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. Water-vapor-weighted mean temperature, Tm , is the key variablefor estimating the mapping factor between GPS zenith wet delay (ZWD) andprecipitable water vapor (PWV). For the near-real-time GPS–PWV retrieval,estimating  Tm from surface air temperature  Ts is a widely usedmethod because of its high temporal resolution and fair degree ofaccuracy. Based on the estimations of  Tm and  Ts at each reanalysisgrid node of the ERA-Interim data, we analyzed the relationshipbetween  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 wereestablished and evaluated by comparisons with the radiosonde data at723 radiosonde stations in the Integrated Global Radiosonde Archive (IGRA).Results show that our global gridded Ts – Tm equations have prominentadvantages over the other globally applied models. At over 17 % of thestations, errors larger than 5 K exist in the Bevis equation (Bevis etal., 1992) and in the latitude-related linear model (Y. B. Yao etal., 2014), while these large errors are removed in our time-varying Ts – Tm models. Multiple statistical tests at the 5 % significancelevel show that the time-varying global gridded model is superior to theother models at 60.03 % of the radiosonde sites. The second-best model isthe 1 ∘ ×  1 ∘ GPT2w model, which issuperior at only 12.86 % of the sites. More accurate Tm can reducethe contribution of the uncertainty associated with Tm to the totaluncertainty in GPS–PWV, and the reduction augments with the growth ofGPS–PWV. Our theoretical analyses with high PWV and small uncertainty insurface pressure indicate that the uncertainty associated with Tm cancontribute more than 50 % of the total GPS–PWV uncertainty when using theBevis equation, and it can decline to less than 25 % when using ourtime-varying Ts – Tm model. However, the uncertainty associated withsurface pressure dominates the error budget of PWV (more than 75 %) whenthe surface pressure has an error larger than 5 hPa. GPS–PWV retrievals usingdifferent Tm estimates were compared at 74 International GNSSService (IGS) stations. At 74.32 % of the IGS sites, the relative differences ofGPS–PWV are within 1 % by applying the static or the time-varying globalgridded Ts – Tm equations, while the Bevis model, thelatitude-related model and the GPT2w model perform the same at37.84 %, 41.89 % and 29.73 % of the sites. Compared with theradiosonde PWV, the error reduction in the GPS–PWV retrieval can be around 1–2 mm when using a moreaccurate Tm parameterization, whichaccounts for around 30 % of the total GPS–PWV error.

atmospheric measurement techniques

Development of time-varying global gridded &amp;lt;i&amp;gt;T&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;s&amp;lt;/sub&amp;gt;–&amp;lt;i&amp;gt;T&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;m&amp;lt;/sub&amp;gt; model for precise GPS–PWV retrieval