ESCI 344 – Tropical Meteorology

Lesson 6 – Local and Diurnal Circulations

 

References:  Forecaster’s Guide to Tropical Meteorology (updated), Ramage

 Climate Dynamics of the Tropics, Hastenrath

 Climate and Weather in the Tropics, Riehl

 “The interaction of trade wind and sea breeze, Hawaii,” Leopold, J. Meteor., 6, 312-320, 1949

 “The effects of a large island upon the trade-wind air stream,” Malkus, Quart. J. Roy. Meteor. Soc., 81, 538-550, 1955

 

Reading:  McGregor and Nieuwolt, pp 111 - 115

“The interaction of trade wind and sea breeze, Hawaii” (e-reserve)

 “The effects of a large island upon the trade-wind air stream” (e-reserve)

 

GENERAL

˜  Diurnal variation of solar radiation and temperature in Tropics is much larger than annual variation.

ˇ  In midlatitudes, annual variation is larger than diurnal variation.

˜  Diurnal and mesoscale process are very dominant in the Tropics.

 

LAND-SEA BREEZE

˜  Land-sea breezes caused by the differential heating across land-water boundaries occur both in the midlatitudes and the tropics.  However, the influence of land-sea breezes is generally more pronounced in the tropics.

˜  Inland or offshore extent of land-sea breeze is greater in the tropics compared with the midlatitudes (~100 km, vs. ~10 km).

˜  The difference in extent is explained by the weaker Coriolis acceleration in the tropics.

ˇ  Rossby radius of deformation is the ratio of the gravity wave speed divided by the Coriolis parameter,

.

ˇ  The radius of deformation is essentially a measure of how large a circulation must be in order for the effects of the earth’s rotation to be important.

ˇ  Circulation smaller than the radius of deformation are not affected by the earths rotation.

ˇ  Coriolis helps to limit the horizontal effects of the land-sea breeze by turning the wind and therefore limiting its inland-offshore influence.

ˇ  In the tropics the radius of deformation is nearly an order of magnitude larger than in the midlatitudes, so a circulation such as the sea breeze can have a much larger horizontal extent without being turned by the Coriolis acceleration.

˜  The type of cloudiness and/or precipitation associated with the land-sea breeze depends on the orientation of the coast with respect to the prevailing wind, as well as the inland topography.

˜  In a 1949 study, Leopold categorized four main types of land-sea breeze interactions with the trade winds over islands.  His four categories are:

ˇ  Lanai type – Appropriate for small, low island in which the trade winds blow up and over the peaks, and do not split.

n  Sea-breeze front is narrow and perpendicular to trades.

ˇ  Maui type – Appropriate when peaks are tall enough to split trade wind flow.

n  Flanks of sea-breeze front exhibit shear lines trailing downwind.

ˇ  Mauna Kea type – Appropriate for windward side of island with very tall peaks.

n  Cloudiness forming on windward side during daytime as trades are reinforced by sea breeze.

n  At night, land breeze front causes convergence offshore, with offshore cloudiness and showers.

ˇ  Kona type – Appropriate for leeward side of islands with very tall, broad peaks.

n  During daytime, sea breeze moves in unimpeded by trade flow, which has been blocked by tall, broad peaks.

n  Convective showers form inland as sea breeze brings in moist air that is heated from below.

n  At night, down-slope land breeze suppresses clouds.

˜  Leopold’s categories likely apply to other islands besides Hawaii.

˜  Land-breeze interaction with trades has been documented as producing enhanced rainfall at night and early morning on the east coasts of Brazil and the Ivory Coast.

˜  Large, mountainous islands (such as Puerto Rico) often have a clear ring of suppressed cloudiness surrounding them in the afternoon.

ˇ  This is due to the compensating subsidence from the vigorous convection over the interior.

˜  Land-lake breeze circulations occur over and around large lakes, such as Lake Victoria in Africa.

 

MOUNTAIN CIRCULATIONS

˜  Mountain-valley breezes are common in the tropics.

ˇ  Valley breezes occur in afternoon, leading to cloudiness over mountains, with clear skies in subsiding air over valleys.

ˇ  Mountain breezes occur at night., leading to clearing over the mountains, and cloudiness in valleys.

˜  Mountain rainfall patterns in tropics often show a band of maximum rainfall somewhere at around 1000 meters above the valley floor, with decreasing rainfall at the foot or toward the top of the mountain.

˜  This is mainly associated with areas where convective clouds dominate the rainfall.          

ˇ  Lower slopes don’t experience as much orographic lift, and also experience evaporation of rain below cloud base.

ˇ  Upper slopes suffer from lower moisture availability.

˜  Where stratiform clouds dominate, the rainfall maximum tends to be near the top of the mountain.

 

STRESS-DIFFERENTIAL INDUCED DIVERGENCE

˜  An abrupt change in friction (surface stress) can result in convergence or divergence and impact rainfall and cloudiness patterns.

˜  In balanced flow the cross-isobar angle is given by

where Fr is the deceleration from friction.  If friction is considered to be linear in V, then

where CD  is the drag coefficient.

˜  Where the drag coefficient is larger, there is more cross-isobar flow.

˜  For the same drag coefficient, there is more cross-isobar flow at low latitudes than at higher latitudes.

˜  An air stream paralleling a coast-line will result in

ˇ  Divergence and drier conditions over the coastline if the lower pressures are inland.

ˇ  Convergence and moister conditions over the coastline if higher pressures are inland.

˜  This may partially explain the dry conditions over certain coastal regions such as the North coast of Venezuela, and other locations in the Tropics.