Theories on the Optimal Conditions of Long-Lived Squall Lines

Transcription

1 Theories on the Optimal Conditions of Long-Lived Squall Lines References: Thorpe, A. J., M. J. Miller, and M. W. Moncrieff, 1982: Two-dimensional convection in nonconstant shear: A model of midlatitude squall lines. Quart. J. Roy. Meteor. Soc., 108, Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for strong long-lived squall lines. J. Atmos. Sci., 45, Lafore, J.-P., and M. W. Moncrieff, 1989: A numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines. J. Atmos. Sci., 46, Lafore, J.-P., and M. W. Moncrieff, 1990: Reply to Comments on "A numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines". J. Atmos. Sci., 47, Xue, M., 1990: Towards the environmental condition for long-lived squall lines: Vorticity versus momentum. Preprint of the AMS 16th Conference on Severe Local Storms. Amer. Meteor. Soc., Alberta, Canada, Xue, M., 2000: Density current in two-layer shear flows. Quart. J. Roy. Met. Soc., 126, A Schematic Model of a Thunderstorm and Its Density Current Outflow Downdraft Circulation -Density Current in a Broader Sense (Simpson 1997) 1

2 Schematic of a Thunderstorm Outflow (Goff 1976, based on tower measurements) Rotor A Conceptual Model of Quasi-Steady 2D Squall Lines (Thorpe, Miller and Moncrieff 1982) Cold Pool In a Broad Sense 2-D Time-Averaged Flow -- Numerical Simulation 2

3 Theories of Intense / Long-lived Squall Lines Thorpe, Miller and Moncrieff (1982) TMM Theory Rotunno, Klemp and Weisman (1987) RKW Theory Perspective The RKW theory for long-lived squall lines, though widely cited, remains controversial We try to look at more careful look at the theory here 3

4 Key Findings of Thorpe, Miller and Moncrieff TMM82 P0 P-10 P-5 P5 P10 P0 Total Rainfall = 449 P0 is quasi-stationary and produced maximum total precipitation Thorpe, Miller and Moncrieff TMM82 All cases required strong low-level shear to prevent the gust front from propagating rapidly away from the storm; TMM concluded that low-level shear is a desirable and necessary feature for convection maintained by downdraught. 4

5 RKW Theory RKW s Vorticity Budget Analysis to Obtain the optimally balanced condition 5

6 RKW s Vorticity Budget Analysis to Obtain the optimally balanced condition In the above, c is defined by H 2 θ ρ c = 2 ( B 0 L) dz= 2g H = 2g H θ0 ρ0 ρ c= 2g H ρ0 which is exactly the density current propagation speed we derived earlier! Therefore the optimal condition obtained based on RKW s vorticity budget analysis says that the shear magnitude in the low-level inflow should be equal to the cold pool propagation speed. RKW Optimal Shear Condition Based On Vorticity Budget Analysis η < 0 w η > 0 u=0 d H u L R u= ur,0 = c ( uη ) ( wη ) B + = x z x 6

7 RKW Optimal Shear Condition X R Vorticity Budget Analysis of RKW ( wη ) ddx= 0 η < 0 X L d ( uη ) Ldz = 0 0 H<d w? η > 0 d u ( uη ) ( wη ) B + = x z x 0 0 d d R R H 0 0 L ( uη) dz ( uη) dz+ ( wη) dx R L d 0 0 L ( wη ) dx = ( B B ) dz L 0 R H 0 L L Bdz g θ θ H c 2 / 0 /2 d 0 R u ( uη ) Rdz = 2 2 R,0 u= ur,0 = c RKW Numerical Experiment of a Spreading Cold Pool Area To be Shown 7

8 θ' Line-Relative Vectors η, Div (shaded) RKW Density Current Simulation Results u=c Circulation are induced by cold pool propagation, NOT vorticity or shear Questions What is the Flow Pattern at the Gust Front? Does the Low-level Inflow have to Contain Shear or Vorticity? Does RKW s Vorticity Balance Argument Adequately Explain the Behavior of Low-level Lifting, Updraft Orientation and Squall Line Intensity? If Not, What Are the Most Important Factors for Strong Low-level Lifting and a Deep Updraft? We will Attempt to Answer These Questions by Using Idealized Density Current Models Numerical Simulations of Realistic Squall Lines Observations 8

9 First, Theoretical Models of Density Currents Inviscid Model of Benjamin (1968, JFM) Density Current Inviscid fluids No vertical shear; Apply Bernoulli s theorem (energy conservation), flow-force balance and mass conservation: h = d/2, c1 = 2 gh ρ/ ρ0 9

10 Convention for Environmental Shear β β α α Shear > 0 Shear < 0 Introduction of Constant Shear (Xu 1992 JAS; Xu, Xue and Droegemeier 1996 JAS) Benjamin s Solution Depth and speed of cold pool increase with increasing positive shear; h > 0.5 H for α > 0; h < 0.5 H for α < 0. Solution verified using timedependent numerical model by Xu, Xue and Droegemeier (1996 JAS). 10

11 Inviscid Density Current Models in Variable Vertical Shear (Xue, Xu, Droegemeier, 1997 JAS; Xue 1999) Two shear layers allow for more flexibility with inflow configuration, e.g., Solution Procedure From the four principles, we obtain four equations They reduce the 7 free parameters to 3 We specify the lowlow-level and upperupper-level inflow shear (α (α and β) and the interface height (d0), and seek solutions of the other parameters which describe the cold pool depth, speed etc. The cold pool depth determines, to a large extent, the steepness of the frontal interface and therefore the erectness of the updraft. updraft. 11

12 Summary of Theoretical Results Positive inflow shear, either at low-levels or at upperlevel, supports a deep cold pool, steep frontal interface, and therefore a deep updraft. A deep updraft can be supported even without lowlevel inflow shear The RKW Theory, however, considers the low-level shear essential for deep updraft to form Verification of Theory With a Timedependent Numerical Model Is a Steady State Solution Realizable in a Time- Dependent, Fully Nonlinear Model? What Is the Shape of the Frontal Interface and the Orientation of the Updraft? What Effects Do Baroclinically-Generated Frontal Eddies Have on the Flow Structure In the Head Region? We Answer These Questions Using a Time- Dependent Numerical Model, the ARPS. 12

13 Zero Upper-Level Shear, Different Low-Level Shear β =0 α= -1 β =0 No Cold Pool Induced Internal Circulation α= +1 Figures Plotted to Scale Zero Low-level Shear with Opposite-Sign UpperLevel Shears β=+2 α=0 β=-2 α=0 Cold pool structure strongly influenced by upperlevel shear too; not considered by RKW 13

14 Conclusions from Numerical Experiments Theoretical model solutions are verified to a high degree of accuracy by a fully nonlinear, time-dependent numerical model; Kelvin-Helmholtz eddies are explicitly resolved by the numerical model. However, their effect on the density current head is relatively small. The model solutions are robust and are independent of the initial conditions as long as the cold pool air supply is sufficient for the expected depth to be achieved; When insufficient cold air is available, the flow is highly unsteady. However, the flow pattern in a time-averaged sense is still close to the steady solution. This non-steady scenario is more typical of atmospheric squall lines. The overall flow is dictated by the overall vorticity distribution in the domain. Low-level shear is not necessary to establish a deep cold pool (which supports an effective trigger mechanism), as RKW theory suggests. Numerical Simulations of Squall Lines in Support of Our Last Argument 14

15 2-D Squall Line Simulations of Xue (1989, 1991) Linear Low-level Shear Step Inflow Profile with Zero Vorticty Step Profile Cases Time (0-10 Hours) -10m/s Stationary -18m/s Constant Speed Propagation -15m/s -25 m/s 15

16 Line is Quasi-Stationary Low-level Linear Shear Inflow Cases (0-4 hours) 12m/s 15m/s 20m/s 28m/s 16

17 Conceptual Model of of Xue (1991) c = cloud-relative cold pool speed Line Relative Inflow Profiles RKW Simulation Results θ Line-Relative Vectors η, Div (shaded) Optimal Condition No need for Cold Pool Circulation or Inflow Shear 17

18 Summary Point #1 RKW theory considers the circulation at the edge of the cold pool (via baroclinic vorticity generation) to be the essential ingredient in generating a circulation to counter the inflow shear Our high-resolution model simulations show that negative vorticity is confined to a thin vortex sheet that is mostly advected to the rear of the system. The circulation resulting from the vorticity is insignificant at the frontal interface. The cold pool internal circulation has little effect on the frontal slope. The primary role of the cold pool is to decelerate the ground level inflow, rather than generating the so called cold pool circulation. Summary Point #2 RKW optimal condition is based on a vorticity budget analysis that gives a zero flux condition at the top of the control volume, which assumes an updraft profile that may not be achievable RKW assumes that the upper-level air will be calm relative to the gust front, which is not guaranteed by the vorticity balance condition. Our approach tries to solve for the flow around the gust front; Our results also show that upperlevel front-relative flow plays an equally important role in determining the slope of the updraft. 18

19 RKW s optimal balance condition requires that the inflow directly interacting with the cold pool contain positive vorticity that matches the negative vorticity generated by the gust front; Summary Point #3 Our theoretical model, confirmed by numerical simulations of both density currents and squall lines, shows that, despite the presence of a cold pool, deep, long-lasting and quasi-steady updrafts can be established without low-level inflow shear. Conclusions Low-level inflow shear is NOT necessary for establishing deep, steady updrafts Baroclinically generated cold pool circulation does not appear to have significant effect on the structure of density current head Rather, the shear between the ground level and the steering level is a more important factor in determining the propagation of cold pool relative the cloud system above The updraft orientation is a function of vorticity distribution throughout the entire domain, and a global solution should be obtained by solving the vorticity equation with proper boundary conditions. To determine the behavior of the updraft branch of inflow over the cold pool, we need to know the vorticity distribution in the entire domain and the boundary conditions. Vorticity in an air parcel alone cannot tell us its trajectory. 19

20 Conclusions In general, a cold pool that propagates at the speed of, or slightly faster than, the steering level wind (or the propagation speed of a cloud) creates an optimal condition for intense, long-lasting squall lines. The role of the low-level system relative inflow is to prevent the cold pool from propagating away from the overhead cloud. The surface system-relative wind speed, rather than the shear, is most important. Our optimal condition based on front propagation speed and surface and steering level winds makes few assumptions and is more generally valid. 20