## 3 Results and Discussion  
In this section, we evaluate the tomography solution for the year 2017, with a focused analysis on the month of August during Hurricane Harvey's passage over Houston. The analysis will take three independent observational datasets as the reference: 30 min GNSS retrieved PWV at each station in the Houston network and, when available, high-resolution GNSS-RO water vapor density profiles and InSAR PWV maps. Those three datasets will allow an assessment of temporal, vertical and horizontal accuracy of the tomography against state-of-the-art NWP results. Figure 2 compares the PWV values estimated by GNSS observations (taken as the reference) versus the PWV simulated by WRF, ERA5, and tomography (TOMO) at GNSS station locations. The WRF data show the worst RMSE value, reaching 5.18 kg/m\({}^{2}\), the ERA5 2.70 kg/m\({}^{2}\), and the TOMO 0.96 kg/m\({}^{2}\). The bias is positive for all three datasets, 2.19, 0.10, and 0.38 kg/m\({}^{2}\) for WRF, ERA5, and TOMO. The correlation coefficient is very high in all cases, in the same order: 0.947, 0.982, and 0.998. The PWV retrieved from tomography is very close to that observed by GNSS. These findings are consistent with those presented by [19]. The spread observed in WRF results (up to 40 kg/m\({}^{2}\) away from GNSS observations), which were driven by ERA5 boundary conditions, results from small-scale variability (more evident in later figures with InSAR data) that may be out of phase with the real world and is partially filtered out by GNSS processing. A complementary analysis is presented in Figure 3a, which shows the average PWV over the tomographic domain. If we take TOMO results as the reference, as direct GNSS observations are only available at specific grid points, we find that ERA5 is much closer to tomography than WRF. Figure 3b displays a zoom over August when the maximum PWV value reached 76.4 kg/m\({}^{2}\) (observed by GNSS) during Hurricane Harvey's passage (25 August to 1 September). The green stars represent the GNSS-RO PWV. Figure 2: TOMO-, WRF-, and ERA5-PWV derived values at the GNSS station’s location versus the PWV estimated from GNSS observations. The black line corresponds to the perfect fit. Legend indicates the correlation and the RMSE in kg/m\({}^{2}\) (within []). Figure 4 uses 30 profiles from GNSS-RO (stars in Figure 3) to assess the vertical distribution of water vapor density. Mean profiles for the all cases (Figure 4a) are close, with ERA5 being wetter than GNSS-RO, WRF, and tomography in the lower troposphere. Again, the profiles of tomography compare better against the reference profile (GNSS-RO) with an RMSE well below 1 g/m\({}^{3}\) at all levels (Figure 4b). All retrieved profiles have a negative bias in the low troposphere (smaller in the case of tomography) and a less clear signal aloft. Tomography shows the best temporal correlation with GNSS-RO, exceeding 0.9 at all levels. Due to the sparse distribution of these observations, these results are not robust and show some noise. While GNSS-RO has been found to be a critical source of data for reanalysis [31], with unique high vertical resolution information, its reliability as an independent observation is not as strong as PWV from GNSS or InSAR. The good match with tomography is reassuring but does not substitute the need for in situ radiosonde observations, which are not available in Houston. The final assessment uses 85 available InSAR observations. Figure 5 shows the InSAR PWV map obtained on 29 August at 00:26 UTC (at the end of Hurricane Harvey's passage by Houston). With a temporal difference of approximately 5 min, we compared the corresponding PWV maps for WRF, ERA5 data, and the tomographic solution, the latter on  
Figure 4: Vertical profiles of TOMO, ERA5, and WRF assessed against 30 available GNSS-RO observations: (**a**) average water vapor density; (**b**) RMSE; (**c**) BIAS; and (**d**) temporal correlation coefficient. Figure 3: Temporal series of the average PWV over the topography domain.