for studying the climatic characteristics of convective storms in the Beijing-Tianjin region and its vicinity.

Installation of a Doppler Radar Monitoring System at Merapi Volcano, Indonesia

Merapi Volcano, Indonesia, is one of the most active dome building volcanoes worldwide. Instabilities at the growing dome cause rockfalls and hot block and ash flows, which can reach run-out distances of several kilometers. Therefore, Merapi has been monitored extensively for many years. However, direct visual observation of the dome is often impossible due to cloud coverage of the summit. In October 2001, a first prototype Doppler radar system was installed to overcome this shortcoming. The system is able to penetrate clouds and observe material movements at the dome, giving valuable information about dynamic processes in the dome area. The system also allows detection of rainfall in several distance intervals. For precise positioning of the beam, the system was equipped with an electromechanical mounting in 2003. A charge-coupled device (CCD) camera attached to the radar mirror documents the radar beam position and provides visual observation of the dome. Recorded data, camera images, and status information are telemetered to the Merapi Volcano Observatory, where they can be processed and interpreted. Status information is also sent via short message service via a global system for mobile communications (GSM) modem. By processing the Doppler radar data, we are able to discriminate between three different types of instability events: sliding dome material, dome material gravitationally breaking off the dome, and explosive outbursts of dome material due to expansion of volcanic gas. In order to independently verify our observations, we compared rockfall events detected by the radar system to seismic recordings and found a good correlation.

Diagnostic analysis of the asymmetric structure of the simulated landfalling typhoon “Haitang”

The landfalling processes of Typhoon “Haitang” near Lianjiang of Fujian Province of China from 00 UTC 19 to 12 UTC 20 July 2005 were reproduced by using the nesting non-hydrostatic WRF model and data assimilation technology (Level II Doppler radar data of Changle of Fujian Province are assimilated to the simulation every one hour from 01 to 06 UTC 19 July 2005). The mesoscale structure and evolution of the typhoon before, during, and after its landfall were discussed. The simulation data show that the assimilation experiment with the high temporal-and-spatial-resolution radar radial velocity and reflectivity data can produce much better simulation of the typhoon track, intensity evolution and landfalling location than the control experiment without assimilating radar data. By using the assimilation experimental data, the mesoscale fine-mesh structure and evolution before and after typhoon landfall were analyzed. Because of the influence of sea–land thermodynamic difference, two asymmetric convective regions were located at ocean and land, respectively. To better understand and investigate the asymmetric-structure characteristics of the landfalling typhoon, several dynamical diagnostic tools, the helicity ( H), the moist potential vorticity (MPV), the convective vorticity vector (CVV), the moist vorticity vector (MVV), which are associated with the development of strong convections, are introduced. Further analysis illuminates that the distributions of these physical diagnostic parameters are totally asymmetric, and subsequently, the associated convections also show distinct asymmetry.

Kinematic structure of convective‐scale elements in the rainbands of Hurricanes Katrina and Rita (2005)

Airborne Doppler radar data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) show the convective‐scale air motions embedded in the principal rainbands of hurricanes Katrina and Rita. These embedded convective cells have overturning updrafts and low‐level downdrafts (originating at 2–4 km) that enter the rainband on its radially outward side and cross over each other within the rainband as well as a strong downdraft emanating from upper levels (6+ km) on the radially inward side. These vertical motion structures repeat from one convective cell to another along each rainband. The resulting net vertical mass transport is upward in the upwind portion of the band and greatest in the middle sector of the principal rainband, where the updraft motions contribute generally to an increase of potential vorticity below the 3–4 km level. Because the convective cells in the middle sector are systematically located radially just inside the secondary horizontal wind maximum (SHWM), the local increase in vorticity implied by the convective mass transport is manifest locally as an increase in the strength of the SHWM at midlevels (∼4 km). The overturning updrafts of the convective cells tilt, stretch, and vertically transport vorticity such that the convergence of the vertical flux of vorticity strengthens the vorticity anomaly associated with the SHWM. This process could strengthen the SHWM by several meters per second per hour, and may explain how high wave number convective‐scale features can influence a low wave number feature such as the principal rainband, and subsequently influence the primary vortex.

4D velocity of Strombolian eruptions and man-made explosions derived from multiple Doppler radar instruments

We used a novel system of three continuous wave Doppler radars to successfully record the directivity of i) Strombolian explosions from the active lava lake of Erebus volcano, Antarctica, ii) eruptions at Stromboli volcano, Italy, and iii) a man-made explosion in a quarry. Erebus volcano contains a convecting phonolite lava lake, presumably connected to a magma chamber at depth. It is one of the few open vent volcanoes that allow a direct observation of source processes during explosions. Its lava lake is the source of frequent violent Strombolian explosions, caused by large gas bubbles bursting at the lake surface. The exact mechanism of these bubble bursts is unclear, as is the mechanism of the creation of the infrasound signal accompanying the explosions. We use the Doppler radar data to calculate the directivity of Strombolian eruptions at Erebus. This allows us to derive information about the expected type of infrasound source pattern (i.e. the role of a dipole in addition to the monopole signature) and the physical structure of the volcano. We recorded 10 large explosions simultaneously with three radars, enabling us to calculate time series of 3D directivity vectors (i.e. effectively 4D), which describe the direction of preferred expansion of the gas bubble during an explosion. Such directivity information allows a comparison to dipole infrasound radiation patterns recorded during similar explosions only a few weeks later. Video observations of explosions support our interpretation of the measurements. We conclude that at Erebus, the directivity of explosions is mainly controlled by random processes. Since the geometry of the uppermost conduit is assumed to have a large effect on the directivity of explosions, the results suggest a largely symmetrical uppermost conduit with a vertical axis of symmetry. For infrasound recordings, a significant dipole signature can be expected in addition to the predominant monopole signature.

An analysis of the axisymmetric three‐dimensional low level wind field in a tornado using mobile radar observations

The purpose of this letter is to present an analysis of the three‐dimensional wind field retrieved from mobile Doppler radar data of the 12 May 2004 Harper, KS tornadoes. The derived three‐dimensional vortex structure and its surrounding flow show marked temporal and spatial variability. This has significant implications on the numerical modeling of tornado‐scale flow.