An examination of the differences between surface and free-air temperature trend at high-elevation sites: relationships with cloud cover, snow cover and wind

Contrasts in high-elevation surface and free-tropospheric temperatures between 1971 and 1996 are examined by comparing surface temperatures from a subset of 72 stations in the GHCN (Global Historical Climate Network) and CRU (Climatic Research Unit) homogeneity adjusted surface data sets with free-air temperatures interpolated to the same locations from NCEP/NCAR Reanalysis R1. The selected stations are above the mean elevation of the surrounding topography, often located on mountain summits. Surface temperatures, free-air temperatures, and their difference (ΔT) are found to be related to independent surface cloud observations from the NDP-026C archive, local wind speed, satellite records of snow cover (NSIDC), and reanalysis wind components. Significant correlations are observed at most stations, and correlation spatial patterns are consistent for different subperiods of the record (e.g., presatellite era versus satellite era). Stepwise regression models built to predict surface temperatures, free-air temperatures, and ΔT from the above meteorological parameters typically explain 20–40% of the temperature variability on an annual basis and more for individual seasons. The stationarity of relationships between temperature and snow/cloud/wind is examined by comparing the temporal trends in the original temperatures with predicted trends from the best fit regression model and trends in model residuals. This provides an assessment of how much of any ΔT trends can be accounted for by changes in meteorology. Significant daytime ΔT residual trends occur primarily in Turkey and eastern China, but significant nighttime ΔT residual trends are more geographically widespread. While daytime residual trends may be the result of surface radiative cooling by increasing anthropogenic aerosol, attribution of nighttime residual trends is uncertain.