The Surface-Based Temperature Inversion on the Antarctic Plateau Stephen R. Hudson Richard E. Brandt University of Washington

Data and Sources Temperatures at 2, 13, and 22 m, winds at 10 m, and downward infrared fluxes at the surface, measured at South Pole from 1994—2003 by CMDL and provided by Thomas Mefford and Ellsworth Dutton Soundings collected by SPMO from 1994—2003 (AMRC) Surface and 20, 50, 100 and 200 cm air temperatures at SPO from winter 2001 (UW) Case study of 1992 inversion destruction at SPO (Warren) Surface and 2 and 30 m air temperatures at Dome C from summer 2004/05 (UW) Soundings at Dome C from summer 2003/04 (UI) Hudson, S.R. and R.E. Brandt, 2005: A Look at the Surface-Based Temperature Inversion on the Antarctic Plateau, J. Climate in Press.

Geography

What and Why Is the Inversion? During most of the year air temperatures increase significantly with height above the surface of the Antarctic Plateau

What and Why Is the Inversion? During most of the year air temperatures increase significantly with height above the surface of the Antarctic Plateau

What and Why Is the Inversion? During most of the year air temperatures increase significantly with height above the surface of the Antarctic Plateau With little or no sun: So T s < T a as long as  a is significantly less than  s — only violated under clouds 4 ass 4 a TT  + SH + LH + SSH

South Pole Inversion—2-22 m Data are 2 and 22-m temperatures from from CMDL’s South Pole Observatory

South Pole Inversion—Surface-2 m Inversion extends to surface with strongest gradient below 20 cm Surface temperature is usually 0-2 K lower than 2-m temperature No lapses under clear skies

Winds and the South Pole Inversion Minimum temperatures and strongest median inversions occur with winds of 3-5 m s -1, not with calm winds May be due to the inversion wind, which makes it difficult to have an inversion and calm winds on sloped terrain

Radiation and the South Pole Inversion Inversion strength is mainly sensitive to IR flux at intermediate values At fluxes less than about 80 W m -2, or brightness temperatures less than about –79°C, the median inversion strength is about 3 K At fluxes greater than about 135 W m -2 the median inversion strength is zero.

The Diurnal Cycle and the Inversion Data are from 22 December 2004 to 31 January 2005 Mean DTR is 13 K at the surface but only 3 K at 30 m Temperature cycle lags the sun by about 1.5 hours at the surface and about 4.5 h at 30 m Weak lapses around midday, but strong inversions at night

Summary An inversion exists over the Antarctic Plateau because of the small amount of solar energy absorbed at the surface and the smaller emissivity of the atmosphere, compared to the surface. An inversion is present between 2 and 22 m at South Pole well over half the time in summer and nearly 90% of time in winter. Winter inversions are variable, with a median strength of 1.7 K and sometimes greatly exceeding 10 K. The inversion continues below 2 m, with a median temperature difference between the surface and 2 m of over 1 K in winter; the strongest gradient is between the surface and 20 cm.

Summary The minimum temperatures and strongest inversions occur with winds of 3 to 5 m s -1, not with calm winds as often expected; this could be the result of a thermal wind created by the inversion over sloped terrain. As expected, the inversion strength generally decreases with higher amounts of infrared flux reaching the surface, but this relationship is largely limited to a range of fluxes of about 80 to 135 W m -2. Away from the Pole the diurnal cycle changes the character of summertime inversions; mean 2-to-30-m inversions on a summer night at Dome C are stronger than almost any observed between 2 and 22 m at South Pole in summer.