1JUNE 2005 HUDSON AND BRANDT 1673 A Look at the Surface-Based Temperature Inversion on the Antarctic Plateau STEPHEN R. HUDSON AND RICHARD E. BRANDT Department of Atmospheric Sciences, University of Washington, Seattle, Washington (Manuscript received 25 June 2004, in final form 2 November 2004) ABSTRACT Data from radiosondes, towers, and a thermistor string are used to characterize the temperature inversion at two stations: the Amundsen-Scott Station at the South Pole, and the somewhat higher and colder Dome C Station at a lower latitude. Ten years of temperature data from a 22-m tower at the South Pole are analyzed. The data include 2- and 22-m temperatures for the entire period and 13-m temperatures for the last 2 yr. Statistics of the individual temperatures and the differences among the three levels are presented for summer (December and Janu- ary) and winter (April–September). The relationships of temperature and inversion strength in the lowest 22 m with wind speed and down- ward longwave flux are examined for the winter months. Two preferred regimes, one warming and one cooling, are found in the temperature versus longwave flux data, but the physical causes of these regimes have not been determined. The minimum temperatures and the maximum inversions tend to occur not with calm winds, but with winds of 3–5 m sϪ1, likely due to the inversion wind. This inversion wind also explains why the near-surface winds at South Pole blow almost exclusively from the northeast quadrant. Temperature data from the surface to 2 m above the surface from South Pole in the winter of 2001 are presented, showing that the steepest temperature gradient in the entire atmosphere is in the lowest 20 cm. The median difference between the temperatures at 2 m and the surface is over 1.0 K in winter; under clear skies this difference increases to about 1.3 K. Monthly mean temperature profiles of the lowest 30 km of the atmosphere over South Pole are presented, based on 10 yr of radiosonde data. These profiles show large variations in lower-stratospheric temperatures, and in the strength and depth of the surface-based inversion. The near-destruction of a strong inversion at South Pole during7hon8September 1992 is examined using a thermal-conductivity model of the snowpack, driven by the measured downward longwave flux. The downward flux increased when a cloud moved over the station, and it seems that this increase in radiation alone can explain the magnitude and timing of the warming near the surface. Temperature data from the 2003/04 and 2004/05 summers at Dome C Station are presented to show the effects of the diurnal cycle of solar elevation over the Antarctic Plateau. These data include surface temperature and 2- and 30-m air temperatures, as well as radiosonde air temperatures. They show that strong inversions, averaging 10 K between the surface and 30 m, develop quickly at night when the sun is low in the sky, but are destroyed during the middle of the day. The diurnal temperature range at the surface was 13 K, but only 3 K at 30 m. 1. Introduction radiation. A significant inversion exists at the surface of the Antarctic Plateau in every month but December It has been known for decades that a surface-based and January (e.g., Schwerdtfeger 1970a). temperature inversion exists over the Antarctic Plateau The inversion exists because the snow-surface emis- and that its existence is not only a major feature of this sivity is greater than the atmospheric emissivity. When region’s climate, but also a driver of some aspects of its the energy absorbed at the surface from solar radiation climate, including near-surface winds and atmospheric is small, this inequality in emissivities results in a tem- perature inversion. Given that the sum of sensible, latent, and subsurface Corresponding author address: Stephen R. Hudson, Depart- ment of Atmospheric Sciences, University of Washington, Box heat fluxes provides only about 10%–15% of the en- 351640, Seattle, WA 98195-1640. ergy the surface emits in winter (Schlatter 1972; Carroll E-mail: [email protected] 1982) and that the surface temperature is nearly con- © 2005 American Meteorological Society JCLI3360 1674 JOURNAL OF CLIMATE VOLUME 18 stant during the 6-month winter (Schwerdtfeger 1984), with height in these inversions (Schwerdtfeger 1984) we can see that the upward longwave emission by the these radiosondes may not adequately capture the de- surface must be nearly balanced by the downward long- tails of the inversion near the surface (Mahesh et al. wave radiation reaching the surface from the atmo- 1997). sphere. This implies that, in winter, More recently, Connolley (1996) compared the cli- matology produced by Phillpot and Zillman with that ␧ ␴ 4 Ϸ ␧ ␴ 4 ͑ ͒ 0.85 s T s a T a, 1 produced by the Met Office Unified Model, a global climate model. Connolley’s results are similar to those ␧ Ϸ ␧ where s 1.0 is the snow-surface emissivity, a is the of Phillpot and Zillman, but show a dependence of in- effective emissivity of the atmosphere, ␴ ϭ 5.6696 ϫ version strength on terrain slope, which was not in- Ϫ8 Ϫ2 Ϫ4 10 Wm K is the Stefan–Boltzmann constant, Ts cluded by Phillpot and Zillman. This approach is useful is the surface temperature, and Ta is the effective tem- in that it can help to fill in the many data voids that exist perature of the atmosphere (i.e., at levels where most of over large portions of Antarctica. On the other hand, the emission originates); the factor 0.85 comes from the we are a long way from being able to replace observa- fact that about 15% of the energy emitted by the sur- tions with model results. face is replenished by nonradiative mechanisms. There- Towers have been used to study the inversion as well; Ϸ ␧ 0.25 fore, in winter, (Ts /Ta) ( a/0.85) , so the surface these have the benefit of allowing for more accurate temperature will have to be lower than the air tempera- measurements to be made in the lowest portion of the ture so long as the emissivity of the atmosphere is less atmosphere, where the inversion is most pronounced. than 0.85, a criterion violated only under optically thick Perhaps the best-known tower-based study of the Ant- ϭ clouds. For a typical wintertime inversion, with Ts arctic Plateau boundary layer was conducted at Plateau ϭ ␧ Ϸ 213 K and Ta 233 K, Eq. (1) implies a 0.6. Station, where temperatures and winds were measured The inversion is of interest to researchers in a variety at numerous levels on a 32-m tower (Riordan 1977). of fields. Waddington and Morse (1994) and Van Lipzig Dalrymple et al. (1966) also presented data collected on et al. (2002) showed that the inversion can affect ice- an 8-m tower at South Pole in 1958. core records, and therefore must be considered for This paper presents data collected at South Pole from proper retrieval of paleoclimate from ice cores. The radiosondes and from a 22-m tower over a 10-yr period. ability of inversion layers to resist vertical mixing af- For most of the period the ground-based data include fects atmospheric boundary layer chemistry and snow only 2- and 22-m temperatures, along with 10-m wind chemistry (Harder et al. 2000; Davis et al. 2001). One of and downward infrared flux, but some interesting fea- the rapidly growing fields of research on the Antarctic tures of the inversion can still be examined with this Plateau is ground-based astronomy, which can benefit limited dataset. Data from a string of thermistors be- from the high elevation and cold, dry atmosphere. tween the surface and 2 m that was operated during the However, Marks (2002) and Travouillon et al. (2003) winter of 2001 are used to examine the inversion very showed that the presence of strong temperature gradi- near the surface at South Pole. The larger-scale char- ents in the inversion layer can hinder atmospheric “see- acteristics of the inversion are captured in the radio- ing,” while its shallow depth allows for good use of sonde data. Additionally, some comparisons are made adaptive optics. Significant temperature gradients in to tower-based and radiosonde data collected in the the lowest2moftheatmosphere associated with the summers of 2003/04 and 2004/05 at Dome C, where a inversion mean that satellite retrievals of surface tem- significant diurnal cycle is observed. perature in Antarctica will be lower than the “surface” The aim of this paper is to present an overview of the air temperature measured at a station, which is usually temperature profile over the Antarctic Plateau at dif- measured around 2 m above the surface. ferent scales, with particular focus on the structure of Many studies of the inversion have tried to examine the surface-based temperature inversion and on some it through radiosonde data. Among others, these have of the factors that influence this inversion. The use of 10 resulted in an analysis of the relationship between near- yr of both radiosonde and tower-based data from South surface winds and the inversion by Dalrymple et al. Pole allows for analyses of the temperature profile that (1966) and Lettau and Schwerdtfeger (1967) and in a come closer to an actual climatology than previous climatology of inversion strength across the continent works. The longer period of record also allows for more by Phillpot and Zillman (1970), which showed that in- robust analyses of the relationships between the inver- versions over South Pole average about 20 K in winter, sion and the winds and downward longwave radiation, and those over the highest parts of the plateau average which are believed to influence the inversion.