A Guide for Interpreting Doppler Velocity Patterns (2024)

4. INTERPRETATION OF DOPPLER VELOCITY PATTERNS WITHIN CONVECTIVE STORMS

4.1 Introduction

In this chapter, we display simulated flow fields and the correspondingDoppler velocity patterns within a 30 x 30 km (16 x 16 n mi) window located100 km (54 n mi) due north of the radar. It is assumed that the Doppler radarscans horizontally through each flow field, which is a reasonable approximationgiven the small scale of the display. Except as noted, the simulated flowfields represent basic features without the addition of environmental windsand storm motion. Unlike the simulations in the previous chapters, no noisewas added to the Doppler velocity data points. Since these Doppler velocitypatterns are blown up, the blockiness associated with 250 m by 1.0° samplingbecomes readily apparent.

A typical radar antenna produces a beam that isabout 1.0° wide. Since the antenna typically scans horizontally while itcollects a sufficient number of samples (minimum of about 30–50) to producea representative mean Doppler velocity value, the beam is effectively broadenedto about 1.5°. However, in order to clearly portray the essence of the patternssimulated in this chapter, the radar beam is assumed to have zero width.Consequently, we do not simulate the gradual degradation of convective-scalefeatures that occurs when the linear width of the beam increases (constantangular width) as distance from the radar increases (e.g., Wood and Brown1997).

With a beam of zero width, small-scale features like tornadoes arenot degraded as they would be with a 1.5° effective beamwidth. Instead, wesimulate tornadoes in Section 4.9 using the broader and weaker tornadic vortexsignature (TVS) that would have resulted from sampling with a 1.5° effectivebeamwidth.

4.2 Patterns Associated with Constant Wind Speed and Direction

In regionsof a convective storm not directly affected by updrafts and downdrafts, theDoppler velocity pattern primarily reflects the relatively undisturbed environmentalflow. It is difficult to deduce from the Doppler velocity pattern in Fig.4.2.1 that the wind is blowing from the southwest at 25 m s^-1 (49 kt). Aboutall one can tell is that the flow has a component away from the radar becausethe Doppler velocities in the window are positive.

The Doppler velocity patternin Fig. 4.2.2 is somewhat easier to interpret. When the radar points duenorth, the Doppler velocity value is zero. Negative velocities to the leftof the zero band indicate a component of the wind toward the radar and positivevalues to the right indicate a component away from the radar. We can statewith confidence, then, that the wind is blowing from the west, but we areunable to determine the wind speed. For the situations shown in Figs. 4.2.1and 4.2.2, we would have been able to determine both wind speed and directionif we were able to see the entire PPI display, as in the figures shown inChapter 2.

4.3 Patterns Associated with Axisymmetric Vortices

Axisymmetric flow arounda vertical axis can be approximated by the overly simplified Rankine combinedvelocity profile, as discussed in the previous chapter. An anticyclonic (clockwise)circulation (mesoanticyclone) is simulated in Fig. 4.3.1. The zero Dopplervelocity band lies along the radial direction from the radar through thecirculation center because flow at all ranges is perpendicular to that viewingdirection. (If the radar beam were not directed exactly toward the circulationcenter, but slightly offset, the positive and negative values would be adjacentto each other with no intervening zero band.) The negative Doppler velocityextreme on the right represents peak tangential velocity toward the radarat the core radius. The positive Doppler velocity extreme on the left, alsoat the core radius, represents peak tangential velocity away from the radar.Mesoanticyclones often are found in the left-moving members of splittingsevere storms.

A cyclonic circulation representing a thunderstorm mesocycloneis simulated in Fig. 4.3.2. The circulation, as well as the Doppler velocitypattern, is a mirror image of the anticyclonic case. If the mesocyclone weremoving toward the north at 18 m s^-1 (35 kt), Fig. 4.3.3 indicates that theflow field and colors (representing Doppler velocity values) are differentbut that the overall Doppler velocity pattern within and surrounding thecore region remains basically the same. In practice, a Doppler velocity displaywill look more like Fig. 4.3.3 than Fig. 4.3.2 because there typically willbe a Doppler component of mesocyclone/storm motion present. When the estimatedmesocyclone/storm component of motion is subtracted from the Doppler velocitydisplay, the resulting pattern (representing storm-relative flow) becomesmore balanced (as in Fig. 4.3.2).

4.4 Patterns Associated with Axisymmetric Radial Flow

A Rankine combinedvelocity profile also can be used to simulate axisymmetric convergent anddivergent flow. In this case, the core radius is the radius at which theinflow or outflow wind speed is a maximum. An example of simulated convergentflow is shown in Fig. 4.4.1. The zero band indicates the portion of the windfield that is perpendicular to the radar viewing direction. At infinite distancefrom the radar, the zero velocity band would be a straight line. However,at finite distances, the zero band is curved with the radar on the concaveside (e.g., Wood and Brown 1992). Convergent flow is toward the radar onthe far side of the zero band and away from the radar on the near side. Thetwo regions with extreme Doppler velocities are located at distances fromthe convergence center equal to the core radius.

The divergence pattern inFig. 4.4.2 is analogous to the convergence pattern but with the colors—andflow directions—reversed. It represents divergence that is found within theupper portions of an updraft or within a microburst beneath a downdraft atthe earth's surface. When the feature is moving toward the north at 18 ms^-1 (35 kt) or has a uniform flow field of 18 m s^-1 from the south superimposedon it (Fig. 4.4.3), the divergent flow pattern becomes distinctly differentwith the apparent divergence center displaced to the south; the true divergencecenter is indicated by the black dot. Also, the Doppler velocities are distinctlydifferent, with only a small region of negative velocities and a larger regionof aliased positive velocities. However, comparison of Figs. 4.4.2 and 4.4.3reveals that the overall Doppler velocity pattern within and surroundingthe core region remains essentially unchanged with the addition of a uniformmotion/flow field.

4.5 Mesocyclone and Divergence Patterns Viewed from Four Different Directions

It is frequently necessary for weather decision makers to view storm featuresusing multiple radars. Knowing each radar's location relative to a particularfeature is critical for proper signature recognition. For example, the Dopplervelocity patterns representing mesocyclonic and divergent flows in Sections4.3 and 4.4 are nearly the same except for a rotational difference of 90°.Thus, in order to distinguish between rotation and divergence, it is vitalthat the pattern be interpreted relative to the viewing direction from theradar.

Figures 4.5.1 and 4.5.2 are presented in order to emphasize the importanceof the viewing direction. In all cases, the display window is oriented withnorth toward the top. In Fig. 4.5.1, when a radar scans past a mesocyclone,flow away from the radar always is on the right and flow toward the radaralways is on the left relative to the radar viewing direction. In Fig. 4.5.2,when a radar scans past a divergence region, flow toward the radar alwaysis on the near side and flow away from the radar always is on the far side.

The mesocyclone pattern to the south of the radar (Fig. 4.5.1), for example,is similar to the divergence pattern to the west of the radar (Fig. 4.5.2).Without knowing the location of the radar relative to the window, it is notpossible to properly interpret the type of flow field that produced the Dopplervelocity pattern.

4.6 Distortion of Doppler Velocity Patterns Owing to Proximity to the Radar

For the mesocyclone and divergence Doppler velocity patterns discussedin Sections 4.3 through 4.5, the radar was located 100 km (54 n mi) fromthe center of the flow features. When the radar is closer to the features,the patterns become more distorted like those for the tropical cyclone discussedin Chapter 3.

In Fig. 4.6.1, the radar is only 10 km (5 n mi) south of themesocyclone center. The weaker Doppler velocities associated with the signatureextend southward and converge at the radar position. The boundaries betweencolors on the inside of the mesocyclone's core region are along imaginarylines radiating out from the radar location; this feature is not as obviouswith vortices that have assumed profiles different from a Rankine profile.

At a range of 30 km (16 n mi), the mesocyclone signature is less distorted(Fig. 4.6.2). The Doppler velocity areas outside the mesocyclone's core regionextend southward to some extent toward the radar.

The zero Doppler velocityband for axisymmetric convergence located 10 km from the radar (right partof Fig. 4.6.3) has a unique shape—the center of the zero band is a circlepassing through the radar and the convergence center. Note that the chordconnecting the two points is the diameter of the circle. From plane geometrywe know that any angle inscribed in a semicircle is a right angle. Therefore,at that point along the radar viewing direction where the radial line intersectsthe zero line, the radial line is perpendicular to the convergent (or divergent)streamline flowing straight into (out of) the center of the convergence (divergence)signature.

At a range of 30 km (Fig. 4.6.4), the zero Doppler velocity bandis less curved because the center of the band now is part of a circle whosediameter is 30 km. Consequently, the extreme positive and negative Dopplervelocity regions are more nearly symmetric.

At a range of 100 km (54 n mi),the distorted character of the mesocyclone (Fig. 4.3.2) and convergence (Fig.4.4.1) patterns is less noticeable. However, one may note that there is evidencethat the color boundaries within the mesocyclone core region are orientedalong imaginary lines radiating from the radar. For the convergence pattern,there is a suggestion that the center of the zero band is part of a circle100 km in diameter.

4.7 Patterns Associated with a Convergent/Divergent Mesocyclone

When therotation and convergence/divergence fields of Sections 4.3, 4.4, and 4.6are combined and have the same core radius, the resulting Doppler velocitypattern resembles a Rankine combined velocity profile. The primary distinctionis that the zero band is neither parallel nor perpendicular to the radarviewing direction. Instead, the zero band is at an intermediate angle dependingon the relative peak velocities of the two flow field components. If boththe peak velocities and core radii are different, the resulting Doppler velocitypattern is more complicated.

Figure 4.7.1 shows the combination of convergenceand cyclonic rotation where the core radii are the same and the peak velocitiesare the same; the feature center is 10 km (5 n mi) north of the radar.In this case, the pattern is rotated 45°, midway between the Doppler velocitypatterns for cyclonic rotation and convergence (see Figs. 4.6.1 and 4.6.3).The center of the zero Doppler velocity band is a circle that passes throughthe radar and the center of the flow feature, but the diameter of the circleno longer passes through both points.

At a greater range of 30 km (16 nmi), the pattern in Fig. 4.7.2 is midway between the cyclonic rotationand convergence patterns in Figs. 4.6.2 and 4.6.4. The same is true forthe pattern at 100 km (54 n mi) distance in Fig. 4.7.3 (compare with Figs.4.3.2 and 4.4.1).

The combination of divergence and cyclonic rotation inFig. 4.7.4 is midway between that for divergence (Fig. 4.4.2) and for cyclonicrotation (Fig. 4.3.2). Combinations of anticyclonic rotation with convergenceand with divergence having the same core radii and peak velocities producesimilarly rotated patterns that are midway between the respective individualDoppler velocity patterns.

When the convergence/divergence and rotationfields are not of the same size and strength, curious Doppler velocitypatterns are produced. Figure 4.7.5 shows the results of combining strongerand smaller cyclonic rotation with weaker and larger convergence. The reverseis shown in Fig. 4.7.6. In both situations, the overall orientation ofthe zero Doppler velocity band indicates a combination of convergence andcyclonic rotation (as in Fig. 4.7.3). The clue in Fig. 4.7.5 that the strongerfield is rotation is that the pattern of extreme Doppler velocity valuesat the center of the pattern is only slightly rotated from a pure rotationpattern. Analogously, the central pattern of extreme Doppler velocity valuesin Fig. 4.7.6 is only slightly rotated from a pure convergence pattern.

4.8 Patterns Associated with Two Mesocyclones having the Same Size and Strength

Mesocyclones associated with supercell thunderstorms undergo a periodicevolution at roughly 45-minute intervals, where the mesocyclone core regionweakens and a new core region concurrently forms on its right flank (e.g.,Burgess et al. 1982). This phenomenon is simulated at a height of about 5km (16 kft) in Figs. 4.8.1 and 4.8.2. The centers of the two core regionsare separated by a distance equal to three core radii.

In Fig. 4.8.1, theradar viewing direction is normal to an imaginary line connecting the circulationcenters. The magnitudes of the peak Doppler velocities of each mesocyclonecore region are decreased between the two centers owing to the opposing flowinduced by the other core region. On the other hand, the magnitudes of thepeak Doppler velocities on the outer portions of each core region are increasedowing to flow in the same direction from both circulations. At first glance,one might mistaken the pattern to be one for a single mesocyclone.

When theradar viewing direction is at a 45° angle to an imaginary line connectingthe circulation centers (Fig. 4.8.2), the two circulations are offset enoughfor the signatures of two separate mesocyclones to be more apparent. Onemight erroneously deduce the presence of convergence at the center of thewindow; however, this feature is simply deformation arising from the juxtapositionof the two rotational fields viewed at this angle (see left side of Fig.4.8.2). Note that the prevailing orientation of the zero band indicates theoverall presence of cyclonic rotation.

4.9 Patterns Associated with a Tornadic Vortex Signature withinthe Parent Mesocyclone

Nearly all significant tornadoes form within a pre-existingparent mesocyclone and typically are found within the mesocyclone's coreregion. Since all but the largest and closest tornadoes are smaller thanthe radar's beamwidth, the tornado's tangential velocities are greatly smoothed(degraded) within the radar beam. Consequently, the Doppler velocities withinthe resulting tornadic vortex signature (TVS) do not reflect either the sizeor strength of the tornado but rather some indeterminable combination ofthe two parameters (see Brown et al. 1978). The one consistent feature ofa TVS is that peak Doppler velocities toward and away from the radar areapproximately one beamwidth apart.

A strong TVS is simulated at the centerof the core region of the parent mesocyclone in Fig. 4.9.1; a tornado isnot simulated because the simulated radar beam has zero width and thereforea tornado would not be properly degraded into a TVS. A comparison of thepattern in Fig. 4.9.1 with the one in Fig. 4.3.2 for a mesocyclone by itselfshows that higher velocities extend toward the center of the mesocycloneowing to the presence of the TVS. Figure 4.9.2 illustrates the situationwhere the TVS is slightly to the right of the mesocyclone center. Again,the TVS makes its presence known because significant velocities extend acrossthe center of the mesocyclone signature where one would expect Doppler velocitiesto otherwise approach zero when the center of the radar beam coincides withthe mesocyclone center. When a strong TVS is located away from the mesocyclonecenter (Fig. 4.9.3), it takes on a signature more of its own, but each circulationsignature is affected by the presence of the other.

4.10 Patterns Associated with Two Divergence Regions

Air diverging out froma point source is observed in the atmosphere near storm top in the upperportions of an updraft and near the earth's surface beneath a downdraft (amicroburst). When two updrafts or downdrafts are near each other, the correspondingdivergent flow fields interact with each other.

Shown in Fig. 4.10.1 aretwo identical divergence regions, whose centers are separated by three coreradii. The individual divergence patterns are not symmetric owing to interactionof the flow fields. The positive Doppler velocity portion of the southwesterndivergence region strengthens the positive portion and weakens the negativeportion of the northeastern divergence region. Analogously, the negativeDoppler velocity portion of the northeastern divergence region strengthensthe negative portion and weakens the positive portion of the southwesterndivergence region. Note the presence of aliased Doppler velocity values wherethe flow in one divergence region was enhanced by flow in the other. Theregion of apparent clockwise rotation at the center of the window actuallyrepresents deformation as shown in the left panel.

The Doppler velocity patternin Fig. 4.10.2 reflects the proximity of two unequal divergence regions withthe larger and stronger one located southwest of the smaller and weaker one.Though the larger and stronger divergence region dominates, the interactionof the flow fields still produces a mutual modification of the Doppler velocitypatterns for the two outflow regions.

4.11 Patterns Associated with Flow Fields beneath Supercell Thunderstorms

Two dominant surface features of a mature supercell thunderstorm are (a)overall cyclonic flow around the storm's rotating updraft (mesocyclone) and(b) the gust front at the leading edge of the cold air expanding outwardfrom the rear flank downdraft. These features are simulated in the left portionsof Figs. 4.11.1–4.11.3. A third (secondary) feature is included in the simulationto represent the frequently-observed increase in speed of air converginginto the updraft ahead of the gust front.

The simulated flow fields and associatedDoppler velocity displays in Figs. 4.11.1–4.11.3 represent storms nominallymoving toward the northeast, east, and southeast, respectively, when viewedby a Doppler radar located 50 km due south of the circulation center. Theoverall Doppler velocity pattern for a storm moving toward the northeast(Fig. 4.11.1) resembles that for a mesocyclone. The pattern is modified bystrong flow away from the radar ahead of the gust front. The small negativearea south of the mesocyclone center is due to a small region of diffluentair behind the southern end of the gust front that has a component towardthe radar.

With the storm moving toward the east, the overall flow patternis rotated by 45° (Fig. 4.11.2). The increased component of flow toward theradar immediately behind the gust front results in a secondary peak in thenegative Doppler velocity values. The strong winds ahead of the front enhancethe positive Doppler velocity portion of the mesocyclone signature.

Whenthe pattern is rotated an additional 45° (Fig. 4.11.3), representing stormmotion toward the southeast, strong winds ahead of and behind the gust frontform a misshapen mesocyclone signature. Flow behind the front enhances thenegative Doppler velocity portion of the mesocyclone signature.

4.12 Patterns Associated with Midaltitude Flow around a Thunderstorm UpdraftRegion

The updraft region in the upwind portion of a thunderstorm typicallyinteracts with midaltitude flow that overtakes the slower-moving storm (e.g.,Brown 1989; Brown and Torgerson 2005). Evidently, the vertical momentum ofthe strong individual updrafts and pressure perturbations associated withthe updraft region present enough resistance to the approaching flow thatair slows down as it passes through the semiporous updraft region. As airfarther upstream approaches the area of slower-moving air with its higherperturbation pressure, some of the air is diverted around the updraft region.The diverted air increases speed as it is forced to flow around the edgesof the updraft region. A developing area of wake flow—that forms directlydownstream of the updraft region—includes a narrow swath of low-speed air.The center of the wake contains a train of speed minima. Each minimum formsat the downstream edge of the dominant updraft within the updraft regionwhere some of the diverted air converges from both sides into a localizedlow-pressure region and descends as a weak downdraft. When the updraft dies,the minimum moves downstream with air continuing to converge into it. Successivedominant updrafts produce the train of successive minima that eventuallyflow out the downstream end of the storm.

This midaltitude flow pattern issimulated in the left portions of Figs. 4.12.1–4.12.4. The circle approximatesthe updraft region. The dot near the upstream edge of the circle is the centerof axisymmetric divergence that simulates diverging flow around the updraftregion. The other two dots within the circle represent a vortex pair (cyclonicon right, anticyclonic on left) that is used to produce increased speedsaround the updraft region and decreased speeds within the updraft region.The pairs of downstream dots represent successively weaker vortex pairs thatare used to simulate the low-speed wake region.

The Doppler velocity patternin the right portion of Fig. 4.12.1 represents the type of midaltitude patternthat one might expect to see when a Doppler radar scans across a thunderstormfrom an upstream location. Doppler velocity maxima occur on the lateral flanksof the overall updraft region and Doppler velocity minima occur through thecenter of the storm. It is typical for localized minima to occur within thewake and for localized maxima to occur along the flanks of the wake region.

When radar views the midaltitude flow at 30o and 60o angles (Figs. 4.12.2and 4.12.3, respectively), the pattern of the low-speed wake region is stillevident downstream of the updraft region. However, the Doppler velocity maximumon the right flank of the updraft region is greatly reduced because mostof the strong flow on that flank is perpendicular to the radar viewing direction.At these viewing angles, the radar is sensing some of the divergent componentof flow around the updraft region, which is indicated by the extreme positiveand negative Doppler velocity areas on the fringes of the updraft region.

When the viewing direction is perpendicular to the overall flow, the presenceof a wake is no longer readily evident (Fig. 4.12.4). Instead, there is aprominent divergence signature associated with air diverging around the updraftregion. Farther downstream, the wake region is represented by weak convergencesignatures associated with air converging into the localized speed minima.

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