Sea-breeze - Key Parameters

by FMI and KNMI

Temperature difference between land and sea

 
19 May 2009/13.00 UTC - T (up) + Td (down) reports over The Netherlands in Sea-Breeze circulation
11 - 16 May 2009 - Meteosat 9 Sea Surface Temp.
19 May 2009/14.00 UTC - Meteosat 9 HRV image

The images above are an example of the traditional explanation of the development of Sea-Breeze circulation as a result of temperature differences over sea and over land, the general scheme is in the image top left. The image top right is an example of Sea-Breeze as a result of low-level geostrophic flow over The Netherlands. Under a ridge of high pressure with axes just to the west of Holland the Sea-Breeze developed in the late morning and moved more the 50 km inland. With 13 degrees at the coast and almost 21 degrees Centigrade in the east the differences were not that high.

The lower images are the average sea surface temperature as measured and computed by satellite info (OSI-SAF). In the satellite image on the right side below the convective clouds of the Sea-Breeze over the west part of Holland are difficult to recognize due to the overlaying high level clouds.

Low-level geostrophic flow

13 June 2001/12.00 UTC - 850 hPa wind (barbs) and geopotential height (black) on Iberian peninsula

Surface winds

 
19 May 2009/11.40 UTC - Windreports over The Netherlands at the start of a Sea Breeze
13 June 2001/12.00 UTC - surface synoptic wind observations with an estimated position of Sea-Breeze front (dashed line)
13 June 2001/12.00 UTC - Meteosat 8 VIS image

A Sea-Breeze observed over the Spanish south-east coast. The line of the Sea-Breeze front cannot be delineated accurately due to insufficient surface observations, but (almost acute angle) onshore wind is apparent at the coast, while inland the wind direction is more random. Some convective clouds appear on the VIS - image along the coast, from south east to east and north east on the See-Breeze front.
Even along the west and south coast of Portugal and the north coast of Spain the surface wind has much similarity with See-Breeze circulation. The Meteosat VIS - image to the right is showing hardly any clouds (low RH), so a maximum solar radiation can be expected over land with high surface temperatures. Along the north coast of Spain the See-Breeze is accompanied by low clouds from the Bay of Biscay, depicted as a large area of strong reflecting white clouds.

Convergence near surface

31 May 2001/12.00 UTC - convergence at 1000 hPa, magenta dashed: divergence, magenta solid: convergence

The classical explanation of Sea Breeze is the heated and expanding air over land, causing convergence aloft over land at first, pressure falling over land and divergent spreading out towards the sea ( offshore), then descending air over sea, pressure rising, onshore winds at the surface with low level divergence over sea and low level convergence again at the Sea-Breeze front over land where the cooler air meets the warm air. This scheme is depicted in the image above, top left.

Sea-Breeze and sensible heat flux over sea and land

A study was done by using a 2D mesoscale model to calculate the maximum sensible heat flux on a specific day and a relation was found with the strength of the Sea-Breeze : Um = 0.35 H1/2max. (Um = max Sea-Breeze, perpendicular to the coastline; H1/2max = max sensible heat flux). The start of the Sea-Breeze is of course dependant on the strength of the existing and opposite surface wind Ug. A relation was found between Um and Ug called the Sea-Breeze index : Isb = Ug/Um, and with Im < 1 a high probability exist for the onset of the Sea-Breeze.

A second and important relation ( X = ts (µm Um - µg Ug) was found by comparing the Sea-Breeze and the opposite wind for the calculation of the distance (X), the Sea Breeze can travel inland given a specific opposite wind. (ts = time, µm and µg are coefficients = 0.7 and 0.8). An explanation of the graphs is available in the chapter Weather Events.

Inland penetration of Sea-Breeze front in relation to opposite wind