Development of time-varying global gridded T s – T m model for precise GPS–PWV retrieval
Abstract. Water-vapor-weighted mean temperature, Tm , is the key variable
for estimating the mapping factor between GPS zenith wet delay (ZWD) and
precipitable water vapor (PWV). For the near-real-time GPS–PWV retrieval,
estimating  Tm from surface air temperature  Ts is a widely used
method because of its high temporal resolution and fair degree of
accuracy. Based on the estimations of  Tm and  Ts at each reanalysis
grid node of the ERA-Interim data, we analyzed the relationship
between  Tm and Ts without data smoothing. The analyses demonstrate that the
 Ts – Tm relationship has significant spatial and temporal variations.
Static and time-varying global gridded Ts – Tm models were
established and evaluated by comparisons with the radiosonde data at
723 radiosonde stations in the Integrated Global Radiosonde Archive (IGRA).
Results show that our global gridded Ts – Tm equations have prominent
advantages over the other globally applied models. At over 17 % of the
stations, errors larger than 5 K exist in the Bevis equation (Bevis et
al., 1992) and in the latitude-related linear model (Y. B. Yao et
al., 2014), while these large errors are removed in our time-varying
 Ts – Tm models. Multiple statistical tests at the 5 % significance
level show that the time-varying global gridded model is superior to the
other models at 60.03 % of the radiosonde sites. The second-best model is
the 1 ∘ ×  1 ∘ GPT2w model, which is
superior at only 12.86 % of the sites. More accurate Tm can reduce
the contribution of the uncertainty associated with Tm to the total
uncertainty in GPS–PWV, and the reduction augments with the growth of
GPS–PWV. Our theoretical analyses with high PWV and small uncertainty in
surface pressure indicate that the uncertainty associated with Tm can
contribute more than 50 % of the total GPS–PWV uncertainty when using the
Bevis equation, and it can decline to less than 25 % when using our
time-varying Ts – Tm model. However, the uncertainty associated with
surface pressure dominates the error budget of PWV (more than 75 %) when
the surface pressure has an error larger than 5 hPa. GPS–PWV retrievals using
different Tm estimates were compared at 74 International GNSS
Service (IGS) stations. At 74.32 % of the IGS sites, the relative differences of
GPS–PWV are within 1 % by applying the static or the time-varying global
gridded Ts – Tm equations, while the Bevis model, the
latitude-related model and the GPT2w model perform the same at
37.84 %, 41.89 % and 29.73 % of the sites. Compared with the
radiosonde PWV, the error reduction in the GPS–PWV retrieval can be around 1–2 mm when using a more
accurate Tm parameterization, which
accounts for around 30 % of the total GPS–PWV error.