# Evaluation of the Urban Weather Generator on the City of Toulouse (France)
## 4 Discussion
### Sensitivity Analysis  
From the sensitivity analysis presented in Section 3.2, three main observations can be made:  
1. First, as shown in Figure 8 concerning elementary effects, forcing atmospheric parameters are the main drivers of the simulation. Parameters _ubnight_ associated with the nighttime urban boundary layer and _Inref_ associated with reference height (see Table 4) are indeed the most represented (_ublinight_ represents almost 50% of the contribution of all parameters to the sensitivity of UWG at night, versus almost 35% for _Inref_ during the day) and have an important weight in the simulation. In practice, unlike the other forcing parameters of the rural station, these values are fixed at initialization (the temporal evolution is not provided by the user). Given their influence in the simulation, this is a main limitation of UWG. This has already been observed in [60] who performed an urban boundary layer height empirical estimation (based on Paris data). Such values are difficult to acquire in practice. Meso-NH [61] is a way to simulate these data. However, this is what we would like to bypass, to avoid time-consuming calculations.
2. As a corollary to the previous observation, the surface parameters have a much smaller influence. Among them, facade-to-site ratio and build density have a slightly greater impact than the others, the latter being negligible. However as shown in Figure 10e-h, only the building density has a discernible influence (Figure 10e,**f**) since very low variations of the simulated temperature are observable over the facade-to-site ratio parameter. This is in contradiction with [45] where, in their study over Toulouse and Basel, building density, facade-to-site ratio, and vegetation cover are the main drivers of the simulated air temperature. Following our observations, this point is also a limitation of UWG since it is recognized that these parameters have an influence on the heat islands.
3. From Figure 9 related to the standard deviation of the elementary effects vs. overall sensitivity measure, it can be shown that the atmospheric reference height parameter (_Inref_) has a non-linear interaction since it is the only parameter that has a significant standard deviation from its average. As stated above, this is either because this parameter interacts with another, or because it has a non-linear influence. The first option is impossible since no other parameter has a similar behavior. This further emphasizes the importance of this parameter, which appears critical for performing consistent simulations. Some studies have tried to estimate _lnref_ empirically from sets of measurements [60] and this point is obviously a challenge for future improvements of UWG.  
A direct consequence of these observations is that the spatial variation of UWG simulations is insignificant since surface parameters have a low influence. Therefore, for the analysis of the urban heat island, UWG does not seem the most adapted. Following our quantitative preliminary study in [58], UWG performs better in densely constructed city centers.