4. INTERPRETATION OF DOPPLER VELOCITY PATTERNS WITHIN CONVECTIVE STORMS

4.1 Introduction

In this chapter, we display simulated flow fields and the corresponding Doppler velocity patterns within a 30 x 30 km (16 x 16 n mi) window located 100 km (54 n mi) due north of the radar. It is assumed that the Doppler radar scans horizontally through each flow field, which is a reasonable approximation given the small scale of the display. Except as noted, the simulated flow fields represent basic features without the addition of environmental winds and storm motion. Unlike the simulations in the previous chapters, no noise was added to the Doppler velocity data points. Since these Doppler velocity patterns are blown up, the blockiness associated with 250 m by 1.0° sampling becomes readily apparent.

A typical radar antenna produces a beam that is about 1.0° wide. Since the antenna typically scans horizontally while it collects a sufficient number of samples (minimum of about 30–50) to produce a representative mean Doppler velocity value, the beam is effectively broadened to about 1.5°. However, in order to clearly portray the essence of the patterns simulated in this chapter, the radar beam is assumed to have zero width. Consequently, we do not simulate the gradual degradation of convective-scale features that occurs when the linear width of the beam increases (constant angular width) as distance from the radar increases (e.g., Wood and Brown 1997).

With a beam of zero width, small-scale features like tornadoes are not degraded as they would be with a 1.5° effective beamwidth. Instead, we simulate tornadoes in Section 4.9 using the broader and weaker tornadic vortex signature (TVS) that would have resulted from sampling with a 1.5° effective beamwidth.

4.2 Patterns Associated with Constant Wind Speed and Direction

In regions of a convective storm not directly affected by updrafts and downdrafts, the Doppler velocity pattern primarily reflects the relatively undisturbed environmental flow. 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). About all one can tell is that the flow has a component away from the radar because the Doppler velocities in the window are positive.

The Doppler velocity pattern in Fig. 4.2.2 is somewhat easier to interpret. When the radar points due north, the Doppler velocity value is zero. Negative velocities to the left of the zero band indicate a component of the wind toward the radar and positive values to the right indicate a component away from the radar. We can state with confidence, then, that the wind is blowing from the west, but we are unable to determine the wind speed. For the situations shown in Figs. 4.2.1 and 4.2.2, we would have been able to determine both wind speed and direction if we were able to see the entire PPI display, as in the figures shown in Chapter 2.

Doppler velocity pattern corresponding to a uniform horizontal wind blowing from the southwest toward the northeast

Fig. 4.2.1.  Doppler velocity pattern (right) corresponding to a uniform horizontal wind blowing from the southwest toward the northeast at a speed of 25 m s-1 (49 kt) (left). Positive Doppler velocities represent flow away from the radar, which is located 100 km (54 n mi) south of the display center. (larger image)

Same as Fig. 4.2.1, except the wind is blowing from the west

Fig. 4.2.2.  Same as Fig. 4.2.1, except the wind is blowing from the west at a speed of 25 m s-1 (49 kt). Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

4.3 Patterns Associated with Axisymmetric Vortices

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

A cyclonic circulation representing a thunderstorm mesocyclone is simulated in Fig. 4.3.2. The circulation, as well as the Doppler velocity pattern, is a mirror image of the anticyclonic case. If the mesocyclone were moving toward the north at 18 m s-1 (35 kt), Fig. 4.3.3 indicates that the flow field and colors (representing Doppler velocity values) are different but that the overall Doppler velocity pattern within and surrounding the core region remains basically the same. In practice, a Doppler velocity display will look more like Fig. 4.3.3 than Fig. 4.3.2 because there typically will be a Doppler component of mesocyclone/storm motion present. When the estimated mesocyclone/storm component of motion is subtracted from the Doppler velocity display, the resulting pattern (representing storm-relative flow) becomes more balanced (as in Fig. 4.3.2).

Doppler velocity pattern (right) of a mesoanticyclone that has peak tangential velocities of 25 m s-1 at a radius of 3 km from the circulation center

Fig. 4.3.1.  Doppler velocity pattern (right) of a mesoanticyclone (left) that has peak tangential velocities of 25 m s-1 (49 kt) at a radius of 3 km (1.6 n mi) from the circulation center (black dot); radius of maximum winds is indicated by circle. Arrow length is proportional to wind speed. (larger image)

Same as Fig. 4.3.1, except that the Doppler velocity pattern corresponds to a mesocyclone at a radius of 3 km from the circulation center

Fig. 4.3.2.  Same as Fig. 4.3.1, except that the Doppler velocity pattern (right) corresponds to a mesocyclonic (left) that has peak tangential velocities of 25 m s-1 (49 kt) at a radius of 3 km (1.6 n mi) from the circulation center (black dot). (larger image)

Same as Fig. 4.3.2, except that the mesocyclone is moving toward the north

Fig. 4.3.3.  Same as Fig. 4.3.2, except that the mesocyclone is moving toward the north at 18 m s-1 (35 kt). Though the Doppler velocity pattern (right) remains essentially unchanged, the apparent circulation center in the flow field (left) is displaced to the left of the true circulation center. Note that Doppler velocity values exceeding 30 m s-1 on the right side of the circulation are aliased.(larger image)

4.4 Patterns Associated with Axisymmetric Radial Flow

A Rankine combined velocity profile also can be used to simulate axisymmetric convergent and divergent flow. In this case, the core radius is the radius at which the inflow or outflow wind speed is a maximum. An example of simulated convergent flow is shown in Fig. 4.4.1. The zero band indicates the portion of the wind field that is perpendicular to the radar viewing direction. At infinite distance from 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 concave side (e.g., Wood and Brown 1992). Convergent flow is toward the radar on the far side of the zero band and away from the radar on the near side. The two regions with extreme Doppler velocities are located at distances from the convergence center equal to the core radius.

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

Doppler velocity pattern corresponding to axisymmetric convergent flow.

Fig. 4.4.1.  Doppler velocity pattern (right) corresponding to axisymmetric convergent flow (left). The maximum radial velocity of 25 m s-1 (49 kt) is at a core radius of 3 km (1.6 n mi); the radius of maximum winds is indicated by the circle. Black dot represents the center of the flow. Arrow length is proportional to wind speed. Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

Same as Fig. 4.4.1, except that the Doppler velocity pattern corresponds to axisymmetric divergent flow.

Fig. 4.4.2.  Same as Fig. 4.4.1, except that the Doppler velocity pattern (right) corresponds to axisymmetric divergent flow (left). (larger image)

Same as Fig. 4.4.2, except that the divergence feature is moving to the north

Fig. 4.4.3.  Same as Fig. 4.4.2, except that the divergence feature is moving to the north at 18 m s-1 (35 kt) or a uniform flow field of 18 m s-1 from the south is superimposed on it. Note that Doppler velocity values exceeding 30 m s-1 on the far side of the divergence center are aliased. (larger image)

4.5 Mesocyclone and Divergence Patterns Viewed from Four Different Directions

It is frequently necessary for weather decision makers to view storm features using multiple radars. Knowing each radar's location relative to a particular feature is critical for proper signature recognition. For example, the Doppler velocity patterns representing mesocyclonic and divergent flows in Sections 4.3 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 vital that the pattern be interpreted relative to the viewing direction from the radar.

Figures 4.5.1 and 4.5.2 are presented in order to emphasize the importance of the viewing direction. In all cases, the display window is oriented with north 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 radar always 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 always is 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 not possible to properly interpret the type of flow field that produced the Doppler velocity pattern.

Doppler velocity patterns for a mesocyclone viewed by a radar (located at the center) from four different directions.

Fig. 4.5.1.  Doppler velocity patterns for a mesocyclone viewed by a radar (located at the center) from four different directions. (larger image)

Doppler velocity patterns for axisymmetric divergence viewed by a radar (located at the center) from four different directions.

Fig. 4.5.2.  Doppler velocity patterns for axisymmetric divergence viewed by a radar (located at the center) from four different directions. (larger image)

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

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

In Fig. 4.6.1, the radar is only 10 km (5 n mi) south of the mesocyclone center. The weaker Doppler velocities associated with the signature extend southward and converge at the radar position. The boundaries between colors on the inside of the mesocyclone's core region are along imaginary lines radiating out from the radar location; this feature is not as obvious with 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 region extend southward to some extent toward the radar.

The zero Doppler velocity band for axisymmetric convergence located 10 km from the radar (right part of Fig. 4.6.3) has a unique shape—the center of the zero band is a circle passing through the radar and the convergence center. Note that the chord connecting the two points is the diameter of the circle. From plane geometry we 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 intersects the 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 band is less curved because the center of the band now is part of a circle whose diameter is 30 km. Consequently, the extreme positive and negative Doppler velocity 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 evidence that the color boundaries within the mesocyclone core region are oriented along 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 circle 100 km in diameter.

Doppler velocity pattern of a mesocyclone when the center is located 10 km (5 n mi) north of the radar

Fig. 4.6.1.  Doppler velocity pattern of a mesocyclone when the center (dot at center of window) is located 10 km (5 n mi) north of the radar; the radar is located at the point where radial lines intersect near bottom center of the window. Core radius is 3 km (1.6 n mi) and peak velocity is 25 m s-1 (49 kt). Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

Same as Fig. 4.6.1, except that the center of the mesocyclone is 30 km north of the radar

Fig. 4.6.2.  Same as Fig. 4.6.1, except that the center of the mesocyclone is 30 km (16 n mi) north of the radar. (larger image)

Doppler velocity pattern of axisymmetric convergence when the center is located 10 km north of the radar

Fig. 4.6.3.  Doppler velocity pattern of axisymmetric convergence when the center is located 10 km (5 n mi) north of the radar; the radar is located at the point where radial lines intersect near bottom center of the window. Core radius is 3 km (1.6 n mi) and peak velocity is 25 m s-1 (49 kt). (larger image)

Same as Fig. 4.6.3, except that the convergence center is 30 km north of the radar.

Fig. 4.6.4.  Same as Fig. 4.6.3, except that the convergence center is 30 km (16 n mi) north of the radar. (larger image)

4.7 Patterns Associated with a Convergent/Divergent Mesocyclone

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

Figure 4.7.1 shows the combination of convergence and cyclonic rotation where the core radii are the same and the peak velocities are 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 velocity patterns 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 through the radar and the center of the flow feature, but the diameter of the circle no longer passes through both points.

At a greater range of 30 km (16 n mi), the pattern in Fig. 4.7.2 is midway between the cyclonic rotation and convergence patterns in Figs. 4.6.2 and 4.6.4. The same is true for the 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 in Fig. 4.7.4 is midway between that for divergence (Fig. 4.4.2) and for cyclonic rotation (Fig. 4.3.2). Combinations of anticyclonic rotation with convergence and with divergence having the same core radii and peak velocities produce similarly rotated patterns that are midway between the respective individual Doppler velocity patterns.

When the convergence/divergence and rotation fields are not of the same size and strength, curious Doppler velocity patterns are produced. Figure 4.7.5 shows the results of combining stronger and smaller cyclonic rotation with weaker and larger convergence. The reverse is shown in Fig. 4.7.6. In both situations, the overall orientation of the zero Doppler velocity band indicates a combination of convergence and cyclonic rotation (as in Fig. 4.7.3). The clue in Fig. 4.7.5 that the stronger field is rotation is that the pattern of extreme Doppler velocity values at the center of the pattern is only slightly rotated from a pure rotation pattern. Analogously, the central pattern of extreme Doppler velocity values in Fig. 4.7.6 is only slightly rotated from a pure convergence pattern.

Doppler velocity pattern corresponding to combination of convergence and cyclonic rotation fields having the same core radius

Fig. 4.7.1.  Doppler velocity pattern (right) corresponding to combination of convergence and cyclonic rotation fields (left) having the same core radius (3 km or 1.6 n mi); maximum inflow velocity and maximum rotational velocity are the same with maximum resultant velocity being 25 m s-1 (49 kt). Black dot represents the feature center and the circle indicates radius of maximum winds. Arrow length is proportional to wind speed. The feature center is 10 km (5 n mi) due north of the radar. Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

Same as Fig. 4.7.1, except that feature center is 30 km due north of the radar.

Fig. 4.7.2.  Same as Fig. 4.7.1, except that feature center is 30 km (16 n mi) due north of the radar. (larger image)

Same as Fig. 4.7.1 (convergence and cyclonic rotation), except that the feature center is 100 km due north of the radar.

Fig. 4.7.3.  Same as Fig. 4.7.1 (convergence and cyclonic rotation), except that the feature center is 100 km (54 n mi) due north of the radar. (larger image)

Same as Fig. 4.7.3, except that the flow field is a combination of divergence and cyclonic rotation

Fig. 4.7.4.  Same as Fig. 4.7.3, except that the flow field is a combination of divergence and cyclonic rotation. (larger image)

Doppler velocity pattern corresponding to cyclonic rotation that is stronger and smaller than axisymmetric convergence

Fig. 4.7.5.  Doppler velocity pattern (right) corresponding to cyclonic rotation that is stronger and smaller (peak tangential velocity is 20 m s-1 or 39 kt, core radius is 1.5 km or 0.8 n mi) than axisymmetric convergence (peak radial velocity is 10 m s-1 or 19 kt, core radius is 3 km or 1.6 n mi). The feature center is 100 km (54 n mi) due north of the radar. (larger image)

Similar to Fig. 4.7.5, except that the convergence is stronger and smaller than cyclonic rotation

Fig. 4.7.6.  Similar to Fig. 4.7.5, except that the convergence is stronger and smaller (peak radial velocity is 20 m s-1 or 39 kt, core radius is 1.5 km or 0.8 n mi) than cyclonic rotation (peak tangential velocity is 10 m s-1 or 19 kt, core radius is 3 km or 1.6 n mi). (larger image)

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

Mesocyclones associated with supercell thunderstorms undergo a periodic evolution at roughly 45-minute intervals, where the mesocyclone core region weakens 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 5 km (16 kft) in Figs. 4.8.1 and 4.8.2. The centers of the two core regions are separated by a distance equal to three core radii.

In Fig. 4.8.1, the radar viewing direction is normal to an imaginary line connecting the circulation centers. The magnitudes of the peak Doppler velocities of each mesocyclone core region are decreased between the two centers owing to the opposing flow induced by the other core region. On the other hand, the magnitudes of the peak Doppler velocities on the outer portions of each core region are increased owing 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 the radar viewing direction is at a 45° angle to an imaginary line connecting the circulation centers (Fig. 4.8.2), the two circulations are offset enough for the signatures of two separate mesocyclones to be more apparent. One might erroneously deduce the presence of convergence at the center of the window; however, this feature is simply deformation arising from the juxtaposition of 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 the overall presence of cyclonic rotation.

Doppler velocity pattern corresponding to two identical mesocyclones whose centers are three core radii apart.

Fig. 4.8.1.  Doppler velocity pattern (right) corresponding to two identical mesocyclones (peak tangential velocity is 25 m s-1 or 49 kt, core radius is 3 km or 1.6 n mi) whose centers are three core radii apart (left). Black dots represent the mesocyclone centers. Arrow length is proportional to wind speed. Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

Same as Fig. 4.8.1, except that an imaginary line through the mesocyclone centers is oriented 45 deg to the radar viewing direction.

Fig. 4.8.2.  Same as Fig. 4.8.1, except that an imaginary line through the mesocyclone centers is oriented 45° to the radar viewing direction. (larger image)

4.9 Patterns Associated with a Tornadic Vortex Signature within the Parent Mesocyclone

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

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

Doppler velocity pattern corresponding to a TVS located at the mesocyclone center

Fig. 4.9.1.  Doppler velocity pattern (right) corresponding to a TVS (not the tornado itself; peak TVS velocity is 40 m s-1 or 78 kt, core radius is 0.9 km or 0.5 n mi) located at the mesocyclone center (peak velocity is 25 m s-1 or 49 kt, core radius is 3 km or 1.6 n mi). Black dot represents the coincident mesocyclone and TVS centers. The mesocyclone's core region is within the circle (left panel). Aliased velocities occur at the TVS location (right panel). (larger image)

Same as Fig. 4.9.1, except that the TVS center is located 1.5 km east of the mesocyclone center.

Fig. 4.9.2.  Same as Fig. 4.9.1, except that the TVS center is located 1.5 km east of the mesocyclone center. The larger and smaller circles in the left panel represent the extent of the mesocyclone and TVS core regions, respectively. (larger image)

Same as Fig. 4.9.2, except that the TVS center is located at the edge of the core region 3.0 km northeast of the mesocyclone center

Fig. 4.9.3.  Same as Fig. 4.9.2, except that the TVS center is located at the edge of the core region 3.0 km northeast of the mesocyclone center. (larger image)

4.10 Patterns Associated with Two Divergence Regions

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

Shown in Fig. 4.10.1 are two identical divergence regions, whose centers are separated by three core radii. The individual divergence patterns are not symmetric owing to interaction of the flow fields. The positive Doppler velocity portion of the southwestern divergence region strengthens the positive portion and weakens the negative portion of the northeastern divergence region. Analogously, the negative Doppler velocity portion of the northeastern divergence region strengthens the negative portion and weakens the positive portion of the southwestern divergence region. Note the presence of aliased Doppler velocity values where the flow in one divergence region was enhanced by flow in the other. The region of apparent clockwise rotation at the center of the window actually represents deformation as shown in the left panel.

The Doppler velocity pattern in Fig. 4.10.2 reflects the proximity of two unequal divergence regions with the larger and stronger one located southwest of the smaller and weaker one. Though the larger and stronger divergence region dominates, the interaction of the flow fields still produces a mutual modification of the Doppler velocity patterns for the two outflow regions.

Doppler velocity pattern corresponding to two divergence regions having the same size and strength

Fig. 4.10.1.  Doppler velocity pattern (right) corresponding to two divergence regions having the same size (core radius of 4 km or 2.2 n mi) and strength (30 m s-1 or 58 kt). The centers of the two divergence centers are 12 km (6.5 n mi) apart and are oriented at a 45° angle to the radar viewing direction. Black dots represent divergence centers. Arrow length is proportional to wind speed. Negative (positive) Doppler velocities represent flow toward (away from) the radar. (larger image)

Same as Fig. 4.10.1, except that the divergence center to the northeast is weaker and smaller

Fig. 4.10.2.  Same as Fig. 4.10.1, except that the divergence center to the northeast is weaker and smaller with a peak radial velocity of 15 m s-1 (29 kt) and a core radius of 2 km (1.1 n mi). (larger image)

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 outward from the rear flank downdraft. These features are simulated in the left portions of Figs. 4.11.1–4.11.3. A third (secondary) feature is included in the simulation to represent the frequently-observed increase in speed of air converging into the updraft ahead of the gust front.

The simulated flow fields and associated Doppler velocity displays in Figs. 4.11.1–4.11.3 represent storms nominally moving toward the northeast, east, and southeast, respectively, when viewed by a Doppler radar located 50 km due south of the circulation center. The overall Doppler velocity pattern for a storm moving toward the northeast (Fig. 4.11.1) resembles that for a mesocyclone. The pattern is modified by strong flow away from the radar ahead of the gust front. The small negative area south of the mesocyclone center is due to a small region of diffluent air behind the southern end of the gust front that has a component toward the radar.

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

When the pattern is rotated an additional 45° (Fig. 4.11.3), representing storm motion toward the southeast, strong winds ahead of and behind the gust front form a misshapen mesocyclone signature. Flow behind the front enhances the negative Doppler velocity portion of the mesocyclone signature.

Doppler velocity pattern corresponding to cyclonic surface flow into the supercell’s updraft with enhanced inflow ahead of the gust front

Fig. 4.11.1.  Doppler velocity pattern (right) corresponding to cyclonic surface flow into the supercell's updraft with enhanced inflow ahead of the gust front (left). The storm is assumed to be moving toward the northeast. Note aliasing of the peak positive Doppler velocity values. (larger image)

Same as Fig. 4.11.1, except that the storm is assumed to be moving toward the east.

Fig. 4.11.2.  Same as Fig. 4.11.1, except that the storm is assumed to be moving toward the east. (larger image)

Same as Fig. 4.11.1, except that the storm is assumed to be moving toward the southeast.

Fig. 4.11.3.  Same as Fig. 4.11.1, except that the storm is assumed to be moving toward the southeast. Note slight aliasing of both the peak positive and negative Doppler velocity values. (larger image)

4.12 Patterns Associated with Midaltitude Flow around a Thunderstorm Updraft Region

The updraft region in the upwind portion of a thunderstorm typically interacts with midaltitude flow that overtakes the slower-moving storm (e.g., Brown 1989; Brown and Torgerson 2005). Evidently, the vertical momentum of the strong individual updrafts and pressure perturbations associated with the updraft region present enough resistance to the approaching flow that air slows down as it passes through the semiporous updraft region. As air farther upstream approaches the area of slower-moving air with its higher perturbation pressure, some of the air is diverted around the updraft region. The diverted air increases speed as it is forced to flow around the edges of the updraft region. A developing area of wake flow—that forms directly downstream 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 forms at the downstream edge of the dominant updraft within the updraft region where some of the diverted air converges from both sides into a localized low-pressure region and descends as a weak downdraft. When the updraft dies, the minimum moves downstream with air continuing to converge into it. Successive dominant updrafts produce the train of successive minima that eventually flow out the downstream end of the storm.

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

The Doppler velocity pattern in the right portion of Fig. 4.12.1 represents the type of midaltitude pattern that one might expect to see when a Doppler radar scans across a thunderstorm from an upstream location. Doppler velocity maxima occur on the lateral flanks of the overall updraft region and Doppler velocity minima occur through the center of the storm. It is typical for localized minima to occur within the wake 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. 4.12.2 and 4.12.3, respectively), the pattern of the low-speed wake region is still evident downstream of the updraft region. However, the Doppler velocity maximum on the right flank of the updraft region is greatly reduced because most of 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 component of flow around the updraft region, which is indicated by the extreme positive and negative Doppler velocity areas on the fringes of the updraft region.

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

Doppler velocity pattern corresponding to ground-relative midaltitude flow around a strong thunderstorm updraft region

Fig. 4.12.1.  Doppler velocity pattern (right) corresponding to ground-relative midaltitude flow (left) around a strong thunderstorm updraft region (circle). Arrow length is proportional to wind speed. The radar viewing direction is parallel to the overall environmental wind. See text for an explanation of the dots in the left panel. Note that Doppler velocity aliasing occurs in the strong flow around the circled updraft region. (larger image)

Same as Fig. 4.12.1, except that the radar viewing direction is at a 30 deg angle to the overall environmental flow.

Fig. 4.12.2.  Same as Fig. 4.12.1, except that the radar viewing direction is at a 30° angle to the overall environmental flow. (larger image)

Same as Fig. 4.12.1, except that the radar viewing direction is at a 60 deg angle to the overall environmental flow.

Fig. 4.12.3.  Same as Fig. 4.12.1, except that the radar viewing direction is at a 60° angle to the overall environmental flow. (larger image)

Same as Fig. 4.12.1, except that the radar viewing direction is perpendicular to the overall environmental flow.

Fig. 4.12.4.  Same as Fig. 4.12.1, except that the radar viewing direction is perpendicular to the overall environmental flow. (larger image)

 

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