Global estimates of water-vapor-weighted mean temperature of the atmosphere for GPS applications

Tm has an annual range of similar to 2-4 K in the tropics and 20-35 K over much of Eurasia and northern North America. The day-to-day Tm variations are 1-3 K over most low latitudes and 4-7 K (2-4 K) in winter (summer) Northern Hemispheric land areas. Diurnal variations of Tm are generally small, with mean-to-peak amplitudes less than 0.5 K over most oceans and 0.5-1.5 K over most land areas and a local time of maximum around 16-20 LST.

The commonly used Tm-Ts relationship from Bevis et al. (1992) is evaluated using the ERA-40 data. Tm derived from this relationship (referred to as Tmb) has a cold bias in the tropics and subtropics (-1 similar to -6 K, largest in marine stratiform cloud regions) and a warm bias in the middle and high latitudes (2-5 K, largest over mountain regions). The random error in Tmb is much smaller than the bias.

A serious problem in Tmb is its erroneous large diurnal cycle owing to diurnally invariant Tm-Ts relationship and large Ts diurnal variations, which could result in a spurious diurnal cycle in GPS-derived PW and cause 1-2% day-night biases in GPS-based PW.