Use of remotely-sensed land surface temperature as a proxy for air temperatures at high elevations: findings from a 5000 metre elevational transect across Kilimanjaro

High elevations are thought to be warming more rapidly than lower elevations, but there is a lack of air temperature observations in high mountains. This study compares instantaneous values of land surface temperature (1030/2230 and 0130/1330 local solar time) as measured by MODIS MOD11A2/MYD11A2 at 1 km resolution from the TERRA and AQUA platforms respectively with equivalent screen level air temperatures (in the same pixel). We use a transect of 22 in situ weather stations across Kilimanjaro ranging in elevation from 990 to 5803 m, one of the biggest elevational ranges in the world. There are substantial differences between LST and T_air, sometimes up to 20˚C. During the day/night LST tends to be higher/lower than T_air. LST-T_air differences (ΔT) show large variance, particularly during the daytime, and tend to increase with elevation, particularly on the NE slope which faces the morning sun. Differences are larger in the dry seasons (JF and JJAS), and reduce in cloudy seasons. Healthier vegetation (as measured by NDVI) and increased humidity lead to reduced daytime surface heating above air temperature and lower ΔT, but these relationships weaken with elevation. At high elevations transient snow cover cools LST more than T_air. The predictability of ΔT therefore reduces. It will therefore be challenging to use satellite data at high elevations as a proxy for in situ air temperatures in climate change assessments, especially for daytime T_max. ΔT is smaller and more consistent at night, so it will be easier to use LST to monitor changes in T_min.