Convective Roll Effects on Sea Breeze Fronts

1. Convective Roll Effects on Sea Breeze Fronts Benton Whitesides EAS 6792: Air Pollution Meteorology December 1, 2003

2. Introduction A Sea Breeze Front (SBF) is created by the interaction of a sea breeze and a background breeze over land. Often Horizontal Convective Rolls (HCR), large eddies forced by diabatic heating, interact with the SBF. This interaction impacts: moisture distribution, u, v, w, , turbulent fluxes and stability. HCR/SBF interaction may allow for cloud development when perhaps neither phenomena could independently.

3. Instability From Diabatic Heating Diabatic surface heating may cause adiabatic vertical motion. In this case, only a 5C increase in the parcel at this location needed.

4. HCR Illustration Diabatic heating creates HCR (large eddies) HCR circulation impacts SBF circulation and cloud formation Note SBF inland from shore. Dailey 1999*

5. Dailey*HCR Model Z>2km: +w => - -w => + Z<2km: +w => + -w => - Z<3km +w => +qv -w => -qv

6. Analysis of Turbulent Properties Using parameters established in the previous model, other parameters were calculated including: u’, u’w’, w’q’, w’’, d(w’’)/dt, and zstability. To assess instability and circulation induced by HCR motion and their impact on cloud formation along the SBF. Focusing on interactions between 2 HCRs

7. U’ and Momentum Flux Divergence at top of updraft, convergence below. u’ = 0 along axis of updraft.

8. U’ and Momentum Flux A B C D Momentum flux (u’w’) peaks directly above & below downdrafts regions A-D.

9. Water Vapor and Thermal Fluxes -w’q’ peaks in regions of large vertical velocity (regardless of direction) Because q’ and w’ always have same sign and are max along the same axes. -w’’ also has greatest magnitude along the updraft and downdraft axes with a maximum near the surface and minimums at greater height due to complex ’ structure.

10. Potential Temperature Tendency calculated from the equation:  = -w’’ t z • changes along updraft / downdraft regions with maximum magnitude where w’ is greatest halfway up the HCRs.

11. Stability Stability was also determined by the calculation of z: (stable z>1; unstable z,1) z = -gz0(w’’)avg u*3 Z found to be positive (stable) on downdraft axes. Z found to be zero (neutral) in centers of each HCR. Expected instability along updraft axis, however calculation could not be used in this region because u* = 0. z0 = 0.001m (coastal region)

12. Conclusions Stability: Generally from *Dailey’s model we can infer that stability decreases in the atmosphere above the axis of updraft and increases along the axis of a downdraft. These perturbations help explain observations of regions of enhanced cloud development flanked buy resistance to development along the sea breeze front. Usefulness of model with calculations: In this case, the only dynamic perturbations were associated with the HCR, therefore, w’ and u’ did not co exist along the axes up updrafts and downdrafts making calculations less effective. In reality, there would be other perturbations preventing this problem and facilitating the use of these calculations.

13. Questions? *Dailey, Peter S., and Robert G. Fovell. The sea-breeze and horizontal convective rolls: Numerical simulation of the interaction between the sea-breeze front and horizontal convective rolls. Part I: Offshore ambient flow. Monthly Weather Review. May 1999. 127. p 858.