NOTES and CORRESPONDENCE High-Resolution Airborne Radar Observations of Mammatus

JANUARY 2001 NOTES AND CORRESPONDENCE 159 NOTES AND CORRESPONDENCE High-Resolution Airborne Radar Observations of Mammatus NATHANIEL S. WINSTEAD,J.VERLINDE,S.TRACY ARTHUR,FRANCINE JASKIEWICZ, MICHAEL JENSEN,NATASHA MILES, AND DAVID NICOSIA Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania 14 January 2000 and 20 June 2000 ABSTRACT High-resolution Doppler radar observations of mammatus clouds coupled with soundings of the preanvil and anvil environments provide a unique opportunity to examine previously reported observations of, and evaluate various hypotheses of, mammatus formation. These observations con®rm the general hypothesis for mammatus formation advanced by Ludlam and Scorer, and provide detail of the cloud interior structure. Speci®cally, the radar observations indicate that mammatus elements are reminiscent of eddy circulations with a weak downdraft core ¯anked by horizontal convergence and divergence at the top and base of the cloud, respectively. Doppler spectral width measurements, however, yielded values of only 2±3 m s21, indicating only weak turbulent motions within individual mammatus elements. Re¯ectivity analyses of mammatus elements indicate a ®rm link to the parent anvil. A dual-Doppler analysis of the parent anvil indicates that the larger-scale environment where the mammatus exist is characterized by the existence of gravity waves or shear overturning. It is hypothesized that these circulations might play a role in the initiation of this particular outbreak of mammatus. 1. Introduction matus clouds. However, studies to verify these hypoth- Mammatus are one of the most striking cloud features eses have been hard to come by. Some studies, such as seen in the atmosphere. Often observed beneath the an- those by Hlad (1944), Martner (1995), and Stith (1995), vils of thunderstorms, these clouds have served as a even presented observations that, on ®rst appearance, visual warning to pilots that turbulence is likely to be seem to bring into question some aspects of the early present (Lankford 1990). Early hypotheses of the pro- hypotheses. Martner presented observations indicating cesses leading to the development of mammatus were that individual clouds have their root deep in the anvil, presented by Wagner (1948), Ludlam and Scorer (1953), while Stith observed positive temperature perturbations and Scorer (1958), and our understanding has not inside the cloud. changed signi®cantly since that time. Three related pro- In this paper, we report results from high-resolution, cesses were identi®ed: subsidence of a cloud interface airborne, radar observations of a large ®eld of mam- layer, fallout of precipitation, and evaporation of pre- matus clouds taken in a severe thunderstorm outbreak cipitation (Scorer 1958). These three processes render during the Veri®cations of the Origins of Rotation in the (stable) subcloud layer slightly unstable and result Tornadoes Experiment (VORTEX95; Rasmussen et al. in downward convection with smooth, usually less 1994). As with several of the previous mammatus stud- sharply outlined, surfaces. In the years since these studies, these observations are from a target of opportunity. ies, additional hypotheses, mostly based on isolated ob- The aircraft was in transit between two severe thun- servational studies, have furthered our understanding of derstorms in the Oklahoma±Texas region when it passed the details of both the environment and structure of beneath a large ®eld of mammatus clouds. The obser- mammatus. Clarke (1962) suggested that gravity waves vations from this single transect are the focus of the produced the vertical motion destabilizing cloud base. study. Martner (1995) provided further evidence that gravity The high spatial resolution within individual mam- waves play an important role in the initiation of mammatus elements, coupled with the along-track mesoscale view offered by the airborne platform and two Cross- chain Linked Atmospheric Sounding System (CLASS) soundings, allow us to evaluate results reported in pre- Corresponding author address: Nathaniel S. Winstead, Space De- partment, Applied Physics Laboratory, The Johns Hopkins Univer- vious papers. In particular, these radar observations are sity, 11100 Johns Hopkins Rd., Laurel, MD 20723. used to examine the different hypotheses presented in E-mail: [email protected] previous studies of mammatus and to address questions q 2001 American Meteorological Society Unauthenticated | Downloaded 09/27/21 09:51 PM UTC 160 MONTHLY WEATHER REVIEW VOLUME 129 point separation is 400 m in the horizontal and 500 m in the vertical. Each individual radar scan was time± space adjusted with the propagation speed of the storm. A Cressman ®lter (Cressman 1959) was used in the interpolation process with a radius of in¯uence of 500 m in the horizontal and 600 m in the vertical. Horizontal velocities were calculated using Custom Editing and Display of Reduced Information in Cartesian Space soft- ware (Mohr and Miller 1983). These horizontal velocities were smoothed using a two-pass, ®ve-point ®lter (Liese 1982), which damped wavelengths up to 1.8 km and eliminated those less than 1.2 km. The vertical velocities were obtained through downward integration of the anelastic continuity equation starting half a grid- point above the last detectable re¯ectivity at cloud top. Following Hildebrand et al. (1996), the vertical velocities were not corrected for hydrometeor fall speed un- der the assumption that the fall speeds of the mostly ice particles are small. All horizontal velocities are presented as anvil-relative velocities with an anvil- mean wind vector of 15.4 m s21 from 2548 subtracted. Data from two CLASS radiosonde launches are also presented. Both radiosondes were released from the FIG. 1. Composite re¯ectivity derived from the ELDORA re¯ec- same location, also indicated in Fig. 1. The ®rst sound- tivities as the NCAR Electra was ¯ying along the anvil containing ing, launched at 2112 UTC, is representative of the pre- the mammatus clouds. The locations of the two CLASS soundings storm environment while the second sounding, launched are indicated by the 3. The box on the plot indicates the dual-Doppler at 2144 UTC, passed directly through the anvil of the analysis domain. observed storm. While there is no ®rm evidence that the second sounding passed directly through the mam- that have arisen from aircraft measurements of mammatus ®eld (it passed through the anvil approximately matus. 50 km from the aircraft track), it does pass through the anvil of the same storm and thus is representative of the anvil environment. Comparisons with aircraft as- 2. Data and methods cend±descend temperature pro®les con®rmed that this The radar data analyzed in this study were obtained sounding was representative of the larger anvil region. by the National Center for Atmospheric Research (NCAR) Electra Doppler radar (ELDORA). The design 3. Overview and capabilities of the radar can be found in Hildebrand et al. (1994, 1996). Important to this study is the high On 8 June 1995, a cluster of severe thunderstorms spatial resolution that can be achieved with the ELDORA. over northern Texas and the Oklahoma panhandle were For the case presented here, the VORTEX Convective observed by the ELDORA during VORTEX. These I scanning mode (Wakimoto et al. 1996) was used to storms consisted of a line of convection in the northern collect the data. In this mode, the along-track resolution Texas panhandle and a supercell storm in the Oklahoma (as de®ned by the antenna rotation rate) was approxi- panhandle. The out¯ow anvils of the Texas line merged mately 330 m, with a 150-m gate spacing along the with the Oklahoma storm, creating a mesoscale precip- beam. The crossbeam resolution, in the sweep plane, is itation region of stratiform rain between the two con- approximately 125 m at a distance of 5 km from the vective regions. An extensive ®eld of mammatus clouds radar (where most of the mammatus were observed) and protruded from the base of this anvil between the two 620 m at 25 km from the radar. storms. These mammatus clouds were observed with Re¯ectivity and Doppler velocity measurements ELDORA along the southeastern edge of the anvil as were edited in radar space [using Solo; Oye et al. the Electra ¯ew from the cell in Oklahoma toward the (1995)] to remove ground clutter and sidelobe returns cells in Texas. A photograph (Fig. 2) showing some of before being interpolated onto a Cartesian grid [using these elements was taken from the Electra during this REORDER; Oye (1994)]. Figure 1 shows the location ¯ight leg. of this grid (labeled mesoscale box in Fig. 1) in relation Figure 3 shows re¯ectivity contours on the horizontal to the track of the aircraft and the location of the anvil. slice through the base of the anvil (Z 5 7 km), the slope It covers a large section of that region of the anvil in of which is upward toward the south. The southern part which the mammatus clouds were embedded. The grid- of this domain is below cloud. A cut through the domain Unauthenticated | Downloaded 09/27/21 09:51 PM UTC JANUARY 2001 NOTES AND CORRESPONDENCE 161 FIG. 2. Photograph from the ELECTRA of the bottom of the anvil containing the mammatus (courtesy W.-C. Lee, NCAR). from south to north ®rst shows the mammatus layer and then the deeper parts of the anvil. Small-scale cellular features, varying in sizes between 1 and 3 km, char- acterize the mammatus layer. Deeper into the anvil these cellular structures gradually merge into a more uniform re¯ectivity pattern (shown in Fig. 6 later in the paper). Some suggestion of a mesoscale organization in the ar- rangement of the cells can be seen. The dashed lines in FIG. 3. Horizontal crosssection at Z 5 7 km through the anvil Fig. 3 indicate an orientation approximately parallel to showing the mammatus elements.

Load more