1. Quasi-Stationary, Extreme-Rain- Producing Convective Systems Associated with Midlevel Cyclonic Circulations Russ S. Schumacher* and Richard H. Johnson Department of Atmospheric Science Colorado State University 14th Cyclone Workshop 23 September 2008 Research supported by NSF Grant ATM-0500061 Computing resources provided by NCAR *Present affiliation: Postdoctoral Fellow, Advanced Study Program, NCAR

2. Purpose To understand the processes responsible for initiating, organizing, and maintaining convective systems that produce excessive rainfall and lead to flash flooding To understand the processes responsible for initiating, organizing, and maintaining convective systems that produce excessive rainfall and lead to flash flooding To investigate convective systems that initiate near midlevel circulations (such as mesoscale convective vortices) and remain nearly stationary for 6-12 hours To investigate convective systems that initiate near midlevel circulations (such as mesoscale convective vortices) and remain nearly stationary for 6-12 hours To determine how convection becomes (and stays) linearly organized within very moist environments that do not support the development of strong cold pools To determine how convection becomes (and stays) linearly organized within very moist environments that do not support the development of strong cold pools Gainesville, TX, 18 June 2007 AP Photo

3. Background: MCVs Latent heating in mesoscale convective systems (MCSs) redistributes potential vorticity (PV), sometimes resulting in a positive PV anomaly (and cyclonic circulation) at midlevels, referred to as a mesoscale convective vortex (MCV) Latent heating in mesoscale convective systems (MCSs) redistributes potential vorticity (PV), sometimes resulting in a positive PV anomaly (and cyclonic circulation) at midlevels, referred to as a mesoscale convective vortex (MCV) Raymond and Jiang (1990) showed how MCVs in shear can initiate and/or maintain convection via isentropic upglide on the downshear side of the vortex Raymond and Jiang (1990) showed how MCVs in shear can initiate and/or maintain convection via isentropic upglide on the downshear side of the vortex Trier et al. (2000) showed that this lifting also leads to destabilization and moist absolutely-unstable layers Trier et al. (2000) showed that this lifting also leads to destabilization and moist absolutely-unstable layers When a low-level jet approaches the MCV, the downshear side is upwind: convection that develops moves slowly (e.g., Fritsch et al. 1994) When a low-level jet approaches the MCV, the downshear side is upwind: convection that develops moves slowly (e.g., Fritsch et al. 1994) thanks to Dan Lindsey of CSU/CIRA for IR imagery

4. Case selection We identified six warm-season extreme rain events associated with MCVs or other midlevel circulations We identified six warm-season extreme rain events associated with MCVs or other midlevel circulations The selection of cases was somewhat subjective, but all had the following in common: The selection of cases was somewhat subjective, but all had the following in common: Gauge-observed rainfall in excess of 200 mm (7.9 in) in less than 12 hours Gauge-observed rainfall in excess of 200 mm (7.9 in) in less than 12 hours Heavy rainfall produced by a convective system that had “back-building/quasi-stationary” organization at some point Heavy rainfall produced by a convective system that had “back-building/quasi-stationary” organization at some point The presence of a preexisting midlevel circulation near where the convection developed The presence of a preexisting midlevel circulation near where the convection developed Flash flooding resulting from the rainfall Flash flooding resulting from the rainfall

5. 6-7 May 2000 Max rainfall: 309 mm (12.1 in); 2 fatalities and $100M in damage Max rainfall: 309 mm (12.1 in); 2 fatalities and $100M in damage More on this case later… More on this case later…  Average vorticity in 700-500-hPa layer at 00Z  Most-unstable CAPE (colors), 900-hPa winds and isotachs (m/s) at 07Z X

6. 20 August 2007 Formed from remnants of TS Erin after surface low weakened Formed from remnants of TS Erin after surface low weakened Max rainfall: 266 mm (10.5 in); most damage was to rural roads Max rainfall: 266 mm (10.5 in); most damage was to rural roads  Average vorticity in 700-500-hPa layer at 00Z  Most-unstable CAPE (colors), 900-hPa winds and isotachs (m/s) at 10Z X

7. 25 June 2008 Occurred after we performed the analysis, but fits the bill… Occurred after we performed the analysis, but fits the bill… Max rainfall: 219 (8.6 in) Max rainfall: 219 (8.6 in)  Average vorticity in 700-500-hPa layer at 00Z  Most-unstable CAPE (colors), 900-hPa winds and isotachs (m/s) at 07Z X

8. Composite analysis: Midlevel vorticity Composites computed from RUC 0-h analyses, centered at location of maximum rainfall Composites computed from RUC 0-h analyses, centered at location of maximum rainfall Results projected onto a map centered at Springfield, MO (black box denotes heavy rainfall location) Results projected onto a map centered at Springfield, MO (black box denotes heavy rainfall location) Average vorticity in the 700-500-hPa layer, 600-hPa heights and winds Average vorticity in the 700-500-hPa layer, 600-hPa heights and winds

9. Composite: Midlevel vorticity & LLJ 700-500-hPa abs. vorticity (colors), 900-hPa winds and isotachs (thick contours) Heavy rainfall occurs at nose of nocturnally- enhanced LLJ, and almost directly underneath the vorticity max

10. Composite CAPE & CIN Most-unstable CAPE (thick lines), CIN for most- unstable parcel (colors) Little CAPE in heavy rain area in the afternoon Little CAPE in heavy rain area in the afternoon LLJ brings high- θ e air into area, and vortex- related destabilization also takes place LLJ brings high- θ e air into area, and vortex- related destabilization also takes place By the time the MCS develops, over 1000 J/kg of CAPE and no CIN By the time the MCS develops, over 1000 J/kg of CAPE and no CIN

11. Composite sounding Calculated at RUC grid point and time nearest heavy rainfall Calculated at RUC grid point and time nearest heavy rainfall Stable near surface, but elevated parcels have 1149 J/kg of CAPE and no CIN Stable near surface, but elevated parcels have 1149 J/kg of CAPE and no CIN Nearly saturated at low levels; PW=49 mm (1.93”) Nearly saturated at low levels; PW=49 mm (1.93”) ~15 m/s LLJ and weak midlevel winds produces “hairpin” hodograph – shear vector turns sharply with height ~15 m/s LLJ and weak midlevel winds produces “hairpin” hodograph – shear vector turns sharply with height

12. Summary: Extreme rainfall near MCVs Schumacher and Johnson (2008), WAF, accepted pending minor revisions

13. In some back-building MCSs, lifting along an outflow boundary/cold pool causes the repeated cell development, consistent with Maddox et al.’s (1979) “mesohigh” flash flood type In some back-building MCSs, lifting along an outflow boundary/cold pool causes the repeated cell development, consistent with Maddox et al.’s (1979) “mesohigh” flash flood type In others, it was difficult to identify any boundaries at the surface, yet the convection kept linear organization and remained nearly stationary for extended periods In others, it was difficult to identify any boundaries at the surface, yet the convection kept linear organization and remained nearly stationary for extended periods Schumacher and Johnson (2005), MWR ? Convective organization

14. 6-7 May 2000 MCS Surface observations and WRF simulations reveal a convectively- generated mesolow and pressure trough, no cold pool Surface observations and WRF simulations reveal a convectively- generated mesolow and pressure trough, no cold pool More details: see Schumacher and Johnson (2008), MWR, October issue More details: see Schumacher and Johnson (2008), MWR, October issue Surface analysis, 0700 UTC WRF simulation: simulated reflectivity and MSLP

15. Low-level structure Simulations show that there is no surface cold pool… Simulations show that there is no surface cold pool… Simulated composite reflectivity 1000 UTC θ v on lowest model level (colors) Δθ v < 1 K

16. Low-level structure Instead, the low-level structure is a wave Instead, the low-level structure is a wave Simulated composite reflectivity 1000 UTC θ v at ~0.7 km AGL cool warm

17. Idealized simulations with Bryan cloud model Convection initiated with low-level momentum forcing (based on Crook and Moncrieff 1988; Loftus et al. 2008) Convection initiated with low-level momentum forcing (based on Crook and Moncrieff 1988; Loftus et al. 2008) Convection becomes linearly-organized, but no cold pool at surface Convection becomes linearly-organized, but no cold pool at surface Instead, the low-level structure is a gravity wave Instead, the low-level structure is a gravity wave Vertical velocity at 7 km θ ’ at 1 km above ground

18. Run with 500-m grid spacing, t=5 h This looks just like the low-level structure in the case- study simulation This looks just like the low-level structure in the case- study simulation θ ’ at 1 km above ground θ v at ~0.7 km AGL in case-study simulation

19. Vertical structure The low-level updraft is associated with a gravity wave instead of a density current The low-level updraft is associated with a gravity wave instead of a density current θ ’, w, flow vectors

20. Conclusions Analyzed 6 events where extreme local rainfall occurred near a midlevel cyclonic circulation Analyzed 6 events where extreme local rainfall occurred near a midlevel cyclonic circulation Lifting associated with the interaction between a strong low-level jet and the midlevel vortex helped to initiate and maintain deep convection Lifting associated with the interaction between a strong low-level jet and the midlevel vortex helped to initiate and maintain deep convection Strong LLJ, weak midlevel winds  “hairpin” hodograph Strong LLJ, weak midlevel winds  “hairpin” hodograph Thermodynamic environment: very high RH, moderate CAPE, little CIN Thermodynamic environment: very high RH, moderate CAPE, little CIN Simulations show that the low-level organizing mechanism is a gravity wave, rather than a cold pool/density current Simulations show that the low-level organizing mechanism is a gravity wave, rather than a cold pool/density current The reverse-shear profile causes the waves to remain nearly stationary The reverse-shear profile causes the waves to remain nearly stationary Convection back-builds, is linearly organized, and moves slowly: very conducive to excessive local rainfall Convection back-builds, is linearly organized, and moves slowly: very conducive to excessive local rainfall

23. Background: MCVs in shear Raymond and Jiang (1990) described how MCVs (or other midlevel PV anomalies) in shear can help to initiate and/or maintain additional convection via isentropic upglide on the downshear side of the vortex Raymond and Jiang (1990) described how MCVs (or other midlevel PV anomalies) in shear can help to initiate and/or maintain additional convection via isentropic upglide on the downshear side of the vortex Trier et al. (2000) showed that this region is destabilized and that moist absolutely unstable layers (MAULs; Bryan and Fritsch 2000) can form by the lifting of conditionally unstable air to saturation Trier et al. (2000) showed that this region is destabilized and that moist absolutely unstable layers (MAULs; Bryan and Fritsch 2000) can form by the lifting of conditionally unstable air to saturation From Trier et al. (2000), based on Raymond and Jiang (1990) shear

24. Background: MCVs and heavy rainfall Bosart and Sanders (1981), Fritsch et al. (1994), and Trier and Davis (2002) studied significant flash flood events near MCVs Bosart and Sanders (1981), Fritsch et al. (1994), and Trier and Davis (2002) studied significant flash flood events near MCVs Fritsch et al. showed that when a strong low-level jet approaches the MCV, convection is favored near the center of the vortex Fritsch et al. showed that when a strong low-level jet approaches the MCV, convection is favored near the center of the vortex This leads to slow system motion and heavy rainfall This leads to slow system motion and heavy rainfall Fritsch et al. (1994) Deep-layer shear

25. Soundings Deep moisture, high PW, moderate CAPE “Hairpin”-shaped hodographs: strong shear reversal w/height

26. Rainfall totals Observed (gauges)Model Precip amounts and locations are very accurate Precip amounts and locations are very accurate Model underestimates stratiform rain region Model underestimates stratiform rain region

27. Summary: 6-7 May 2000 MCS Schumacher and Johnson (2008), MWR, in press

28. Parcel trajectories along gravity wave Thin trajectories originate below 1 km Thick trajectories originate between 1-2 km Blue shading shows location of wave Near-surface (stable) air rises over upward branch of wave, sinks on other side Elevated (destabilized) air rises over wave and parcels reach LFC SN SW NE

29. Idealized simulations Bryan and Fritsch (2002) cloud model is used for more idealized simulations Bryan and Fritsch (2002) cloud model is used for more idealized simulations Initial state is the composite sounding from the six events Initial state is the composite sounding from the six events To initiate convection, a low- level momentum forcing is applied, based on Crook and Moncrieff (1988) and Loftus et al. (2008, MWR) To initiate convection, a low- level momentum forcing is applied, based on Crook and Moncrieff (1988) and Loftus et al. (2008, MWR) This method of initiation is meant to simulated MCV-related lifting This method of initiation is meant to simulated MCV-related lifting

30. Simulation with 1-km grid spacing Domain translating at ~7.9 m/s

31. Idealized simulations with Bryan cloud model Convection initiated with low-level momentum forcing Convection initiated with low-level momentum forcing Convection becomes linearly-organized, but no cold pool at surface Convection becomes linearly-organized, but no cold pool at surface Vertical velocity at 7 km θ ’ on lowest model level t = 5 h

32. Observational evidence These MCSs are small, so they don’t often pass over a surface station These MCSs are small, so they don’t often pass over a surface station The 20 August 2007 case does provide some evidence for these waves The 20 August 2007 case does provide some evidence for these waves 0745 UTC Pressure and wind vary in phase, consistent with gravity wave dynamics Wave trough Wave crest