Synoptic Environments Associated with the Training of Convective Cells Aylward and Dyer 2010

1. Synoptic Environments Associated with the Training of Convective CellsAylward and Dyer 2010

2. Introduction • The main focus is investigating the dynamics resulting in synoptically forced training convective rainfall • Synoptic conditions necessary for the generation of an SCEPT event • Define a SCEPT event to which precipitation rate and duration period satisfy a given criteria • Gather selected atmospheric variables which have a statistically significant correlation to SCEPT events • Allow forecasters to better locate regions of training convection based on findings

3. SCEPT Synoptically forced Convective Extreme Precipitation Training • Events linked with frontal boundaries (typically warm fronts) • Isentropic lift • Deep convection develops in a baroclinic environment where differential cyclonic vorticity advection is present • Typically ahead of a short-wave trough or jet streak • 500 hPa trough nearly stationary during training events • LLJ helps initiate convection and heavy rainfall

4. SCEPT • Near-neutral to weakly positive differential vorticity advection • Location of 850 hPa maximum moisture convergence • Precipitable water maximum (moisture tongue) good indicator of heavy rainfall • High precipitable water content (>1.42 in) required • Midlatitude and upper-level lows positioned to the W or NW • Strong surface moisture convergence and UVV at 700 hPa

5. Data Acquisition • Multisensor Precipitation Estimates (MPE) using a Z-R relationship to diagnose precipitation rate for each event • SCEPT event location points were gathered from the National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP – NCAR) while atmospheric variables examined for each event were gathered from the North American Regional Reanalysis (NARR)

6. Methodology

7. Methodology • Rainfall rates must be exceeding 17 mm/h for at least 3 consecutive hours to account as an event • Event location determined by where event occurred • If multiple events occurred, location determined by data cluster • NARR data used to analyze synoptic conditions of each event • diagnostic fields included geopotential heights (300, 500, and 850 hPa), 500 hPa differential vorticity, 300 hPa jet streaks, UVV at 700 hPa, 850 hPa temperature fields, and precipitable water

8. Methodology • For each height field, the trough axis either deepening, weakening or neutraland tilted positively, negatively, or neutral • A shortwave (long-wave) trough <8000km (>8000km) in amplitude and wavelength • Closed lows were recorded at the pressure level(s) • Axis of greatest precipitable water was noted as well • 36 SCEPT events during from 2004 - 2006

9. 3 Cases 1) Closed Upper Level Low (CULL) 2) Upper Level Trough (ULT) 3) 850 hPa Trough-Low (850TL)

10. CULL • Contained a 500 hPa closed low • Positively tilted & strengthening trough at 850 and 500 hPa • Lows at 300 and 500 hPa nearly stacked • 850 hPa jet generally over SCEPT event, moving toward the NW • Precipitable water averaged 36.2mm

11. CULL • Moisture convergence occurred along the flow of the 850 hPa jet from SW-NE • The SCEPT event was located along the warm front of the associated MLC • Accompanied by WAA • Generally occurred from 7Z -16Z during the cool season months

12. CULL A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

13. ULT • 500 hPa trough without a closed low • Maximum UVV at 700hPa occurred along the flow • 850 hPa jet advecting moisture from the SSW • Precipitable water average 37.1mm

14. ULT • Accompanied by WAA • 300 hPa trough located 915 km west of event • Generally occurred in the warm sector of an MLC • Strong, positively tilted troughs existed at 300 and 500 hPa

15. ULT A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

16. 850TL • Weak UL flow with an 850 hPa shortwave trough or low present • Trough/low averaged 565km to west of SCEPT event • Moisture convergence was greatest along flow of 850 hPa jet • Advection occurred from the SSW

17. 850TL • Little to no UL forcing existed and no relationship could be found with the 500 hPa and 300 hPa flows • Precipitable water averaged 42.2mm • Primarily a warm season phenomena

18. 850TL A) 300 hPa, B) 500 hPa, C) 850 hPa, D) Surface observations

19. Results

20. Results • Each case has its own characteristics • However, Strong UVV at 700 hPa, WAA and High Moisture content common to all • Of the 36 events, 47% had a closed 500 hPa low and 25% had a longwave 500 hPa trough • Overall, 72% of the SCEPT events were synoptically forced with a long-wave trough • SCEPT event locations were concentrated across the western region of study • Gulf moisture and Atlantic moisture transport influenced events

21. Conclusion • 500 hPa trough was slightly positive or neutrally tilted for CULL and ULT events • SCEPT events developed right of the 300 hPa jet streak • Surface moisture convergence maximum within 230 km of event location, training parallel to 850 hPa flow • 700 hPa UVV maximums within 270 km of SCEPT event with training at the 700 hPa on a WSW to ENE orientation

22. Discussion • Why SCEPT events occurred during the early morning hours associated with the least thermal instability • Possibility of a weakening LLJ during morning leading to system speed reduction • Longer research period needed examining more variables • Finer look surface moisture convergence and UVV to reduce error

23. Questions