Test of Supercell Propagation Theory Using Data from VORTEX 95 Huaqing Cai NCAR/ASP/ATD

Test of Supercell Propagation Theory Using Data from VORTEX 95 n Research Motivation and Objectives n Data and Methodology n Results n Summary

Schematic Visual Appearance of a Supercell Thunderstorm

Why Supercells Rotate ?

Schematic Plan View of an Idealized Supercell as Viewed by Radar Hook Echo

A Typical Supercell Viewed by Doppler Radar

Which direction is the storm moving ? Mean Wind Pure Advection

Burgess, Master Thesis, 1974 Storm Split & Propagation Hodograph Left Mover Right Mover

Illustration of Hodograph u V u V Straight Hodograph Curved Hodograph Surface Upper Level

Newton(1963), Obstacle Flow Analogy P+ P-

ADV VPGF BUOY Linear Nonlinear Buoyancy

Hodograph Environmental Wind Motion Produced by the Storm

Illustration of Linear Effect UpshearDownshear W>0 W<0

Illustration of Nonlinear Effects

Rotunno and Klemp, MWR, 1982 Mesocyclones X X + - Shear Vector -

Hane and Ray, JAS, 1985 Shear Vector Linear Effect H L 40 dBz Storm Relative Flow

LeMone et al, MWR, 1988 P’(hpa) H L W max Shear Vector

Findings and Limitations of Past Studies n The pressure pattern predicted by the linear theory appears to be confirmed n No vertical pressure gradient was obtained by earlier retrievals or aircraft observations n The aircraft observations can only be obtained near the cloud base, there was no direct measurement inside the storm

Research Objectives n There has been no comprehensive observational test of supercell propagation theory n Try to decompose nonlinear perturbation pressure into nonlinear cyclostrophic and nonlinear shear terms and determine which term is more important for the rightward movement of supercells

Data and Methodology n The Garden City storm during VORTEX on May 16, 1995 n Dual-Doppler technique n Pressure-buoyancy retrieval technique

Dual-Doppler Technique Radar 1 Radar 2 3D wind fields can be reconstructed through dual-Doppler technique

ELDORA Airborne Doppler Radar

Pressure-buoyancy Retrievals

Linear and Nonlinear Perturbation Pressure Retrievals

ELDORA Fly Track and Overall Storm Evolution

What is Special about the Garden City Data Set ? n High space resolution (300 m along track) n High time resolution ( ~5 minute) n Full coverage of the whole life cycle of the storm

Analysis Procedure of the Garden City Storm n A total of 11 legs were carefully edited using NCAR software SOLO (~2800 scans) n 3D wind field was obtained using NCAR software REORDER and CEDRIC n Pressure-buoyancy retrievals were performed for each leg using a modified retrieval routine

Hodograph From Sounding and Dual-Doppler Syntheses

Perturbation Pressure Vertical Vorticity Shear Vector

Linear and Nonlinear Perturbation Pressure Vertical Pressure Gradient Shear Vector

Linear and Nonlinear Perturbation Pressure Gradient Buoyancy and Advection Shear Vector

Vertical Velocity Tendency Using Forward Time Difference Vertical Velocity Tendency Using Vertical Momentum Eq Observed Calculated

Nonlinear Cyclostrophic Perturbation Pressure Nonlinear Shear Perturbation Pressure

Nonlinear Cyclostrophic Perturbation Pressure Gradient Nonlinear Shear Perturbation Pressure Gradient

Shear Vector

Summary and Future Work n This is the first comprehensive analysis of the perturbation pressure in a severe storm n The nonlinear terms in the pressure equation are important for the straight hodograph case, consistent with past numerical simulations. The importance of the nonlinear shear term has not been shown in the past, this study has shown that the horizontal circulation is as important as the vertical circulation associated with the mesocyclones.

Summary and Future Works (Continued) n More data will be needed for further verification of supercell propagation theory, especially in a curved- hodograph case