1. Fine-scale vertical structure and dynamics of dryline boundaries observed in IHOP Qun Miao Bart Geerts Department of Atmospheric Science University of Wyoming

2. IHOP_2002 (International H 2 O Project) 13 May and 30 June 2002 in Kansas-Oklahoma Kingair Probes measure air motion, state variables, and fluxes The Wyoming Cloud Radar (WCR) 95 GHz Doppler radar BLH and CI missions

3. Clear-Air Echoes  Echoes observed by radars in atmosphere with no clouds or precipitation, mostly from spring to autumn.  Insects in the well-mixed convective boundary layer (CBL) (Wilson et al. 1994, Russell and Wilson 1997, Riley 1999, Kusunoki 2002 )

4. Patterns of Clear-air echoes  polygonal cells -- light wind, little wind shear, and strong surface heat fluxes  banded echo structures -- helical roll circulations in a sheared CBL  Less well-defined patterns are observed more commonly  The most striking feature: thin lines of stronger radar echoes – radar “ fine-line ”.

5. The convective boundary layer  Thermals are buoyantly driven convective eddies which generate most of the turbulence in the unstable atmospheric BL.  Thermals transport energy from the surface up to the overlaying CBL.  They also contribute to the mixing (entrainment) of free- tropospheric air with the CBL.  The properties of thermals are important

6. Some findings from our previous work  Microinsects tend to subside in the CBL, at an average rate of 0.7 m s -1, relative to the air.  The scatterers tend to oppose updrafts in which they are embedded. This opposition increases with updraft strength.

7. Some findings from our previous work  The CBL depth and its local variations are captured well by the radar reflectivity profiles.  Echo plumes are generally rising and buoyant.  The key implication is that spatial patterns of clear-air echoes within the weakly-sheared CBL depict the distribution of rising, convective plumes.

8. References Geerts, B., and Q. Miao, 2005: Vertical velocity bias of echo plumes in the convective boundary layer, detected by an airborne mm-wave radar. J. Atmos. Ocean. Tech., 22, 225-246. Geerts, B., and Q. Miao, 2005: A simple numerical model of the flight behavior of small insects in the atmospheric convective boundary layer. Environ. Entomol., 34, 353-360. Miao, Q., B. Geerts, and M. LeMone, 2006: Vertical velocity and buoyancy characteristics of coherent echo plumes in the convective boundary layer, detected by a profiling airborne radar. J. Appl. Meteor. Climat., 45, 838–855.

9. Drylines The dryline in the Great Plains Origins of the air masses The orientation of drylines is generally meridional and approximately normal to the terrain gradient. It is believed that drylines are observed on more than 40% of the days from late spring to early summer (Schaefer 1974, Hoch and Markowski 2005). Local severe thunderstorms often initiate in the vicinity of the dryline (e.g. Fujita 1958, Ziegler and Rasmussen, 1998).

10. What causes the convergence that gives rise to the dryline and radar fine-line?  symmetric instability (McGinley and Sasaki 1975; Sun 1987),  solenoidal forcing due to a horizontal gradient of virtual potential temperature or the inland sea-breeze effect (Sun and Ogura 1979; Ziegler and Hane 1993),  vertical transport of westerly momentum (Danielsen 1974; McCarthy and Koch 1982)  waves (Koch and McCarthy 1982; Davies-Jones and Zacharias 1988).

11. The scale examined here is O (1-10 km), and the focus is on the cross-fine-line vertical circulation in the afternoon, when the CBL is well-developed. This study examines the fine-scale radar vertical structure and flight-level data of the drylines in IHOP_2002, with as goal to gain an insight in the dynamic forcing of the fine-scale convergence. The central hypothesis of this work is that this convergence primarily results from a horizontal difference in buoyancy and thus in virtual potential temperature (θ v ).

12. Methodology Two methods are used to estimate the meso-  scale convergence near a baroclinic boundary. A steady solenoidal circulation A density current

13. wind shear dominates horizontal vorticity  Assuming along-boundary uniformity and the steady-state secondary circulation due to baroclinicity:

14. The  θ v driving this circulation is largest near the ground (where it is referred to as  θ v,o ) and decreases with height. where R is the ratio of  θ v at z i,d to  θ v,o For simplicity we use the same slope R for the profiles of  θ v,  u, and  u 2.

15. 1300 m

16. 20020522 2207-2350 UTC

17. (1)

18. Speed of a density current (U dc ) based on laboratory experiments (Simpson and Britter 1980), (2) (Simpson et al. 1977)

19. 22 May Oklahoma Panhandle 2 fine-lines, the primary dryline to the east No convections on May 22.

27. 19 June a prefrontal dryline in NW Kansas clear skies, no deep convection before 20:30 Z. The dryline progresses from west to east. after 2135 UTC, the dryline accelerated towards the northwest. Thunderstorms erupted just east of the dryline in the vicinity of misocyclones.

32. 24 May Texas Panhandle erased by a cold front

37. 18 June southwest Kansas A belt of cumulus clouds developed around 2130 UTC.

40. 4 days below 0.5z i,d 5 legs on 22 May 3 on 24 May 9 on 18 June 4 on 19 June

41. NOAA P-3 data VORTEX 94, 95 10 km averages 6 crossings below 600 m on 7 June 1994 (black) 5 crossings at ~180 m on 6 May 1995 (grey)

42. Conclusions The 22 May primary dryline,  a clearly sloping WCR echo and updraft plume,  a secondary circulation with a denser moist airmass  a  θ v of 1.3K over 10 km, decreasing with height in the CBL. sufficient to explain the observed fine-scale confluence, in terms of a baroclinically-induced circulation. Other fine-lines  all convergent, and all are marked by some low-level  θ v, but less than 1.0K over 10 km.  In some cases the fine-scale confluence exceeds that expected from baroclinicity alone.  Some slope in the echo plume, towards the denser airmass, but no solenoidal circulation.

43. A steady solenoidal circulation and density current theory, are used to estimate the meso-  scale convergence near a baroclinic boundary. Both methods tend to overestimate confluence, especially for well-defined boundaries (large  θ v ). The optimal scale at which  θ v and  u are measured is close to 3 km. In most cases a mesoscale θ v gradient is present. A threshold gradient may exist for the development of one or more radar fine-lines.  θ v exceeds 0.5K over 10 km. Similarly, fine-line boundaries may assume density-current properties once their  θ v exceeds a threshold.  θ v exceeds 1.0K over 10 km.

44. Future work More field measurements with multiple observational platforms Vegetation, soil moisture, and terrain through both observation and land surface models Comparison with the results of some high-resolution modeling work (e.g. Ziegler et al. 1997, Peckham et al. 2004, Xue and Martin 2006).

45. Questions?