Deep Convection A review of processes “Everything we hear is an opinion, not a fact. Everything we see is a perspective, not truth” Marcus Aurelius: AD

Learning Outcomes Review severe weather processes and associated parameters. Review hodograph concepts Understand the role of shear-related processes and parameters in determining propagation and updraft rotation. –Bulk shear; SREH Review radar signatures associated with severe convective weather.

Physical ProcessParameters used to diagnose Radar products Radar signatures Large Hail (> 2cm in diameter) Strong updraft; Hail embryos reside in regions of high super- cooled liquid water in the hail growth region; Minimal melting of hailstone Shear / SREH - CAPE - Lifted Index - Freezing level heights (0 C, -20C) - WBFZL ( km) - Shear / SREH - CAPPI - WER / BWER - low-level reflectivity gradient - storm-top displacement - TBSS - anomolous propagation - mid-level rotation - splitting / -Storm top div - Radar algorithms Damaging Wind > 90kt gust at surface Weak shear - Mid-level flow vector (transfer of momentum) & evaporatively driven downdraft Strong shear - Rear inflow jet - Mid-level wind strength - DMAPE -Mid-level convergence - low-level divergence - low to mid-level velocity maximum - bow echo

Heavy precipitation (based on 1 in 10 year R I) - Strong precipitation - Long lasting convection (slow moving cells / Large cells / Stationary trigger) - Precipitable Water > monthly (ave +1 SD) - Warm Cloud Depth > 3.0km -Adiabatic Liquid Water Content > 12g/kg - weak steering flow - Large accumulations Tornado- Shear (0-1km) [10ms-1] - SREH (0-1km) - EHI - Low LCL heights - 0-1km CAPE - Strong low- level rotation - TVS

Buoyancy and shear processes

Measuring buoyancy - CAPE integrating: Where w is updraft strength in m/s Maximum possible value (excluding super cells) CAPE is the positive area between the parcel and environment virtual temperature curves between the LFC and the EL on the skew T –log P diagram T V cloud parcel curve T V environment curve

Predicted saturated adiabat for updraft parcel Mean moist adiabat hPa Mid-way moist adiabat DMAPE area The method used in Australia to determine the downdraft psuedo-adiabat LFS=Level of Free sinking DMAPE

Mean Layer Flow (MLF) Vector ( hPa) Vertically orientated downdraft vector due to negative buoyancy – magnitude calculated from DMAPE area on sounding The resultant Surface Convective Gust is the Vector addition of the MLF vector and the downdraft vector (calculated via DMAPE). Estimating the convective gust strength

Strong Instability immediately above LFC (strong Lapse Rate) Height of environmental WBFZL Dry slots in mid and low- levels or deep moist layer to 500hPa Stable layer capping moist low-levels (CIN) Low-level moisture 2.7Summary - Special buoyancy factors associated with Severe Convection

Plot the following hodograph. Activity Level (hPa) heightWindDirSpeed (kts) 1010Surf m m m m m m m m27539

Equations of Motion p = total pressure;  = density These equations contain density because they are in height (z) co-ordinates. They appear simpler than those for synoptic-scale motions because the effect of the Coriolis force and Friction is not included. 2. Building tools

The environment in which we will grow a storm - the basic state z x The over-bar denotes basic – state values. 2. Building tools Activity

Vorticity in our (basic-state) environment Right – hand rule: The (environmental) vorticity vector points to the left of the shear vector 2. Building tools

An updraft grows in our environment z x In and around the storm values of pressure, density and wind are perturbed away from their environmental values small 2. Building tools

Cloud modelling results 3a. Linear terms z = 6km, t = 40mins; p’ cont. intervals at 0.5 hPa; Updraft (heavy) contours at 10m/s intervals.

Pressure and vorticity that arise when updraft interacts with a shear layer Consider a slice of atmosphere (say at z = 3km). z u w’ x y 2. Building tools + -

Positive vorticity Negative vorticity Straight line hodograph 3a. Linear terms

Consider these anti-clockwise and clockwise turning hodograph to be composed of 3 shear layers (1. a low-level layer, 2. a mid-level layer and 3. a high-level layer) stacked on top of one another. Determine the pressure perturbations relative to the shear vector for each layer. Where does high perturbation pressure near the ground underlie low perturbation pressure aloft ? Activity Curved hodograph – 3 layer model

When the shear vector turns with height the orientation of the H and L pressure areas turn with height so that an upwardly directed pressure gradient force drives new updraft on the left flank of the storm. This forcing makes the storm propagate to the left of the steering flow. The new updraft is correlated with mid-level cyclonic vorticity. Anti-clockwise turning hodograph 3a. Linear terms

3b. Non-Linear terms

The storm inflow layer and its rotational potential Board exercise. We will develop the accompanying conceptual model “hodograph picture” in the lectures. The mathematical definition of SREH (U – c) is the storm relative flow vector  is the vorticty vector Storm motion vector Storm relative wind vectorVorticity vector * Board derivation 3c. Storm-Relitive Helecity

E sfc 500 hPa W S N 800 hPa SREH – Straight line hodograph Area proportional to the SREH calculated between (0-2km) AGL for left moving versus right moving storms – straight hodograph. In the straight line hodograph case both the left and right moving members of the original split have equal magnitudes of SREH. “Steering” flow vector 3c. Storm-Relitive Helecity

E sfc 500 hPa W S N 800 hPa Area proportional to the SREH calculated between (0-2km) AGL for left moving versus right moving storms – curved hodograph. In this case the hodograph curvature produces a larger magnitude of SREH for the right - moving storm. SREH – Backing shear vector hodograph profile 3c. Storm-Relitive Helecity

Learning Outcomes Review severe weather processes, associated parameters. Review hodograph concepts Understand the role of shear-related processes and parameters in determining propagation and updraft rotation. –Bulk shear; SREH Review radar signatures associated with severe convective weather.