B. Geerts and E. Linacre
Subsiding layers can be identified in an aerological diagram (Note 7.D) by anomalously warm, dry air. The lower limit of the downward motion is the subsidence inversion, where dewpoint and temperature traces converge. Subsidence inversions are found mainly near the subtropical highs, behind cold fronts, at high latitudes (poles and winter continents), and behind mesoscale convective systems.
The subsidence inversions are strongest near or just equatorward of the axis of the subtropical high, because downward motion in the Hadley cell is strongest there. In Australia they typically have a vertical temperature rise of 5 K or more within a layer only 50 - 100 m thick, about 1.5 km high (1). There is also a sharp change of moisture content in the air, with extremely dry air aloft. Subtropical inversions are long-lived, and they are usually do to the combined effect of subsidence (which warms the layer above the inversion) and PBL mixing (which cools the air below the inversion). Hence the large temperature gradient. Over continents in summer, these inversions tend to be higher and weaker, because the extra heat deepens the PBL and erodes the inversion.
Subsidence inversions are often referred to as Trade wind inversions over the subtropical oceans. These inversions are found higher and are weaker towards the ITCZ, and they are closer to the sea surface (sometimes below 500 m) and stronger (temperature rise up to 10 K) around the latitude of the Tropics, especially near the eastern margins in case of cold currents and ocean upwelling (Fig 7.10 in the book). An example of a weak Trade wind inversion can be seen around 870 hPa in the 10 September '98 12UTC sounding at Fortaleza, Brazil (4ēS). More subsidence is evident above 800 hPa
The air behind a cold front tends to subside (cold conveyor belt, Fig 13.4 in the book). A good example is the 24 Jan '99 12UTC sounding at Nashville, Tennessee, USA. Postfrontal subsidence is present between 860 and 600 hPa. The frontal boundary itself appears to be between 400 and 350 hPa (slightly warmer air is wedged above), although it is unusual to find a frontal surface at that level.
Air tends to sink near the poles and the big cold highs over Siberia and Canada in winter. The resulting subsidence inversion usually is hard to discern from the stronger and deeper radiation inversion. An example is the 30 Apr '98 12 UTC sounding at Mould Bay, Canada, near the Arctic Sea (76ēN). A weak subsidence inversion can be seen just above 900 hPa, but it merges directly with the much stronger radiation inversion. The temperature rises from -20ēC on the ground to -4ēC 600 m higher. At the South Pole Station on the Antarctic ice sheet, there is generally a radiation inversion of around 30 K between the surface and 100-500 m, and the effect of subsidence is evident only as a stable layer between 500-3,000 m above ground level (2).
Occasionally a radiosonde will ascend through sinking air in the mesoscale downdraft behind a mesoscale convective system. The resulting sounding looks like an onion, with the inversion forming the root of the onion and the anvil the top of the onion, with an outgrowth slanting to the left along a SALR. Usually the onion is quite skinny. An example is the 8 Jan '99 00 UTC sounding at Manaus, Brazil (3ēS). A large cloud deck had just passed the station, and there is a weak subsidence inversion around 750 hPa.
(2) Jacka, K. and P. Reid 1999. A study of the surface and near surface air temperature over the Antarctic continent. A paper to the 6th National Australian Meteorological and Oceanographic Society, Canberra Conference.