Convective Roll Effects Convective Roll Effects on Sea Breeze Frontson Sea Breeze Fronts

Benton WhitesidesBenton Whitesides

EAS 6792: Air Pollution MeteorologyEAS 6792: Air Pollution Meteorology

December 1, 2003December 1, 2003

IntroductionIntroductionA 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.

Instability From Diabatic HeatingInstability From Diabatic HeatingDiabatic surface heating may cause adiabatic vertical motion.

In this case, only a 5C increase in the parcel at this location needed.

HCR HCR IllustrationIllustrationDiabatic heating Diabatic heating

creates HCR creates HCR (large eddies)(large eddies)

HCR circulation HCR circulation impacts SBF impacts SBF circulation circulation and cloud and cloud formationformation

Note SBF Note SBF inland from inland from shore.shore.

Dailey 1999*

Dailey*Dailey*HCR HCR ModelModel

Z>2km:Z>2km:

+w => -+w => -

-w => +-w => +Z<2km:Z<2km:

+w => ++w => +

-w => --w => -Z<3kmZ<3km

+w => +q+w => +qvv

-w => -q-w => -qvv

Analysis of Turbulent PropertiesAnalysis of Turbulent Properties

Using parameters established in the previous model, Using parameters established in the previous model, other parameters were calculated including:other parameters were calculated including:

u’, u’w’, w’q’, w’u’, u’w’, w’q’, w’’, d(w’’, d(w’’)/dt, and z’)/dt, and zstabilitystability. .

To assess instability and circulation induced by HCR To assess instability and circulation induced by HCR motion and their impact on cloud formation along motion and their impact on cloud formation along the SBF.the SBF.

Focusing on interactions between 2 HCRsFocusing on interactions between 2 HCRs

Divergence at top of updraft, convergence below.Divergence at top of updraft, convergence below.u’ = 0 along axis of updraft.u’ = 0 along axis of updraft.

1 2 3 4 5 6 7 8 9 10 11S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

X (km)

z

u'

-10--5 -5-0 0-5 5-10 1 2 3 4 5 6 7 8 9 10 11S1

S3

S5

S7

S9

S11

S13

-20

-15

-10

-5

0

5

10

15

20

25

30

u'w'

x (km)

z (km)

-20--15 -15--10 -10--5 -5-0 0-5 5-10 10-15 15-20 20-25 25-30

U’ and Momentum Flux

Momentum flux (u’w’) peaks directly above & below Momentum flux (u’w’) peaks directly above & below downdrafts regions A-D.downdrafts regions A-D.

1 2 3 4 5 6 7 8 9 10 11S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

S12

S13

X (km)

z

u'

-10--5 -5-0 0-5 5-10 1 2 3 4 5 6 7 8 9 10 11S1

S3

S5

S7

S9

S11

S13

-20

-15

-10

-5

0

5

10

15

20

25

30

u'w'

x (km)

z (km)

-20--15 -15--10 -10--5 -5-0 0-5 5-10 10-15 15-20 20-25 25-30

A B

C D

U’ and Momentum Flux

-w’q’ peaks in regions of large-w’q’ peaks in regions of largevertical velocity (regardless of direction) Because q’ and w’ vertical velocity (regardless of direction) Because q’ and w’ always have same sign and are max along the same axes. always have same sign and are max along the same axes.

-w’-w’’ also has greatest magnitude along the updraft and ’ also has greatest magnitude along the updraft and downdraft axes with a maximum near the surface and downdraft axes with a maximum near the surface and minimums at greater height due to complex minimums at greater height due to complex ’ structure. ’ structure.

1 2 3 4 5 6 7 8 9 10 11S1

S3

S5

S7

S9

S11

S13

0

5

10

15

20

25

w'q'

x (km)

z (km)

0-5 5-10 10-15 15-20 20-25

1 2 3 4 5 6 7 8 9 10 11S1

S3

S5

S7

S9

S11

S13

-10

-8

-6

-4

-2

0

2

4

6

8

w'theta'

x (km)

z (km)

-10--8 -8--6 -6--4 -4--2 -2-0 0-2 2-4 4-6 6-8

Water Vapor and Thermal Fluxes

Potential Temperature Tendency calculated from the Potential Temperature Tendency calculated from the equation: equation:

= = --w’w’’’t t zz

changes along updraft / downdraft regions with maximum changes along updraft / downdraft regions with maximum magnitude where w’ is greatest halfway up the HCRs.magnitude where w’ is greatest halfway up the HCRs.

1 2 3 4 5 6 7 8 9 10 11

S1

S3

S5

S7

S9

S11

-40

-30

-20

-10

0

10

20

x (km)

Z

Potential Temperature Tendency

-40--30 -30--20 -20--10 -10-0 0-10 10-20

StabilityStability

Stability was also determined by the calculation of z: Stability was also determined by the calculation of z: (stable z>1; unstable z,1)(stable z>1; unstable z,1)

z = -z = -gzgz00(w’(w’’)’)avgavg

uu**33

Z found to be positive (stable) on downdraft axes.Z found to be positive (stable) on downdraft axes.Z found to be zero (neutral) in centers of each HCR. Z found to be zero (neutral) in centers of each HCR. Expected instability along updraft axis, however Expected instability along updraft axis, however

calculation could not be used in this region calculation could not be used in this region because ubecause u** = 0. = 0.

zz00 = 0.001m (coastal region) = 0.001m (coastal region)

ConclusionsConclusionsStability: Generally from *Dailey’s model we can infer Stability: Generally from *Dailey’s model we can infer

that stability decreases in the atmosphere above that stability decreases in the atmosphere above the axis of updraft and increases along the axis of a the axis of updraft and increases along the axis of a downdraft. These perturbations help explain downdraft. These perturbations help explain observations of regions of enhanced cloud observations of regions of enhanced cloud development flanked buy resistance to development flanked buy resistance to development along the sea breeze front.development along the sea breeze front.

Usefulness of model with calculations:Usefulness of model with calculations:In this case, the only dynamic perturbations were In this case, the only dynamic perturbations were associated with the HCR, therefore, w’ and u’ did associated with the HCR, therefore, w’ and u’ did not co exist along the axes up updrafts and not co exist along the axes up updrafts and downdrafts making calculations less effective. In downdrafts making calculations less effective. In reality, there would be other perturbations reality, there would be other perturbations preventing this problem and facilitating the use of preventing this problem and facilitating the use of these calculations.these calculations.

Questions?Questions?

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