Mesoscale and Local Refraction Effects


Learning Objectives



In this last module of this section, we now examine features that have horizontal scales less than 200 km (In meteorology, "mesoscale" refers to features that are 2 - 200 km in size). These are sea and land breezes, and other effects that set up inversions locally over land.

Sea and Land Breezes

Sea breezes occur in coastal areas when the land heats up. Since hot air is lighter it will rise and be replaced with low level air from the ocean. As the heated air rises it can return at higher levels back to the ocean where it sinks, resulting in a complete circulation cell. The sinking area is favorable to inversions and may create or enhance ducting.

Land breezes occur at night when the ocean is warmer than the nearby land areas. The cycle is opposite the sea breeze, with low level air moving from land to ocean while the upper level return flow goes from ocean to land. In this situation ducting can occur over the ocean where masses of dry land air are moistened on the bottom, which can cause surface ducting.

Local Effects: Inversions over Land

Ducts can occur over land also. Here is such a scenario (there are others). Valleys can fill with cold, moist air trapped by an inversion. Sometimes antennas on nearby hills are in a hole region because they are just above the inversion (and associated duct) caused by the cold and moist valley air. Valley-to-mountain communication can be disrupted until the sun heats up the valley, which destroys the inversion and frees the moisture, thus removing the duct and associated hole regions.

Climatology of selected locations

To look a bit further into local conditions, we'll look at the frequency of ducting from climatology at several locations for comparison. Surface-based ducts and elevated ducts are shown separately.


Vandenburg AFB

Vandenburg Air Force Base is located near the coast in southern California (see map at right) and has a high percentage occurence of both surface-based and elevated ducts (see below), with the highest likelihood of elevated ducts in the summer months. No trend in surface-based ducts, but elevated ducts occur about half of the time in the summer ducts.


Bahrain has a very high probably of surface-based ducts in the summer months (left bar chart). Note the change of scales between the two plots - the probability of elevated ducts is highest in the fall months, but still below 40% probability. For maysept ducting occurs more than 80% of hte time. In winter, there is less Opposite in elevated ducts, mostly because there are so many surface based ducts.


In Taipei, there is a very low probability of either surface-based ducting or elevated ducting (see graphs below). 10% in July, but very little either surface-based or elevated, and not much seasonal variability.

Pratas Island

Pratas Island in the South China Sea A bit to the south, March and April there is elevated ducting The difference between the South China sea and Taiwan are probably due to more of a monsoon influence in the South China Sea. e c