Mesoscale Processes and Severe Convective Weather Richard H. Johnson and Brian E. Mapes Presentation by Chris Medjber Severe Convective Storms, Meteorological Monographs, 28, no. 50, American Meteorological Society, pg

Main Topics  Mesoscale Mechanisms for Environment Preconditioning Preconditioning  Convection Triggers

Mesoscale Mechanisms for Environment Preconditioning Introduction  For severe storms to develop, synoptic and/or mesoscale processes must act to provide adequate moisture and instability for convection to initiate.  Once initiation has begun, the interaction of convection with the shear environment produces a pattern of storm evolution that can lead to severe weather.

Mesoscale Preconditioning Processes for Severe Weather Advective Differential Advection Creation of capping inversion Creation of capping inversion Destabilization Destabilization Formation of deep, dry PBL Formation of deep, dry PBL (leading to microbursts) (leading to microbursts) Convergence Lines Fronts Fronts Drylines Drylines Sea/Land/Lake breezes Sea/Land/Lake breezes Mountain/Valley breezes Mountain/Valley breezes Moisture Advection Increase CAPE, lower LFC Increase CAPE, lower LFC Local cumulus moistening Local cumulus moisteningDynamical Secondary Circulations Geostrophic adjustment Geostrophic adjustment Jets Jets Gravity Currents, Waves Cold pool lifting Cold pool lifting Localized reduction of CIN Localized reduction of CIN Modification of vertical shear Modification of vertical shear Mesoscale Instabilities Boundary Layer Processes Horizontal convective rolls Horizontal convective rolls Inertial oscillation (low-level jets) Inertial oscillation (low-level jets)

Advective Processes

Moisture Advection Sources  Low-level jets Results  Increased CAPE  Lowered LFC  Promotes new cloud growth  MCCs

Differential Advection Sources  Low-level jets  Ageostrophic circulations about a mesoscale jet- streak  Transport of clouds and moisture aloft downstream of mountain barriers  Jet-streak circulations and boundary layer heating changing rapidly over short periods of time

Differential Advection (Cont’d) Results  Long-lived bow echoes and MCCs or mesoscale vorticity centers  Dry microbursts  Convective outbreaks  On the synoptic scale, differential advection can cause destabilization, vertical wind shear, or establish capping inversions

Converging Lines Sources  Cross-front circulations along cold, warm, stationary, or quasi-stationary fronts  Precipitation-driven convective downdrafts (“gust fronts”)  Drylines  Sea and land breezes  Mountain/Valley breezes

Converging Lines (Cont’d) Results  Destabilization of the environment  Reduces CIN to the point where severe can occur even in the absence of CAPE  Derecho and bow echo development  Nocturnal MCC development  Waterspout formation

Dynamical Processes

Secondary Circulations Upper-level wind maxima (jet streaks)  Transverse ageostrophic circulations about the jet axis are argued to initiate convection with clouds and precipitation being most prevalent in the right entrance and left exit region of the jet streak Low-level jet  Convection is favorable through the enhancement of moisture and temperature advection, increased low-level convergence, and an increase in vertical wind shear associated with this jet

Triggering of Convection Introduction  Isolated convective-triggered mechanisms (storms along a gust front, drylines, terrain features, etc.)  Combined convective-triggered mechanisms (gust fronts colliding or intersecting other low- level perturbations such as other gust fronts, drylines, cold fronts, terrain features, etc.)

Local Processes  Buoyancy-driven circulations in the convective boundary layer (CBL) * forced and active cumulus  Terrain forcing * cloud initiation from leeside convergence and upslope flow development * “Caprock”, Ozark Mountains, and Wichita Mountains  Surface inhomogeneities from soil moisture or vegetation type

Advective Processes  Boundary layer convergence lines (convective scale)  Cold front lifting  Collision or intersection of advective phenomena (gust fronts, sea/lake breezes, drylines, etc.) * e.g. intersections of drylines and fronts, and gust fronts and sea breezes fronts and sea breezes  CBL thermals

Dynamical Processes  Horizontal convective rolls  Collision and intersection of gravity waves, and bores with other lifting mechanisms is the most common trigger for severe weather

Combined Lifting Processes  Genesis of severe weather most often occurs from the combination of local, advective, and dynamical processes  Boundary layer rolls with convergence lines  Dryline intersection with fronts, boundary layer rolls, and mesoscale low pressure areas  Gust fronts with terrain