Christopher A. Davis and Stanley B. Trier. National Center for Atmospheric Research 1 Boulder, Colorado

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1 1.10 Convection Initiation Near Mesoscale Vortices Christopher A. Davis and Stanley B. Trier National Center for Atmospheric Research 1 Boulder, Colorado 1. Background While the focusing of deep, moist convection by convergence boundaries contained within the planetary boundary layer (PBL) is known to lift air parcels to their level of free convection (LFC), convection initiation depends strongly on processes occurring above the PBL. In particular, mesoscale lifting, while less vigorous, is more persistent and covers a greater area than lifting induced by convergence boundaries. Mesoscale lifting can arise from many different phenomena, including warm and cold fronts, lowlevel jets intersecting fronts and vertical disturbances in the middle and upper troposphere. Generally, the lifting is crucially dependent on the presence of baroclinity and therefore occurs most readily in environments with appreciable shear in the lowest few kilometers. The particular agents responsible for mesoscale lifting discussed in the present paper are midtropospheric mesoscale convective vortices (MCVs). 1 The National Center for Atmospheric Research is sponsored by the National Science Foundation. Corresponding author: Christopher A. Davis National Center for Atmospheric Research P.O. Box 3000 Boulder, Colorado USA (tel) (fax) cdavis@ucar.edu Figure 1. Schematic (after Raymond and Jiang 1990) showing two contributions of quasi-balanced mesoscale lifting due to a mid-tropospheric vortex in vertical shear. These vortices have a radius of maximum tangential wind of km and are a commonly observed structural component of many large mesoscale convective systems (MCSs). Their primary importance as the dynamically balanced remnant of deep convection, is to alter the mesoscale vertical motion in the lower troposphere. Ascent tends to occur downshear, with subsidence upshear relative to the vortex center (Raymond and Jiang, Fig. 1). Previous studies have shown that this ascent can act alone (e.g. Fritsch et al. 1994) or together with lifting by boundary-layer processes (Trier and Davis 2002) to

2 initiate convection. Observations from the Bow Echo and MCV Experiment (BAMEX) offer unprecedented ability to resolve the structure of MCVs and their associated vertical motion. A more extensive treatment of BAMEX objectives and observations can be found in Davis et al. (2004). 2. Data and Analysis As summarized in Davis et al. (2004), BAMEX utilized two P-3 Orion IOP date 24/5 2/6 5/6 11/6 29/6 #snd V m R m Table 1. Summary of the five mature MCVs sampled during BAMEX. The estimated maximum azimuthally averaged tangential wind (V m ) is in m/s; the radius of the maximum wind (R m )isinkm. aircraft, one from the Naval Research Laboratory (NRL) and the other from the National Oceanic and Atmospheric Administration (NOAA), and a Lear jet equipped with dropsondes. In addition, a ground based observing system (GBOS) consisting of three mobile GPS-Loran Profiling System (MIPS) from the University of Alabama, Huntsville, were used. For MCV missions without appreciable precipitation near the vortex, the main deployment was the GBOS and the Lear jet with dropsondes. The Lear jet executed flight legs km long across the vortex circulation. GBOS was deployed in a triangle on the downshear side so that the soundings from the triangles could be used to compute a time series of vertical motion. The number of soundings obtained during MCV missions ranged from about 18 to 31, spanning a 3-6 h period. The objective of the soundings was to sample both the vortex and its environment. To enhance the dropsonde data in the analysis, we included GBOS and National Weather Service soundings, profiler observations from times during the drop periods. Soundings and profilers were time-space corrected relative to approximately the central time of each flight assuming an average translation speed of the MCV. This was estimated using radar animations to track the center. Soundings were interpolated Figure 2. Composite radar reflectivity for, left panel: 1100 UTC 5 June (IOP 5), middle: 0600 UTC 11 June (IOP 8), right: 0500 UTC 29 June (IOP 15). Note, local midnight is 0700 UTC. Red indicates reflectivity greater than 50 dbz; yellow, greater than 40 dbz and light green, greater than 30 dbz. Atmospheric Sounding Systems (MGLASS) and the Mobile Integrated to a 10 hpa interval in the vertical. Pressure levels for profiler data were

3 estimated from the average of all the soundings. 3. Results In Table 1 we summarize each mature MCV case during BAMEX. All cases occurred within the southwestern part of the BAMEX domain. We estimated the maximum azimuthally averaged tangential wind (V m ) and the radius at which it occurred (R m ) for each case. The vortex of IOP 8 was the strongest and had the greatest circulation. The MCV of IOP 4 was the largest, but both IOP 4 and IOP 5 MCVs wind and temperature (Fig. 3), the MCVs appear as small-scale baroclinic waves, with warm and cold advection downshear and upshear from the vortex, respectively. The temperature gradients seldom exceed 1 K km -1. However, it is apparent that the new convection occurs in regions of warm advection. In IOPs 1 and 4, the atmosphere downstream from the MCV remained conditionally stable, and therefore no new convection was generated. As is evident from Fig. 4, MCVs in IOPs 5 (5 June), 8 (11 June) and 15 (29 June) were moving into environments with at least local conditional instability. In IOP 5, Figure 3. Radar composite reflectivity (dbz, scale in center panel), temperature and winds for, left: 800 hpa at 2120 UTC 5 June, center: 900 hpa at 1730 UTC 11 June, right: 750 hpa at 2050 UTC 29 June. were clearly embedded within largerscale troughs making assignment of a scale somewhat arbitrary. Of the five major MCVs, new deep convection was induced in three cases. These cases, IOP5, IOP8 and IOP15 will be studied in more detail herein. Figure 2 shows the MCSs, furing their mature phase, from which the MCVs arose. In all cases, the preceding convection was organized as a nocturnal MCS. The MCV was detected during the dissipation stage of each system, typically around sunrise. For each of the three cases, the wind, temperature, relative humidity and vertical motion were analyzed. From the instability was weak and convection initiated where the warm advection maximized. In IOPs 8 and 15, instability was widespread, and convection initiated farther from the MCV and to a lesser extent where warm advection was pronounced. As shown by the soundings in Fig. 5, taken from IOP 8 (middle panel in Figs 1-4), the thermodynamic character of the atmosphere was altered substantially by the MCV. On the southeast side of the vortex (red lines), the boundary layer was more moist and warmer, mainly in response to poleward advection of heat and moisture induced by the MCV. Significant instability

4 occurred in this area, with lifted indices exceeding 3 o C. The convection here Figure hpa lifted index (negative meaning parcel warmer than environment) for parcels lifted from 950 hpa, top: June 5, middle: June 8, bottom: June 29. (middle panel of Fig. 4) was intense with echoes greater than 55 dbz. To the north of the MCV center (Green lines), the sounding was nearly neutral. Banded precipitation structures developed with 2 hours of the time in Fig. 4. There was no strong convection, however. To the southwest of the MCV, subsidence occurred in the lower and middle troposphere, although the surface layer remained moist. The subsidence appears consistent with isentropic downgliding upshear from the vortex that suppressed convection. The vertical displacements needed to make these changes in the soundings are on the order of 500 m. This requirement is entirely consistent with modeling results of Trier et al. (2000). Vertical velocities of ~5 cm s -1, if sustained for 3 h, result in vertical displacements of about 500 m. Given the size of the vortex L, the characteristic time scale is L/ u, where u is the vertical shear across the depth of the vortex. In the case of IOP 8 (June 11), with L=150 km and u=5 m s -1, this time scale was about 9 hours. Vertical motion, though spatially variable was about 5 cm s -1 in the region of baroclinity to the southeast of the vortex (not shown). Thus, the mesoscale ascent was strong enough and likely sustained long enough to create vertical displacements of 500 m or more. We also point out that the largescale shear in this case was exceedingly weak, but locally, the tangential circulation about the vortex increased the shear dramatically (red wind barbs in Fig. 5). The shear reached 15 m s -1 between the surface and 800 hpa, strong enough to promote severe convection despite the apparently benign synopticscale environment. In other cases, the

5 shear induced by the MCV was only about half as large as in IOP Conclusions In this study we have examined the precipitation patterns within three MCVs of varying strength, each occurring within differing environmental shear and thermodynamic stability regimes. In cases of moderate to strong vertical shear beneath the level of maximum vortex strength (IOPs 5 and 15), the instantaneous 700hPa vertical motion pattern resembles the conceptual model of Raymond and Jiang (1990), which includes isentropic ascent (descent) downshear (upshear) of the MCV circulation center. However, in cases of weak environmental vertical shear (e.g., IOP 8), the observed vertical motion pattern is more complicated. In each of the cases thermodynamic stabilization occurs in the wake (i.e., west and southwest) of the MCV, which corresponds to its upshear quadrants. This region is relatively precipitation free. In one of the cases (IOP 5), the conditional instability (and convection) is localized to the region of warm advection near the center of the MCV, where vortex induced lifting and advection have presumably acted the longest to ripen the thermodynamic state for convection. In the two other cases (IOPs 8 and 15), the heaviest precipitation occurs along the periphery of the MCV circulation where conditional instability is greatest. The effect of the mesoscale ascent (descent) within the MCV circulation in these cases is to increase Figure 5. Selected soundings showing temperature (solid), dewpoint (dashed) and wind (barbs) points on the southwest side of the vortex (red), north of the vortex (green) and southwest of the vortex (blue). Locations appear in Fig. 4.

6 (decrease) the conditional instability by cooling and moistening (warming and drying) the layer extending from the top of the PBL into the middle troposphere. However, the region of greatest conditional instability is determined not only by the vertical motions, which are greatest downshear near the MCV center (i.e., near or within the radius of maximum tangential winds), but also by low-level horizontal temperature and moisture advections and surface heating, which also occur downshear, but result in the greatest PBL θe increases farther from the MCV center. In addition to enhancements in thermodynamic instability, vertical shear is also enhanced in parts of the MCV circulation. The character of shear enhancement depends on details of the vortex structure and its environment, and thus can vary significantly from case to case. Since the strength of the MCV circulation generally increases through the lower troposphere, and the background vertical shear is predominately westerly in the central United States warm season convective environment, enhanced westerly (southerly) shear is often found in the southern (eastern) quadrant of MCVs. The enhanced vertical shear supported severe convection within the MCV circulation of IOP 8, whereas in IOP 15, the greatest vertical shear enhancement occurred closer to the MCV center than did the region of greatest conditional instability where deep convection actually formed. The key point from this paper is the importance of slow, mesoscale lifting above the boundary layer for initiating convection. While MCVs are one type of disturbance capable of inducing such lifting, they are far from the only one. Disturbances generated from flow past orography may prove to be equally (or more) important for focusing lowertropospheric ascent and thermodynamic destabilization. In our design of future observing programs, it is important to consider the mesoscale vorticity distribution in the free troposphere, in the warm season. References Davis, C., N. Atkins, D.Bartels, L. Bosart, M. Coniglio, G. Bryan, W. Cotton, D. Dowell, B. Jewett, R. Johns, D. Jorgensen, J. Knievel, K. Knupp, W.-C. Lee, G. McFarquhar, J. Moore, R. Przybylinski, R. Rauber, B. Smull, J. Trapp, S. Trier, R. Wakimoto, M. Weisman, and C. Ziegler, 2004: The Bow-Echo And MCV Experiment (BAMEX): Observations and Opportunities, Bull. Amer. Meteor. Soc., August issue. Fritsch, J. M., J. D. Murphy, and J. S. Kain, 1994: Warm core vortex amplification over land. J. Atmos. Sci., 51, Raymond, D. J., and H. Jiang, 1990: A theory for long-lived mesoscale convective systems. J. Atmos. Sci., 47, Trier, S.B., C.A. Davis, and W. C. Skamarock, 2000 : Long-lived mesoconvective vortices and their environment. Part II: Induced thermodynamic destabilization in idealized simulations. Mon. Wea. Rev., 128,