'c_ ~ YW· '-( j t.f- Af<d-,/ bservations of a Midlatitud~ Squall Line Boundary Layer Wake Paul J. Hamilton and Richard H. Johnson Observations of a Midlatitude Squall Line Boundary Layer Wake by Paul J. Hamilton and Richard H. Johnson Research supported by the National Science Foundation under grant ATM-8507961 Department of Atmospheric Science Colorado State University Fort Collins, Colorado June 1987 Atmospheric Science Paper No. 414 Abstract Mesoscale pressure perturbations frequently observed with mesoscale convec- tive systems (MCS) are examined with special emphasis on the characteristics, structure, lifecycle and driving mechanism of the "wake depression" found in the wake region of the convection. A severe squall line which traversed the OK PRE- STORM surface and upper air mesonetwork on 10-11 June 1985 is the focus of this observational study. Extensive surface, upper air and digitized radar data collected during the OK PRE-STORM field experiment were used for analysis. Various mesoanalyses of this squall line at the surface and aloft have allowed for intensive examination of three pressure features observed with this squall line: the mesohigh, wake depression and pre-squall mesolow. Their relationship to and interaction with other meteorological parameters such as precipitation, tempera- ture, potential temperature and moisture are explored. Furthermore, the mesoscale system-relative "jets" observed with midlatitude squall lines are examined for their possible influence on the pressure field. The mesohigh develops quickly during the early growth of the squall line and precedes the wake depression by several hours. The predominant mesohigh is linked to the formation of a large cold pool which developed as a result of widespread hail and intense rainfall from a supercell ahead of the young squall line. Analyses show that the wake depression is not a uniform, stagnant feature behind the mesohigh but has embedded small-scale features, a distinct lifecycle and can undergo rapid intensification. The wake depression also is related to some aspects of the squall line's precipitation pattern. The low consistently "hugs" the back edge of the stratiform precipitation and is observed to split into two· separate lows as the convective line splits. Additionally, the wake depression is a hydrostatic response to a layer of warm, dry air (produced by subsidence) found just above the surface. It is suggested that the wake depression is in part a surface manifestation of forced subsidence by the descending rear inflow jet. Acknowledgements I wish to extend sincere gratitude to my advisor, Dr. Richard H. Johnson, whose guidance made this research a success. His "open-door" policy and personal interest in weather combined to make this a unique working experience. My ap- preciation to the other committee members, Dr. Wayne Schubert and Dr. Howard Frisinger. Special thanks to Jim Toth who took much of his time to introduce me to the Atmospheric Science Department's computer system and answered question after question and never grew impatient. I have learned much from him and will be forever grateful. Many thanks to Gail Cordova for typing the manuscript and to Judy Sorbie for drafting some of the figures. I would also like to thank Greg Stumpf and Tammy Taylor for their assistance . I acknowledge and thank the Air Force Institute of Technology who funded much of this research and made this experience possible. This research was sup- ported by the National Science Foundation {NSF) Grant No. ATM-8507961. TABLE OF CONTENTS 1 Introduction 1 2 Background 6 2.1 The Mesohigh ..... 6 2.2 The Wake Depression . 9 2.3 The Pre-Squall Mes~low 11 3 Oklahoma-Kansas PR:F(-STORM 13 3.1 Data Sources ....... 13 3.2 Data Analysis Procedures ...... 15 4 Synoptic Overview 19 5 · Results of Surface Analyses of the 10- 11 June Squall Line 29 5.1 The S.torm's Precipitation Pattern .. 29 5.2 Temperature and Dewpoint Analyses .................. 36 5 .3 Mesoanalysis of the Surface Pressure . 41 5.4 Surface Analyses of Potential and Equivalent Potential Temperatures 59 6 Time Series for Selected PAM stations 65 1 Results of Lower Tropospheric Analyses for the 10-11 June Squall Line 71 8 Summary and Discussion 87 References 90 LIST OF FIGURES Fig. 1. Vertical cross section normal to 22 May 1976 squall line . 2 Fig. 2. Schematic of mesoscale surface pressure and streamlines in vicinity of a squall line . 5 Fig. 3. Schematic section through a squall line illustrating the thunderstrom high and wake depression 8 Fig. 4. Illustration of a cumulonimbus cloud and vertical motion downwind of cloud. 12 Fig. 5. Tbe OK PRE-STORM surface mesonetwork showing the positions of the eighty automated surface observing stations . 14 Fig. 6. Locations of the rawinsonde stations in the immediate OK PRE- STORM network . 16 Fig. 7. Approximate tracks of the 19 MCSs which passed over the OK PRE-STORM mesonetwork in June 1985 . 20 Fig. 8. Visible (upper) and IR (lower) images of the forming squall line at 2100 GMT, 10 June 1985 . 21 Fig. 9. Surface mesoanalysis at 2100 GMT, IO June 1985 . 23 Fig. 10. 850 mb (a), 700 mb (b), 500 mb (c) and 300 mb (d) height and temperature analyses for 0000 GMT, 11 June 1985 ... 24,25 Fig. 11. Pre-squall sounding from Pratt (PTT), KS at 2330 GMT, 10 June 1985 . 26 Fig. 12. IR image of mature squall line at 0300 GMT, 11 June 1985 28 Fig. 13. Diagonal displays of composited low-level reflectivity at hourly intervals 30 Fig. 14. Isochrone analysis of gust front position 33 Fig. 15. Analyses of Total Rainfall (a), Stratiform Rainfall (b) and Stratiform Rain Fraction (c) for the 10-11 June squall line over the PAM network . 34,35 Fig. 16. Surface temperature (a) and dew point (b) analyses for 2300 G~fT, 10 June 1985 over the PRE-STOR:-.f network 37 Fig. 17. As in Fig. 16 except for 0300 GMT, 11 June 1985 38 Fig. 18. As in Fig. 16 except for 0700 GMT, 11 June 1985 39 Fig. 19. Pressure analysis at 518 m ASL at 2300 GMT, 10 June 1985 42 Fig. 20. As in Fig. 19 except for 0100 GMT, 11 June 1985 44 Fig. 21. Severe weather events in southwest Kansas during the late afternoon on 10 June 1985 . 45 Fig. 22. As in Fig. 19 except for 0~00 GMT, 11 June 1985 47 Fig. 23. As in Fig. 19 except for 0300 GMT, 11 June 1985 49 Fig. 24. As in Fig. 19 except for 0400 GMT, 11 June 1985 50 Fig. 25. As in Fig. 19 except for 0500 GMT, 11 June 1985 52 . Fig. 26. As in Fig. 19 except for 0600 GMT, 11 June 1985 . 53 Fig. 27. As in Fig.· 19 except for 0700 GMT, 11 June 1985 54 Fig. 28. As in Fig. 19 except for 0725 GMT, 11 June 1985 56 1 Fig. 29. Surface divergence field (x 10-s s- ) over PAM network at 0720 GMT, 11 June 1985 . ... 57 Fig. 30. Tracks of surface mesohigh (H) and wake depression (L) 58 Fig. 31. Analyses of potential temperature (a) and equivalent potential temperature (b) at 2300 GMT, 10 June 1985 .... 60 Fig. 32. As in Fig. 31 except for 0230 GMT, 11 June 1985 . 62 Fig. 33. As in Fig. 31 except for 0725 GMT, 11 June 1985 . 63 Fig. 34. Time series of pressure (mb) and accumulated rainfall (mm) in (a) and pressure, temperature (°C) and dewpoint (°C) in (b) at PAM station 10 . 66 Fig. 35. As in Fig. 34 except for PAM station 41 . 68 1 Fig. 36. Time series of maximum wind gust (m s- ), pressure (mb) and accumulated· rainfall (mm) at PAM station 23 . 69 Fig. 37. Wake region "onion" sounding from Wichita (IAB), KS at 2100 GMT, 10 June 1985 . 72 Fig. 38. As in Fig. 37 except for 06:!4 G~fT, 11 June 1985 . 7·1 Fig. 39. Composite 850 mb relative humidity (%) with reflectivity at 0600 G~fT, 11 June 1985 . .. 76 Fig. 40. Same as Fig. 39 except for potential temperatures (B) 77 Fig. 41. Same as Fig. 40 except for equivalent potential temperatures (8e) · · • • • • · · · • · • · • • • · • · · · • • . 78 Fig. 42. Composite 700 mb relative humidity (%) with reflectivity 81 Fig. 43. Composite cross section of potential temperature (K) and relative humidity (%) at 0600 GMT, 11 June 1985 . 82 Fig. 44. Vertical cross sections reconstructed from radar volume scan conducted at 0345-0353 GMT, 11 June 1985 . · 84 Fig. 45. Vertical composite cross section of system-relative flow 1 (m s- ) and relative humidity (%) at 0600 GMT, 11 June 1985 . 86 Chapter 1 INTRODUCTION Mesoscale Convective Systems (MCS) such as tropical and midlatitude squall lines and Mesoscale Convective Complexes (MCC) as defined by Maddox (1980) have increasingly become the focus of research in the field of mesoscale meteorology. Many extensive studies of squall lines have been performed using a myriad of data sets or procedures that include conventional and Doppler radar, satellite imagery, atmospheric sounders, wind profilers, surface networks and numerical modelling techniques. These studies have not only raised new and challenging questions but have given further insight to the answers. However, answers to many questions regarding the dynamical and thermodynamical structure in both the vertical and horizo.ntal and the surface characteristics of these systems remain elusive. Figure 1 shows a simplified two-dimensional schematic of a midlatitude squall line (from Smull and Houze, 1986a). The structure of this squall line is quite similar to those observed over the tropics. The storm is most notably characterized by a narrow line, approximately 10 to 50 km in width, of intense convective cells which leads the system. The convective cells are frequently observed to extend well into the upper troposphere and even to push through the tropopause.