Chapter 3 Convective Dynamics Photographs Todd Lindley 3.2 Ordinary or "air-mass storm 3.2.1. Main Characteristics Consists of a single cell (updraft/downdraft pair) Forms in environment characterized by large conditional instability and weak vertical shear Vertically erect built-in self-destruction mechanism Can produce strong straight-line winds or microburst Life cycle is generally < 1 hour, usually 20-45 min These storms form in weakly-forced environments, and are driven primary by convective instability rather than the ambient winds They are sometimes called "air-mass" storm because they form within the same air-masses with more-or-less horizontal homogeneity 1
Photograph by: NSSL This is a single cell storm, looking east from about 15 miles. The storm was moving east (into the photo). Some of the anvil cloud has been left behind the storm, but the greater portion of the anvil is blowing off in advance of the storm and is not observable from this perspective. (May storm in the Texas Panhandle near Amarillo.) This late May storm in Oklahoma, looking northeast from about 20 miles, occurred with weak to moderate vertical wind shear. It did not produce any severe weather. 2
Life cycle of a non-severe single cell storm in weak wind shear Radar history of the severe pulse storm often with larger instability Example of Single-cell Life Cycle 3
3.2.2. Basic Dynamics (forces acting on an air parcel in the vertical) Perturbation Vertical Momentum Equation (base-state hydrostatic equations has been subtracted off on the RHS) water Three stages of single-cell storm development Towering stage Mature Stage Dissipating Stage 4
LCL Lifting condensation level LFC Level of free convection level at which the parcel is warmer than the Environment EL Equilibrium Level level at which the parcel s T becomes the same as the environment again Development of Single Cell Storm Step 1 In the absence of frontal or other forcing, daytime heating of the PBL causes the convective temperature to be reached. Thus, there is no negative area on the skew-t diagram for an air parcel rising from the surface the lid is broken 5
Development of Single Cell Storm Step 2 Updraft forms once the air reaches the LCL, latent heat is released due to condensation: - L dqv = Cp dt For every 1g/kg of water condensed, the atmosphere warms about 3 degrees. This feeds into the buoyancy term through an increase in q (remember earlier vertical momentum equation?). The saturated air parcel ascends following the moist adiabat, along which the equivalent potential temperature q e is conserved. Until the Equilibrium level is reached, the air parcel is warmer than the environment, which keeps the buoyancy positive (without the effect of water loading see later) Development of Single Cell Storm Step 2 continued When cloud forms, part of it is carried upward by the draft and the other part falls off the updraft. The weight of this liquid water makes the air parcel heavier, this water loading effect acts to reduce the positive buoyancy. B = g(q /q) - g L 10 x 3/300 ~ - 10 x 0.01 kg/kg Therefore 10 g/kg of cloud or rain water will offset 3 K temperature surplus. 6
Development and decay of Single Cell Storm When the cloud grows to a stage that the updraft becomes too heavy because of water loading, it will collapse, updraft then turns into downdraft. Another important process that contributes to the collapse is the evaporative cooling. When cloud grows, cloud droplets turn into larger rain drops that fall out of the updraft, reaching the lower level where the air is sub-saturated. The rain drops will partially evaporate in this sub-saturated air, producing evaporative cooling. This cooling enhances the downdraft. In the absence of vertical wind shear, the cell is upright, this downdraft would then disrupt the low-level updraft, causing the cell to dissipate. This is the built-in self-distruction mechanism mentioned earlier The cold downdraft sometimes form a cold pool that propagates away from the cell above, further removing the lifting undernearth the cell The effect of pressure gradient force In addition to buoyancy force and water loading, another force that is also acting on the rising parcel is the vertical pressure gradient force (PGF) When an air parcel rises (due to buoyancy), it has to push off air above it, creating higher pressure (positive p ) above (imaging push yourself through a crowd) Below the rising parcel, a void is created (imagine a vacuum cleaner), leading to lower pressure at the cloud base H L PGF 7
Effect of Pressure Gradient Force The higher pressure above will push air to the side, making room for the rising parcel, while the lower pressure below attracts surrounding air to compensate for the displaced parcel Such a positive-negative pattern of p perturbation creates a downward pressure gradient. The PGF force therefore opposes the buoyancy force, therefore acts to reduce the net upward forcing. Effect of Pressure Gradient Force The degree of opposition to the buoyancy force depends on the aspect ratio of the cloud (L/H), or more accurately that of the updraft The effect is larger for wider/large aspect-ratio cloud, and weaker for narrower/small aspect ratio cloud, because For narrow cloud, a small amount of air has to be displaced/attracted by the rising parcel, therefore the p perturbation needed to achieve this is smaller, so that the opposing pressure gradient is smaller (often <<buoyancy) so a narrow cloud can grow faster PGF is stronger for wide cloud, as a results, the net upward force (buoyancy PGF) is significantly reduced, the cloud can only grow slowly. When B and PGF has similar magnitude, the vertical motion becomes quasi-hydrostatic this is typical of large scale broad ascent. Dynamic stability analysis of inviscid flow shows that the infinitely narrow clouds grow the fastest, but in reality, the presence of turbulent mixing prevents the cloud to become too narrow, hence the typical aspect ratio of clouds is 1. 8
Skew-T analysis and Parcel Theory Skew-T analysis and Parcel Theory typically neglect the effect of PGF induced by vertical motion, essentially they assume that the environment is unchanged by the parcel motion. They also neglect the effect of mixing/friction Therefore, they only provide a, though still very useful, upper-limit for the convection intensity Convective Available Potential Energy (CAPE) CAPE measures the amount of convective instability, or more accurately the potential energy of an environmental sounding the energy that can be converted into kinetic energy when an air parcel rises from LFC to EL It is based on the simple parcel theory which neglects the effect of mixing/friction, PGF and sometimes water loading. From CAPE, we can estimate the maximum vertical velocity that can be reached by a parcel 9
CAPE Skew-T The negative area is equal to CIN (Convective Inhibition) The positive area (where air parcel is warmer than environment) is equal to CAPE Lifted Index temperature excess in 500mb environment over that of a parcel lifted from the low moist layer (negative value indicates instability) 10
ARPS Simulation of a Single Cell Storm May 20, 1977 Del City Supercell Storm Sounding used without environmental wind ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=0 11
ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=15min ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=20min 12
ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=25min ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=30min 13
ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=45min ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=0, 15min 14
ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=20, 25min ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=30, 45min 15
ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=45min High pressure is seen undernearth the cold pool. Gust front circulation produces strong lifting. ARPS Simulation of a Single Cell Storm Animations (at http://twister.ou.edu/mm2002) Buoyancy + Wind + qw + Reflectivity Eqivalent Potential Temperature + Wind + qw + Reflectivity Perturbation Pressure + Wind + qw + Reflectivity 16