Lesson 1 – Ingredients for severe thunderstorms B. Barrett – SO441 Synoptic Meteorology A severe thunderstorm near Lusk, WY 18 May 2014

Two basic ingredients for severe thunderstorms Good buoyancy Provides strong lift Wind shear Keeps warm, buoyant updrafts separate from cold, rainy downdrafts If buoyancy and moisture are limited, often you simply get shallow convection But if both are sufficient and in presence of a lifting mechanism, get deep convection

Lifting mechanisms in the atmosphere Convective heating Convergence along a density gradient Motion up topography Convergence into surface low pressure Source: http://web.gccaz.edu/~lnewman/gph111/topic_units/moisture/moisture_stabil_prec/4_lifting.jpg

More on buoyancy Quantified by Convective Available Potential Energy (CAPE) CAPE quantifies difference in temperatures from the LCL (lifting condensation level) to the EL (equilibrium level): parcel minus environment CAPE depends on many factors: Surface air temperature Surface dew point temperature Environmental temperature throughout the troposphere

Buoyancy climatology Examine global mean CAPE in November versus May What similarities do you see? What differences? Compare mean CAPE to annual lightning flash distribution Similarities? Differences? Source: http://www.metoffice.gov.uk/media/image/o/1/ Lightning_Strikes_map_%28Credit_NASA%29.jpg

More on wind shear Wind shear: a change in wind speed and/or direction with height Speed shear example: 10 kts at surface, 20 kts at 850 mb, 30 kts at 700 mb, 50 kts at 500 mb Directional shear example: Southeast at surface, south-southwest at 850 mb, southwest at 700 mb, west at 500 mb Often wind profile contains both speed and directional shear Sometimes messy though: Speeds increase, then decrease, then increase again Direction veers (like figure at the right), but then backs, then veers again A wind profile favorable for supercellular thunderstorms

Storm-relative helicity Storm-relative helicity (SRH) measures low-level vertical wind shear as “felt” by a thunderstorm Storm motion is removed from the calculation You already understand relative winds. Consider this example: you are jogging to the east at 5 mph and the wind is from the east at 5 mph. You feel a 10 mph “relative” wind. If you were jogging to the west at 5 mph, and the wind was also to the west at 5 mph, you would feel a 0 mph relative wind. SRH can be calculated as:

Storm-relative helicity Storm-relative helicity can be calculated as: It can be approximated as:

Shear and storm-relative helicity Assume storm motion is from 225 degrees (from the SW) at 12 m s-1. Calculate the following for this environment: 0-6 km deep-layer shear 0-3 km SRH 0-1 km SRH

Buoyancy and low-level shear acting together In severe thunderstorms, buoyancy and helicity act together: Low-level helicity gets tilted into the vertical by the thunderstorm updraft! Source: http://tornado.sfsu.edu/geosciences/classes/m500/Shear_Helicity/Helicity.htm

Tilting of vorticity Another view of vorticity being tilted into the vertical Once tilted, buoyancy acts to stretch it Stretching of vorticity increases it (Hang on – later in the semester, we will see the vorticity equation) The greater the buoyancy, the greater the vertical motion and thus greater the stretching Image source: Penn State Univ.