• Boe, B. A., 1994: The North Dakota Tracer Experiment: Tracer applications in a cooperative thunderstorm research program. J. Wea. Modif.,26, 102–112.

  • Charba, J., 1974: Application of gravity current model to analysis of squall-line gust front. Mon. Wea. Rev.,102, 140–156.

  • Cotton, W. R., and R. A. Anthes, 1989: Storm and Cloud Dynamics. Academic Press, 592 pp.

  • Droegemeier, K. K., and R. B. Wilhelmson, 1987: Numerical simulation of thunderstorm outflow dynamics. Part I: Outflow sensitivity experiments and turbulence dynamics. J. Atmos. Sci.,44, 1180–1210.

  • Fankhouser, J. C., 1982: The 22 June 1976 case study: Large-scale influences, radar echo structure and mesoscale circulations. Hailstorms of the Central High Plains, C.A. Knight and P. Squires, Eds., Colorado Associated University Press, 1–34.

  • Glancy, R. T., 1979: Aircraft observations of wind shear accompanying a severe thunderstorm gust front. M.S. thesis, Dept. of Atmospheric Sciences, University of Wyoming, 140 pp. [Available from Dept. of Atmospheric Science, University of Wyoming, Laramie, WY 82071.].

  • Goff, R. C., 1976: Vertical structure of thunderstorm outflows. Mon. Wea. Rev.,104, 1429–1440.

  • Houze, R. A., Jr., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Intrieri, J. M., A. J. Bedard, and R. M. Hardesty, 1990: Details of colliding thunderstorm outflows as observed by Doppler lidar. J. Atmos. Sci.,47, 1081–1098.

  • Klingle, D. L., D. R. Smith, and M. M. Wolfson, 1987: Gust front characteristics as detected by Doppler radar. Mon. Wea. Rev.,115, 905–918.

  • Mahoney, W. P., III, 1988: Gust front characteristics and the kinematics associated with interacting thunderstorm outflows. Mon. Wea. Rev.,116, 1474–1491.

  • ——, and A. R. Rodi, 1987: Aircraft measurements on microburst development from hydrometeor evaporation. J. Atmos. Sci.,44, 3037–3051.

  • Martner, B. E., and F. M. Ralph, 1993: Breaking Kelvin–Helmholtz waves and cloud-top entrainment as revealed by K-band Doppler radar. Preprints, Ninth Conf. on Atmospheric and Oceanic Waves and Stability, San Antonio, TX, Amer. Meteor. Soc., 141–144.

  • ——, J. D. Marwitz, and R. A. Kropfli, 1992: Radar observations of transport and diffusion in clouds and precipitation using TRACIR. J. Atmos. Oceanic Technol.,9, 226–241.

  • Mitchell, K. E., and J. B. Hovermale, 1977: A numerical investigation of the severe thunderstorm gust front. Mon. Wea. Rev.,105, 657–675.

  • Mueller, C. K., and R. E. Carbone, 1987: Dynamics of a thunderstorm outflow. J. Atmos. Sci.,44, 1879–1898.

  • Ralph, F. M., C. Mazaundier, M. Crochet, and S. V. Venkateswaran, 1993: Doppler sodar and wind-profiler observations of gravity-wave activity associated with a gravity current. Mon. Wea. Rev.,121, 444–463.

  • Shapiro, M. A., 1984: Meteorological tower measurements of a surface cold front. Mon. Wea. Rev.,112, 1634–1639.

  • Simpson, J. E., 1987: Gravity Currents in the Environment and the Laboratory. John Wiley and Sons, 244 pp.

  • Sinclair, P. C., and P. M. Kuhn, 1991: Aircraft low altitude wind shear detection and warning system. J. Appl. Meteor.,30, 3–16.

  • Wakimoto, R. M., 1982: The life cycle of thunderstorm gust fronts as viewed with Doppler radarand rawinsonde data. Mon. Wea. Rev.,110, 1060–1082.

  • Wilson, J. M., and W. E. Schreiber, 1986: Initiation of convective storms at radar-observed boundary-layer convergence lines. Mon. Wea. Rev.,114, 2516–2536.

  • View in gallery

    The 1900 CDT Bismarck NWS radiosonde data plotted on a skew T–logp thermodynamic diagram. The sonde was launched approximately 2 h before the gust front was observed near New Salem. The lifted index (LI) was −3.

  • View in gallery

    Time–height images of data from the vertically pointing NOAA/C radar on 20 June 1993. The upper panel is the radar reflectivity factor (dBZ), and the lower panel is vertical Doppler velocity (m s−1). Positive velocities (white and lighter gray shades) indicate upward motion. Locations of the gust front (GF), arcus cloud (AC), and a precipitation roll-like feature (PR) are indicated. For clarity, only a few shades of gray are used in these black and white renditions of the original color images.

  • View in gallery

    Time–height displays of the vertically pointing radar data for an 8-min period encompassing the arrival time of the gust front. Contours are shown for (a) the radar reflectivity factor in increments of 10 dBZ, (b) mean vertical velocity in 2 m s−1 increments, and (c) Doppler spectrum width in 0.5 m s−1 increments. In (a) the region of stronger reflectivity (>10 dBZ) is shaded with diagonal hatching. In (b) the zero velocity isotach is dotted and the region of downward motion is hatched.

  • View in gallery

    An 8-min time series of vertical velocity through the gust front at 1.35 km AGL, which was the height of the maximum updraft in the gust front. The maximum downward motion (not shown) occurred at the next higher range gate.

  • View in gallery

    Comparison of gust front structures: (a) the New Salem case vertical velocities, (b) vector winds in a Colorado gust front from Mahoney (1988), and (c) the schematic diagram of Droegemeier and Wilhelmson (1987). The New Salem data are those of Fig. 3b replotted to match the height scale of Mahoney’s (1988) figure with the timescale reversed; the contour interval is 2 m s−1. The vertical component of velocity in Mahoney’s case was derived from low-elevation-angle scans, and the contour interval is 3 m s−1.

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Vertical Velocities in a Thunderstorm Gust Front and Outflow

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Abstract

Continuous vertically pointing measurements of a thunderstorm outflow, including its gust front, were obtained with a Doppler radar near New Salem, North Dakota. The measurements provide a high-resolution depiction of the vertical structure of reflectivity and vertical velocity within the gust front, the outflow, and the parent storm. Earlier gust front remote sensing studies have used Doppler observations obtained with low-elevation-angle scans to accurately measure the horizontal flow pattern from which vertical velocities were subsequently estimated by integrating the continuity equation. In contrast, the New Salem case provides direct, rather than derived, Doppler measurement of vertical velocities with better vertical resolution and vastly superior temporal resolution. The gust front’s vertical structure is in general agreement with earlier observations and numerical simulations, except that the transition from strong upward to strong downward motion was more abrupt. The maximum updraft, of almost 10 m s−1, was measured in the gust front at 1.35 km above ground level and was followed by equally strong downward motion only 1 min later at a slightly higher altitude.The observations support the earlier use of the continuity method for deriving the basic pattern of vertical motions in density currents from quasi-horizontal scan data.

Corresponding author address: Brooks E. Martner, NOAA/ETL, 325 Broadway, Boulder, CO 80303.

bmartner@etl.noaa.gov

Abstract

Continuous vertically pointing measurements of a thunderstorm outflow, including its gust front, were obtained with a Doppler radar near New Salem, North Dakota. The measurements provide a high-resolution depiction of the vertical structure of reflectivity and vertical velocity within the gust front, the outflow, and the parent storm. Earlier gust front remote sensing studies have used Doppler observations obtained with low-elevation-angle scans to accurately measure the horizontal flow pattern from which vertical velocities were subsequently estimated by integrating the continuity equation. In contrast, the New Salem case provides direct, rather than derived, Doppler measurement of vertical velocities with better vertical resolution and vastly superior temporal resolution. The gust front’s vertical structure is in general agreement with earlier observations and numerical simulations, except that the transition from strong upward to strong downward motion was more abrupt. The maximum updraft, of almost 10 m s−1, was measured in the gust front at 1.35 km above ground level and was followed by equally strong downward motion only 1 min later at a slightly higher altitude.The observations support the earlier use of the continuity method for deriving the basic pattern of vertical motions in density currents from quasi-horizontal scan data.

Corresponding author address: Brooks E. Martner, NOAA/ETL, 325 Broadway, Boulder, CO 80303.

bmartner@etl.noaa.gov

Introduction

Strong downdrafts beneath thunderstorms are deflected laterally upon reaching the ground and often produce cold-air outflow and a gust front that propagate horizontally away from the storm. The gust front is the boundary between the cooler, denser outflow air from the storm and the surrounding warmer environmental air. Abrupt changes in horizontal and vertical winds along this boundary may be hazardous to low-flying aircraft and can cause damage to surface structures. In addition, the strong upward motions caused by convergence along the gust front may help maintain the parent thunderstorm or may initiate the formation of new convective storms (Wilson and Schreiber 1986). Thunderstorm outflow is an example of atmospheric density currents, and the gust front is dynamically similar to other cold flow boundaries, including microbursts and cold fronts. Brief overviews of the dynamics of gust fronts and density currents are presented by Cotton and Anthes (1989), Houze (1993), and Simpson (1987).

Gust fronts have been studied primarily by using measurements from towers or remote sensors and also through numerical modeling and laboratory simulations. Several earlier observations with single or multiple Doppler radars have used low-elevation-angle scans to accurately measure the horizontal wind field. Estimates of vertical velocities are then derived from the horizontal motions through vertical integration of the equation of continuity (e.g., Wakimoto 1982). A gust front study by Intrieri et al. (1990) using Doppler lidar observations relied on the same technique to derive vertical motions. By contrast, this article examines the structure of a gust front that passed over a vertical-pointing Doppler radar, thereby allowing direct and continuous measurements of the vertical velocities to be obtained. Ralph et al. (1993) reported vertical Doppler observations of a density current of unknown origin, possibly outflow from a distant thunderstorm, using a wind profiler and acoustic sodar measurements. Vertical motions in their case were considerably weaker than those presented here.

Instrumentation and conditions

The gust front occurred on the night of 20 June 1993 near New Salem, North Dakota, about 50 km west of Bismarck, North Dakota. The measurements were obtained with the NOAA/C research radar operated by the National Oceanic and Atmospheric Administration’s (NOAA) Environmental Technology Laboratory (ETL). This is a 3.2-cm-wavelength (X band) system that has Doppler, dual-polarization, and full scanning capabilities. It was deployed near New Salem to participate in the North Dakota Tracer Experiment (NDTE), a weather modification research project focused on studying the transport and dispersion of cloud seeding materials in convective storms (Boe 1994).

Almost all coordinated NDTE measurements were obtained during daylight hours, but the NOAA/C radar was also operated many nights in a zenith-pointing, unattended mode, following the conclusion of the daily NDTE operations. In this mode, the radar was fortunate to record high-resolution measurements of a thunderstorm gust front and outflow passing overhead on the night just before the official beginning of the NDTE field season. These fortuitous radar measurements provide a high-resolution Eulerian depiction of the vertical structure of a gust front, including accurate direct (rather than derived) Doppler observations of its vertical velocity structure.

The NOAA/C radar has excellent spatial and temporal resolution and good sensitivity. The radar has a beamwidth of 0.8°, and its range resolution in the NDTE was 112 m. At a height of 2 km, which was the approximate top of the outflow, these parameters yielded a quasi-cylindrical sample volume that was 112 m high and 30 m wide; the minimum detectable signal is about −32 dBZ at this height. A new beam of data was recorded every 0.3 s. At each of the 328 range gates, 250 pairs of pulses were electronically averaged in the 0.3-s dwell time to estimate the mean radial velocity with a very high degree of accuracy (better than ±0.1 m s−1 for typical conditions). The radar’s minimum range is about 150 m.

In addition to detecting precipitation, the radar can also measure the Doppler velocities of airflow in the visually clear boundary layer through the detection of suspended particulates such as insects, bits of vegetation, and large dust particles, and to a lesser degree through the detection of backscatter from refractive index gradients. This “clear-air” capability made the gust front measurements possible. The radar’s circular dual-polarization capability allows inferences of the scattering particles’ shapes to be made (Martner et al. 1992).

The gust front reached the NOAA/C radar near New Salem at 2114 CDT 20 June 1993, and the thunderstorm passed over the radar several minutes later. (The NDTE used central daylight time project-wide as its official time coordinate; add 5 h to obtain UTC). A rain gauge at the site recorded a total accumulation of 2.8 mm of rain. The 1900 CDT National Weather Service (NWS) radiosonde data from Bismarck are plotted in Fig. 1. The sounding shows that moderately high convective instability was present, and a dry-adiabatic layer with a well-mixed moisture profile extended from the surface to about 800 mb. The large dewpoint depressions of about 15°C in this low layer would have supported evaporational enhancement of thunderstorm downdrafts. Winds aloft were moderate, with directions between southwesterly and northwesterly.

The NDTE Doppler surveillance radar at Bismarck was not yet operational, and storm reflectivity images from the NWS radar scope at Bismarck were not recorded on film this night. The project did not operate a mesonet of surface instruments. Thus, unfortunately, there are no data to document the horizontal winds and the propagation speed of the New Salem gust front or the situation of its parent storm. Operator notes from the NWS radar, however, do provide some information about storm cells west of Bismarck. Their movement was consistently noted to be about 10 m s−1 (20 kt) from the west or west-northwest. The echoes were described as areas of cells, rather than lines, suggesting that the New Salem thunderstorm was not part of a squall line.

Vertically pointing measurements

Figure 2 displays time–height images of the NOAA/C vertically pointing data, spanning 27.5 min of time and 12 km of height above ground level (AGL, add 0.67 km to obtain height above mean sea level). This figure reveals the larger storm-scale context for the gust front. The upper panel of the figure shows the pattern of the radar reflectivity factor, and the lower panel shows the pattern of vertical velocity. In the velocity image, white and light gray shadings represent regions of upward motion. The gust front’s location beneath the thunderstorm’s anvil is clearly visible in these images between 0 and 2 km AGL at about 2114 CDT. The velocity image reveals an abrupt change from strong upward to strong downward motions just behind the gust front, with areas of weaker up and down motions extending from there to the leading edge of the precipitation at about 2128 CDT.

An arcus cloud echo can be seen above the gust front at 3.8 km AGL in the velocity panel of Fig. 2. This cloud echo was about 400 m thick and had upward Doppler velocities of 2–3 m s−1. This and other very weak echo velocity features revealed by the radar’s linear receiver were not detectable by the less sensitive logarithmic receiver from which the reflectivity data of Fig. 2 are derived.

Within the thunderstorm echo, the melting-layer height near 3.5 km AGL is discernable by a weak enhancement of reflectivity and a sharp increase of downward velocities. Below this level, downward motions were generally 7–10 m s−1, indicating the presence of large raindrops or strong downdrafts. Vigorous convective vertical motions ranging from −12 to +8 m s−1 were detected at 9 km AGL, high within the core of the storm. The thin, black, horizontal lines at 0.6 and 1.3 km AGL in the reflectivity image are nearby ground clutter targets that were filtered effectively from the velocity data but not from the reflectivity data. At 2138 CDT, a power failure, perhaps caused by a nearby lightning strike, ended the data recording. Prior to this, the storm’s maximum reflectivity directly above the radar was 37 dBZ at 8 km AGL. Although larger reflectivities may have occurred later or at other locations, the available data suggest that this was not an intense storm.

Even assuming steady-state conditions, it is not possible to precisely convert the timescale of Fig. 2 into a horizontal scale because of the absence of supporting measurements, but a crude approximation is nevertheless useful. The NWS radar operator noted that the thunderstorm cells in the area were moving eastward at about 10 m s−1. Because the gust front arrived at the NOAA/C radar approximately 14 min before the thunderstorm, its speed must have been greater than 10 m s−1. By selecting a propagation speed of 13 m s−1, which is not uncommon for gust fronts (Goff 1976; Wakimoto 1982; Mahoney 1988), an approximate horizontal separation of 11 km is derived for the distance between the gust front and the leading edge of the storm precipitation (between 2114 and 2128 CDT).

The radar’s depolarization data (not shown) confirm that, at the time it passed overhead, the New Salem gust front echo was produced by highly depolarizing, very nonspherical targets, such as insects, seeds, and dust particles, rather than by raindrops. Indeed, this is true of all of the outflow echo below 2 km AGL until 2128 CDT, when streamers of rain first reached the surface. These small particulates in the gust front and outflow air have relatively weak or minimal terminal velocities, and therefore they trace the vertical air motions with greater fidelity than raindrops. Their observed vertical motions will only slightly underestimate updraft speeds and overestimate the downdrafts. In contrast, the depolarization data indicate that the arcus cloud echo was composed of spherical particles—namely, cloud droplets.

Figure 3 provides a closer look at the features in the vicinity of the gust front. It shows contoured values of the radar reflectivity factor, mean velocity, and Doppler spectrum width for data spanning 2.8 km of height and 8 min. The contours were mapped from the data by graphics software that applied spline smoothing. Thus, some details and the extreme values are not revealed in these plots.

The reflectivity data (Fig. 3a) show that the gust front region contained values of 0 to +10 dBZ, which was about 15–20 dB greater than the surrounding boundary layer echo. The vertical velocity pattern (Fig. 3b) shows a curl-shaped region of strong upward motion at the outflow’s leading edge, which became broader and stronger in upper levels. This was followed by a slightly longer period of downward motion, which also had its strongest values in the upper region behind the gust front. The extremes of vertical velocity ranged from +9.7 m s−1 at 1.35 km AGL to −9.9 m s−1 at 1.46 km AGL 1 min later. Up- and downdrafts in excess of 2 m s−1 each lasted for about 2 min (∼1.6-km width) above the radar. Thus, it seems likely that these features were part of a large organized circulation, rather than just transient small-scale turbulent eddies.

The Doppler spectrum width value (Fig. 3c) is related to the strength of turbulence at subbeam sample volume scales, among other factors. For zenith-pointing radar observations, the distribution of raindrop terminal fall speeds is often the dominant contributor to this parameter. However, in the New Salem case, the scatterers were not raindrops; therefore, turbulent air motions within the beam are likely to have dominated this measurement. The greatest spectrum width values coincided with the regions of stronger upward and downward motions (compare Figs. 3b,c), suggesting that the most turbulent gust front air occurred there. The extreme value of approximately 3 m s−1 at 1.35 km AGL occurred just before the maximum updraft.

A time series plot of the vertical velocity data at 1.35 km AGL is shown in Fig. 4. This is the height at which the maximum updraft appeared, and it is just below the height of the maximum downdraft. At this height, the transition from updraft to downdraft encompassed a 15 m s−1 change in only 38 s. If the 13 m s−1 propagation speed estimate is applied, this amounts to a 15 m s−1 change over a horizontal distance of 494 m. Most of this change occurred in less than half of the period and distance.

Discussion

Compared with previous radar observations of gust fronts, the NOAA/C data have vastly superior temporal resolution and better vertical resolution, and provide more accurate vertical motion data because of the direct, rather than derived, nature of the measurement. Still, many features of the New Salem gust front closely resemble those of other observations reported in the literature based on low-elevation-angle-scan Doppler techniques. The 2-km outflow depth and the maximum vertical velocities of almost ±10 m s−1 near 1.4 km AGL in the New Salem case are not unusual compared with numerous other gust fronts studied with radar by Wakimoto (1982) in Illinois, Klingle et al. (1987) in Oklahoma, and Mahoney (1988) in Colorado. The pattern and magnitudes of the vertical motions are remarkably similar to those in cases studied by Mueller and Carbone (1987) and Mahoney (1988) using dual-Doppler measurements. These similarities suggest that the use of continuity in the earlier radar studies to derive vertical motions from the horizontal flow fields is reasonably accurate for this application. Specific similarities and differences between these and other studies are discussed in this section.

Figure 5 presents a comparison of the New Salem measurements with those of a gust front examined by Mahoney (1988) and with the schematic summary diagram of Droegemeier and Wilhelmson (1987), which is based on observations by several researchers as well as their own numerical simulations and those of Mitchell and Hovermale (1977). Although the New Salem data reveal more intricate detail in the vertical velocity pattern, basic structural similarities are evident in all three diagrams in the figure. In each, strong updrafts caused by convergence occur at the leading edge of the outflow and slope backward with height toward the storm; maximum updrafts occur more than a kilometer above ground. The updrafts are closely followed by deep downward motion, with the maximum downdrafts located aloft. The scale between the maximum and minimum vertical motion is more compressed in the New Salem case (1 min or about 1 km). This abruptness is not unprecedented for density currents, however. Shapiro (1984) used tower data to document a vertical motion change of +5 to −2 m s−1 in about 15 s for the passage of a scale-contracted cold front.

Based on common visual observations, gust front schematic diagrams such as Droegemeier and Wilhelmson’s place an arcus cloud above the gust front head. The velocity image of Fig. 2 shows that a weak cloud echo with upward velocity did exist at this location for the New Salem case. The modeling simulations of Droegemeier and Wilhelmson (1987) show the development of prominent Kelvin–Helmholtz (KH) instability billows atop the outflow. There is no clear evidence of the existence of KH waves in the New Salem data, however. Although there are fluctuating upward and downward motions in the outflow echo behind the gust front (Fig. 2), the variations do not appear to have the regular periodicity of waves. Vertically pointing radar observations such as these are capable of detecting KH waves, as was demonstrated by Martner and Ralph (1993) for observations of billows at the top of a deep stratiform cloud. Therefore, if KH waves existed in this case, they must have been weak or poorly developed. The spectrum width pattern in Fig. 3c suggests that the most turbulent region was very closely aligned with the region of strongest updrafts and downdrafts, rather than with the wake region farther behind the outflow head, as indicated in the schematic diagram of Droegemeier and Wilhelmson (1987).

In situ measurements of gust fronts have been obtained using instrumented towers and research aircraft. The tower measurements (e.g., Charba 1974; Goff 1976; Intrieri et al. 1990) were limited to heights below 0.5 km AGL and were, therefore, unlikely to reach the levels at which the maximum vertical motions occur. The strongest of the vertical velocities detected by anemometers in these studies was a 6 m s−1 updraft at the top of a 461-m tower reported by Goff (1976); he inferred that stronger vertical motions existed at greater heights. None of these tower studies detected vertical velocity changes that were as abrupt as those of the New Salem case. Aircraft penetrations of gust fronts at 600–700 m AGL reported by Glancy (1979) and Fankhouser (1982) both measured maximum updrafts of about 7 m s−1. Updrafts on the perimeters of microbursts measured with aircraft by Mahoney and Rodi (1987) and Sinclair and Kuhn (1991) were considerably weaker than those observed by radar in the New Salem gust front.

Wakimoto (1982) introduced the concept that a gust front’s reflectivity, in all but its dissipating stage, may be produced in large measure by raindrops from the parent thunderstorm that are suspended in the horizontal vortex at the outflow’s leading edge, rather than by dust or insects swept up from the surface. He called this feature the precipitation roll. However, as mentioned in section 3, the NOAA/C depolarization data conclusively rule out raindrops as the primary scattering targets in the New Salem case. Thus, this case would only seem to fit Wakimoto’s (1982) dissipation stage gust front model, but the New Salem vertical motions were at least a factor of 2 more vigorous than those of his dissipation stage example. At 2128 CDT, a curl-shaped reflectivity streamer connected to the parent storm near the surface passed over the radar shortly before the arrival of the storm core aloft (Fig. 2). This feature might represent a secondary surge of outflow. It has the same shape as Wakimoto’s precipitation rolls, and the depolarization data confirm that these scatterers were raindrops.

The spectral width values shown in Fig. 3c are much smaller than those reported by Klingle et al. (1987) for several Oklahoma gust fronts. Their measurements, however, are of nearly horizontal motions within low-elevation-angle radar beams, in contrast to the New Salem measurements, which are of vertical motions within a much narrower vertical beam. The Oklahoma spectrum width data probably contained large contributions from vertical shear of the horizontal winds across the beam, which can be significant, especially at long ranges.

The sharp reversal of vertical motion illustrated in Fig. 4 would at least have caused discomfort, and perhaps a more serious hazard, to the occupants of any airplane flying through the gust front at this altitude. For an airplane moving at 100 m s−1, the vertical air motion change of 15 m s−1 in 38 s observed by the radar would have been compressed into about 5 s, assuming the 494-m horizontal separation computed from the estimated propagation speed is accurate. Within this period, changes of 11 m s−1 across 208 m would have been encountered in 2 s by the aircraft. This rapid updraft–downdraft transition would have produced a sharp turbulent jolt, similar to those encountered by aircraft flying within vigorously growing convective clouds and mature thunderstorms. Yet the strong downdrafts in this case were located high enough above the ground that a dangerous loss of altitude is not likely to have resulted.

An environment-relative maximum horizontal wind speed of about 20 m s−1 is found for the New Salem case using the 13 m s−1 propagation speed estimate and Mahoney’s (1988) finding that the maximum horizontal outflow winds are, on average, 50% faster than the gust front propagation speed. Unlike microbursts, in which a penetrating aircraft encounters a lift-reducing tailwind shortly after an initially strengthened headwind, a gust front penetration, in either direction, experiences only the relatively increased, lift-enhancing headwind. Thus, horizontal wind changes are also unlikely to have posed a severe flight hazard in this case. Nevertheless, if not anticipated, the sharp vertical and horizontal wind changes across a gust front may be sufficiently abrupt to cause an unsuspecting pilot to lose control for some anxious period of time.

Summary and conclusions

Continuous, vertically pointing measurements were obtained with a Doppler radar in North Dakota as a thunderstorm gust front and outflow passed overhead. The vertical structure of the outflow and its parent thunderstorm were documented in finescale detail. Maximum vertical motions ranged from +9.7 m s−1 in the gust front at 1.35 km AGL to −9.9 m s−1 at 1.46 km AGL in the outflow air behind it 1 min later. Horizontal winds were not measured, but the gust front propagated toward the radar at a speed of at least 10 m s−1.

Compared with other remote sensor studies of gust fronts, these measurements provide better height resolution (112 m) and far better temporal resolution (0.3 s). They also reveal the vertical structure of the density current at greater heights than has been possible with instrumented towers and with more complete height coverage than can be obtained with limited aircraft penetrations. In addition to the finer-scale resolution, the North Dakota case provides more accurate vertical velocity data than the earlier remote sensing studies of gust fronts because they are direct Doppler measurements of the vertical wind instead of continuity-derived estimates from low-elevation-angle measurements of horizontal winds.

Although generalizations should not be drawn from a single case, similarities in the overall structure and in the vertical velocity patterns and magnitudes of this case with earlier remote and in situ observations of gust fronts suggest that this was not an unusual case. The similarities lend support to the earlier use of the continuity method for deriving vertical motions in density currents from quasi-horizontal scan data. The new measurements do differ from earlier studies in that the transition from strong updrafts to strong downdrafts was more abrupt and, therefore, potentially more hazardous to low-flying aircraft.

Acknowledgments

The NDTE was conducted in collaboration with the North Dakota Atmospheric Resource Board. ETL’s participation was funded by NOAA and the state of North Dakota through the Atmospheric Modification Program. Bruce Bartram of ETL single-handedly installed and maintained the radar near New Salem. The skew T sounding diagram was provided by Ben Bernstein of the National Center for Atmospheric Research.

REFERENCES

  • Boe, B. A., 1994: The North Dakota Tracer Experiment: Tracer applications in a cooperative thunderstorm research program. J. Wea. Modif.,26, 102–112.

  • Charba, J., 1974: Application of gravity current model to analysis of squall-line gust front. Mon. Wea. Rev.,102, 140–156.

  • Cotton, W. R., and R. A. Anthes, 1989: Storm and Cloud Dynamics. Academic Press, 592 pp.

  • Droegemeier, K. K., and R. B. Wilhelmson, 1987: Numerical simulation of thunderstorm outflow dynamics. Part I: Outflow sensitivity experiments and turbulence dynamics. J. Atmos. Sci.,44, 1180–1210.

  • Fankhouser, J. C., 1982: The 22 June 1976 case study: Large-scale influences, radar echo structure and mesoscale circulations. Hailstorms of the Central High Plains, C.A. Knight and P. Squires, Eds., Colorado Associated University Press, 1–34.

  • Glancy, R. T., 1979: Aircraft observations of wind shear accompanying a severe thunderstorm gust front. M.S. thesis, Dept. of Atmospheric Sciences, University of Wyoming, 140 pp. [Available from Dept. of Atmospheric Science, University of Wyoming, Laramie, WY 82071.].

  • Goff, R. C., 1976: Vertical structure of thunderstorm outflows. Mon. Wea. Rev.,104, 1429–1440.

  • Houze, R. A., Jr., 1993: Cloud Dynamics. Academic Press, 573 pp.

  • Intrieri, J. M., A. J. Bedard, and R. M. Hardesty, 1990: Details of colliding thunderstorm outflows as observed by Doppler lidar. J. Atmos. Sci.,47, 1081–1098.

  • Klingle, D. L., D. R. Smith, and M. M. Wolfson, 1987: Gust front characteristics as detected by Doppler radar. Mon. Wea. Rev.,115, 905–918.

  • Mahoney, W. P., III, 1988: Gust front characteristics and the kinematics associated with interacting thunderstorm outflows. Mon. Wea. Rev.,116, 1474–1491.

  • ——, and A. R. Rodi, 1987: Aircraft measurements on microburst development from hydrometeor evaporation. J. Atmos. Sci.,44, 3037–3051.

  • Martner, B. E., and F. M. Ralph, 1993: Breaking Kelvin–Helmholtz waves and cloud-top entrainment as revealed by K-band Doppler radar. Preprints, Ninth Conf. on Atmospheric and Oceanic Waves and Stability, San Antonio, TX, Amer. Meteor. Soc., 141–144.

  • ——, J. D. Marwitz, and R. A. Kropfli, 1992: Radar observations of transport and diffusion in clouds and precipitation using TRACIR. J. Atmos. Oceanic Technol.,9, 226–241.

  • Mitchell, K. E., and J. B. Hovermale, 1977: A numerical investigation of the severe thunderstorm gust front. Mon. Wea. Rev.,105, 657–675.

  • Mueller, C. K., and R. E. Carbone, 1987: Dynamics of a thunderstorm outflow. J. Atmos. Sci.,44, 1879–1898.

  • Ralph, F. M., C. Mazaundier, M. Crochet, and S. V. Venkateswaran, 1993: Doppler sodar and wind-profiler observations of gravity-wave activity associated with a gravity current. Mon. Wea. Rev.,121, 444–463.

  • Shapiro, M. A., 1984: Meteorological tower measurements of a surface cold front. Mon. Wea. Rev.,112, 1634–1639.

  • Simpson, J. E., 1987: Gravity Currents in the Environment and the Laboratory. John Wiley and Sons, 244 pp.

  • Sinclair, P. C., and P. M. Kuhn, 1991: Aircraft low altitude wind shear detection and warning system. J. Appl. Meteor.,30, 3–16.

  • Wakimoto, R. M., 1982: The life cycle of thunderstorm gust fronts as viewed with Doppler radarand rawinsonde data. Mon. Wea. Rev.,110, 1060–1082.

  • Wilson, J. M., and W. E. Schreiber, 1986: Initiation of convective storms at radar-observed boundary-layer convergence lines. Mon. Wea. Rev.,114, 2516–2536.

Fig. 1.
Fig. 1.

The 1900 CDT Bismarck NWS radiosonde data plotted on a skew T–logp thermodynamic diagram. The sonde was launched approximately 2 h before the gust front was observed near New Salem. The lifted index (LI) was −3.

Citation: Journal of Applied Meteorology 36, 5; 10.1175/1520-0450(1997)036<0615:VVIATG>2.0.CO;2

Fig. 2.
Fig. 2.

Time–height images of data from the vertically pointing NOAA/C radar on 20 June 1993. The upper panel is the radar reflectivity factor (dBZ), and the lower panel is vertical Doppler velocity (m s−1). Positive velocities (white and lighter gray shades) indicate upward motion. Locations of the gust front (GF), arcus cloud (AC), and a precipitation roll-like feature (PR) are indicated. For clarity, only a few shades of gray are used in these black and white renditions of the original color images.

Citation: Journal of Applied Meteorology 36, 5; 10.1175/1520-0450(1997)036<0615:VVIATG>2.0.CO;2

Fig. 3.
Fig. 3.

Time–height displays of the vertically pointing radar data for an 8-min period encompassing the arrival time of the gust front. Contours are shown for (a) the radar reflectivity factor in increments of 10 dBZ, (b) mean vertical velocity in 2 m s−1 increments, and (c) Doppler spectrum width in 0.5 m s−1 increments. In (a) the region of stronger reflectivity (>10 dBZ) is shaded with diagonal hatching. In (b) the zero velocity isotach is dotted and the region of downward motion is hatched.

Citation: Journal of Applied Meteorology 36, 5; 10.1175/1520-0450(1997)036<0615:VVIATG>2.0.CO;2

Fig. 4.
Fig. 4.

An 8-min time series of vertical velocity through the gust front at 1.35 km AGL, which was the height of the maximum updraft in the gust front. The maximum downward motion (not shown) occurred at the next higher range gate.

Citation: Journal of Applied Meteorology 36, 5; 10.1175/1520-0450(1997)036<0615:VVIATG>2.0.CO;2

Fig. 5.
Fig. 5.

Comparison of gust front structures: (a) the New Salem case vertical velocities, (b) vector winds in a Colorado gust front from Mahoney (1988), and (c) the schematic diagram of Droegemeier and Wilhelmson (1987). The New Salem data are those of Fig. 3b replotted to match the height scale of Mahoney’s (1988) figure with the timescale reversed; the contour interval is 2 m s−1. The vertical component of velocity in Mahoney’s case was derived from low-elevation-angle scans, and the contour interval is 3 m s−1.

Citation: Journal of Applied Meteorology 36, 5; 10.1175/1520-0450(1997)036<0615:VVIATG>2.0.CO;2

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