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

Water‐vapor‐weighted atmospheric mean temperature, Tm, is a key parameter in the retrieval of atmospheric precipitable water (PW) from ground‐based Global Positioning System (GPS) measurements of zenith path delay (ZPD), as the accuracy of the GPS‐derived PW is proportional to the accuracy of Tm. We compare and analyze global estimates of Tm from three different data sets from 1997 to 2002: the European Centre for Medium‐Range Weather Forecasts (ECMWF) 40‐year reanalysis (ERA‐40), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, and the newly released Integrated Global Radiosonde Archive (IGRA) data set. Temperature and humidity profiles from both the ERA‐40 and NCEP/NCAR reanalyses produce reasonable Tm estimates compared with those from the IGRA soundings. The ERA‐40, however, is a better option for global Tm estimation because of its better performance and its higher spatial resolution. Tm is found to increase from below 255 K in polar regions to 295–300 K in the tropics, with small longitudinal variations. Tm has an annual range of ∼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 ∼ −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.