A global comparison of surface and free-air temperatures at high elevations

[ 1] Surface and free-air temperature observations from the period 1948-2002 are compared for 1084 surface locations at high elevations (>500 m) on all continents. Mean monthly surface temperatures are obtained from two homogeneity adjusted data sets: Global Historical Climate Network ( GHCN) and Climatic Research Unit ( CRU). Free-air temperatures are interpolated both vertically and horizontally from the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis R1 2.5degrees grids at given pressure levels. The compatibility of surface and free-air observations is assessed by examination of the interannual variability of both surface and free-air temperature anomalies and the surface/free-air temperature difference (DeltaT). Correlations between monthly surface and free-air anomalies are high. The correlation is influenced by topography, valley bottom sites showing lower values, because of the influence of temporally sporadic boundary layer effects. The annual cycle of the derived surface/free-air temperature difference (DeltaT) demonstrates physically realistic variability. Cluster analysis shows coherent DeltaT regimes, which are spatially organized. Temporal trends in surface and free-air temperatures and DeltaT are examined at each location for 1948-1998. Surface temperatures show stronger, more statistically robust and widespread warming than free-air temperatures. Thus DeltaT is increasing significantly at the majority of sites (>70%). A sensitivity analysis of trend magnitudes shows some reliance on the time period used. DeltaT trend variability is dominated by surface trend variability because free-air trends are weak, but it is possible that reanalysis trends are unrealistically small. Results are sensitive to topography, with mountaintop sites showing weaker DeltaT increases than other sites ( although still positive). There is no strong relationship between any trend magnitudes and elevation. Since DeltaT change is dependent on location, it is clear that temperatures at mountain sites are changing in ways contrasting to free air.