Understanding Extratropical Synoptic-Scale Processes and Severe Convection

1. Extratropical Synoptic-Scale Processes and Severe Convection Part I Austin Cross Doswell, C.A. III, and L.F. Bosart, 2001: Extratropical synoptic-scale processes and severe convection. Severe Convective Storms, Meteor. Monogr ., 28, no. 50, Amer. Meteor. Soc., 27-69.

2. Introduction • Traditional view says synoptic-scale processes simply provide setting for severe convection • Instead, using mesoscale processes as intermediary, synoptic-scale view can be taken

3. Main topics • Overview of QG principles and jet streak processes • Discussion of boundary layer processes and how they relate • Basic climatological distributions of convection

4. Deep, Moist Convection • Deep, moist convection (DMC) requires three ingredients • Moisture • Low Static Stability • Ascent of parcels to LFC • Extratropical cyclones (ETC) play role in first two, but do not provide enough lift • But they provide environment favoring small scale processes (e.g. drylines, fronts)

5. Quasigeostrophic • Value is not in prediction, but qualitative understanding of midlatitude, synoptic-scale processes

6. Static Stability •  is assumed to be a function of pressure; actually varies in space and time. • Rising motion favors decrease in stability • Sinking motion increases stability • Emanuel et al (1987) parameterizations, show that ascent is localized and intense, while descent is weaker and widespread • Static stability is important in cyclogenesis and associated frontogenesis

7. Vertical Wind Shear • Important factor is determining severity of convection • Geostrophic vertical wind shear associated with thermal advection • Strong vertical wind shear is major factor in supercell convection

8. Vertical Wind Shear • Strong vertical wind shear has been viewed as inhibiting factor for convection, as it tends to reduce intensity of updrafts • Actually promotes new cell development by interaction of existing updrafts and shear • For synoptically-evident, major outbreaks favorable wind shear widespread • But wind shear parameters vary substantially in synoptic scales

9. Upper Level Jet Streaks • Jet streams have along-flow variation in wind speed that cannot be only curvature effects, called jet streaks

10. Jet Streaks • Conceptual model used to diagnose ascent areas • Ascent favorable for cyclogenesis and organized DMC • Jet streaks often coupled to low level jet streams, advecting moist, unstable air Figure 2

11. Planetary Boundary Layer • Defined as the tropospheric layer where the effects of the surface are important • PBL not synoptic because time scale of processes as small as an hour or less • Still interacts with synoptic-scale systems

12. Diurnal Variations in PBL • On sunny days, PBL has inversion that ascends and weakens • The erosion of the stable layer is one reason why DMC usually begins in afternoon • Movement of well-mixed, dry layer over a cooler layer can create capping inversion, suppressing convection • Capping can promote convection elsewhere by “storing” parcels with high CAPE

13. Diurnal Variation in PBL • Decoupling of surface and atmosphere creates nocturnal boundary layer wind maximum, especially on sunny days • Diurnal cycle in horizontal temperature gradient, makes for poleward flow increasing with height at night • Combo of both makes low-level jet stream • Diurnal changes in PBL wind profile can modify potential for severe convection

14. Climatology of DMC • Convection develops when heat redistribution on synoptic-scales insufficient • Convective transport is far less, but more rapid than ETCs • Moisture and instability needed is tied to accumulation of sensible and latent heat in lower levels

15. Spatial Distribution • Worldwide average is one meter of precipitation annually • Most falls in the tropics • Hsu and Wallace (1976) show that precip • peaks over continents in warm season in mid and low latitudes • follows sun in tropics, except deep tropics and monsoons • While not direct, vast majority of rainfall in tropics, and in warm season extratropically, is from DMCs

16. Mesoscale Convective Complexes Figure 5 • In Americas, preferentially lee of mountains in mid latitudes, where LLJS common • Elsewhere, mostly in tropics and subtropics • Considerable DMC occurs outside of MCC areas

17. Great Plains Rainfall • High Plains has notable warm season precip peak • Much of rainfall is from nocturnal DMC • These have two sources: • Afternoon storms from the Rockies • Storms that develop locally with mesoscale weather systems

18. Global Distribution • Remote sensing (TRMM) shows most DMC over land, except ITCZ • Most likely due to lower heat capacity • Exceptions include surface warm currents along eastern continent boundaries, often associated with synoptic cyclones • Complex terrain also favors DMC even without significant moisture

19. Temporal Variation • DMC usually is coupled to, but lags, peak solar heating • However, can be tied to synoptic-scale processes, out of phase with heating • LLJS enhanced by nocturnal winds can help convection after sunset • Peak at night over tropical oceans; reasons not understood

20. Seasonality • Seasonal cycle of DMC follows that of conditional instability • However, DMC are at cores of many explosive maritime cyclones • Occasionally in winter cyclones • In warm sector, similar to warm season • High lapse rates over cold, stable, moist air • Usually only severe weather is heavy precipitation