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The arrows show the position of a sea breeze near the Gulf of Carpentaria and a front which has penetrated into the sub-tropics from the south. High resolution -120 kb- |
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The arrows show the position of a sea breeze near the Gulf of Carpentaria and a front which has penetrated into the sub-tropics from the south. High resolution -120 kb- |
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The detection of cold fronts over central Australia is difficult due to the sparse nature of the observational network. However, cloud-free infrared imagery, which shows surface temperatures, can be used to highlight the difference between ground temperatures ahead of and to the rear of cold fronts and sea breezes. During the day infrared images over northern Australia show that ground temperatures to the rear of cold fronts and sea breezes are relatively colder than those ahead. At night-time however, the land ahead of the advancing front and sea breeze cools under light wind conditions and consequently a shallow layer of cold air forms (a temperature inversion) adjacent to the relatively cold ground. As a cold front or sea breeze penetrates these inland areas at night the strong winds that accompany them mix the cold air adjacent to the surface with warmer air from above and surface temperatures subsequently rise. The night-time infrared image shows that relatively warm surface temperatures (green areas) are located immediately to the rear of the both the sea breeze and cold front as they advance inland. Relatively cooler surface temperatures (shown in blue) are seen inland ahead of both the front and sea breeze. |
Data obtained from the Central Australian Fronts Experiment (CAFE '96) show that cold fronts pass through the arid inland areas of northern Australia. The position of such a front traversing central Australia at night during the experiment is clearly evident in the infrared image overleaf, enhanced to detect low-level temperatures. It is consistent with observations from CAFE '96 and diagnostics generated by local forecasting models fields used to locate fronts.
The imagery shows that surface temperatures immediately to the rear of the front are warmer than pre-frontal surface temperatures. The apparent paradox of warm signatures in satellite imagery representing what should be, in this case, a relatively colder post-frontal air-stream has also been noted in imagery showing 'cold' Antarctic katabatic air-streams (D'Aguanno, 1986). CAFE observations suggest that winds ahead of the front are generally light allowing a nocturnal surface temperature inversion to form. The passage of the front and the accompanying strong and gusty post frontal winds mix out the nocturnal surface temperature inversion causing screen height and surface temperatures to rise.
The movement of such fronts and their strong winds over arid inland regions pose forecasting problems such as low-level atmospheric turbulence and sometimes severely reduced visibility due to areas of raised dust. Warm surface temperatures are similarly in evidence where the sea breeze has penetrated inland from the Gulf of Carpentaria and again broken the surface inversion.
| Further reading | D'Aguanno J., 1986: Use of AVHRR data for studying katabatic winds in Antarctica, International Journal of Remote Sensing, 7, 703-713. |
| Sources | Australian Bureau of Meteorology, 150 Lonsdale St., Melbourne. |