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FIG. 5. Passive tracer mixing ratio (mg kg™') in the (left) 40% urbanized and (right) 80% urbanized valleys (see text for details). Isentropes are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.

Figure 5 Passive tracer mixing ratio (mg kg™') in the (left) 40% urbanized and (right) 80% urbanized valleys (see text for details). Isentropes are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.

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Following Rampanelli et al. (2004), the idealized to- pography is represented by the function In summary, we assume that urbanization inherently leads to an areal expansion of both the UHI and the emission area, which is consistent with the idea that, for instance, as the urban land expands so does the number of cars in the city. Intraurban variations of pollution (Wilson et al. 2005) and surface energy fluxes (Offerle et al. 2006) are still very complex issues that are strongly dependent on the particularities of each city. Therefore, we regard our assumption of intraurban homogeneity as the simplest possibility, although other assumptions boundaries far from the top of the sidewalls. The average steepness of the sidewalls is 12.5%, and the transitions between them and the plateaus and the valley floor are smooth in order to prevent abrupt changes in wind direction. The vertical grid is set with terrain-following coordinates stretched with a minimum grid size of 25m at the surface and a maxi- mum grid size of 121m at the top of the simulation domain.
Following Rampanelli et al. (2004), the idealized to- pography is represented by the function In summary, we assume that urbanization inherently leads to an areal expansion of both the UHI and the emission area, which is consistent with the idea that, for instance, as the urban land expands so does the number of cars in the city. Intraurban variations of pollution (Wilson et al. 2005) and surface energy fluxes (Offerle et al. 2006) are still very complex issues that are strongly dependent on the particularities of each city. Therefore, we regard our assumption of intraurban homogeneity as the simplest possibility, although other assumptions boundaries far from the top of the sidewalls. The average steepness of the sidewalls is 12.5%, and the transitions between them and the plateaus and the valley floor are smooth in order to prevent abrupt changes in wind direction. The vertical grid is set with terrain-following coordinates stretched with a minimum grid size of 25m at the surface and a maxi- mum grid size of 121m at the top of the simulation domain.
TABLE 1. Summary of the model setup.
TABLE 1. Summary of the model setup.
FIG. 1. Vertical 6 profiles taken every hour from 0600 to 1800 LST in the center of the valley. Each panel corresponds to a different urban area fraction (ranging from 0% to 100%) and shows the evolution in time from the initial inversion profile on the left to the final profile on the right. The arrow indicates the profile corresponding to 1200 LST, and the horizontal dotted line indicates the altitude of the top of the valley.
FIG. 1. Vertical 6 profiles taken every hour from 0600 to 1800 LST in the center of the valley. Each panel corresponds to a different urban area fraction (ranging from 0% to 100%) and shows the evolution in time from the initial inversion profile on the left to the final profile on the right. The arrow indicates the profile corresponding to 1200 LST, and the horizontal dotted line indicates the altitude of the top of the valley.
FIG. 2. Consecutive longitudinal mean fields of the cross-valley w in the (left) rural, (center) 40% urbanized, and (right) 80% urbanized valleys. Isentropes are shown every 1K. The thick (thin) outline of the orography marks the urban (rural) area.
FIG. 2. Consecutive longitudinal mean fields of the cross-valley w in the (left) rural, (center) 40% urbanized, and (right) 80% urbanized valleys. Isentropes are shown every 1K. The thick (thin) outline of the orography marks the urban (rural) area.
of about 2.0ms |, ina layer approximately 700 m above the mountaintop level (i.e., z ~1700m). These winds advect warm air and thermal energy from the plumes toward the valley center, in turn producing air sub- sidence. This compensating flow is simi by Serafin and Zardi (2010a), and it is in the fact that the ascending motions re flows and thermal plumes must be com scending flows elsewhere, because of mass continuity. However, in contrast to that study, we a subsidence does not reach the valley core since the de- scending winds deviate toward the mountaintops as a consequence of the inversion layer, which restricts the ar to that found agreement with ated to upslope pensated by de- so found that air downward motions. Hence, we found horizontal winds diverging from the valley center just above the inversion layer, so that a circulation cell is evid a layer (Figs. 3d-f). The formation of thi ent above such s circulation cell results from the effect of the inversion layer in decou- pling the wind field above and below it, as the temperature updraft speeds of about 1.0ms | (Fig. 2f). The differ- ence between the more urbanized valley and the other two valleys suggests that the extra heat produced from the extended urban land is resulting in more energetic plumes. On the other hand, the similarity between the rural and the 40% urbanized valleys can be related to the downslope winds appearing in the second case. The higher heating of the surface in the 40% urbanized valley, in comparison to the rural one, does not result in more energetic thermal plumes because the extra energy is expended in the circulation involving the downslope winds, as a result of the interplay between the urban heat island and the temperature inversion. Moreover, such extra energy results in more and larger plumes within the CBL of the 40% urbanized valley than in the rural one.
of about 2.0ms |, ina layer approximately 700 m above the mountaintop level (i.e., z ~1700m). These winds advect warm air and thermal energy from the plumes toward the valley center, in turn producing air sub- sidence. This compensating flow is simi by Serafin and Zardi (2010a), and it is in the fact that the ascending motions re flows and thermal plumes must be com scending flows elsewhere, because of mass continuity. However, in contrast to that study, we a subsidence does not reach the valley core since the de- scending winds deviate toward the mountaintops as a consequence of the inversion layer, which restricts the ar to that found agreement with ated to upslope pensated by de- so found that air downward motions. Hence, we found horizontal winds diverging from the valley center just above the inversion layer, so that a circulation cell is evid a layer (Figs. 3d-f). The formation of thi ent above such s circulation cell results from the effect of the inversion layer in decou- pling the wind field above and below it, as the temperature updraft speeds of about 1.0ms | (Fig. 2f). The differ- ence between the more urbanized valley and the other two valleys suggests that the extra heat produced from the extended urban land is resulting in more energetic plumes. On the other hand, the similarity between the rural and the 40% urbanized valleys can be related to the downslope winds appearing in the second case. The higher heating of the surface in the 40% urbanized valley, in comparison to the rural one, does not result in more energetic thermal plumes because the extra energy is expended in the circulation involving the downslope winds, as a result of the interplay between the urban heat island and the temperature inversion. Moreover, such extra energy results in more and larger plumes within the CBL of the 40% urbanized valley than in the rural one.
Fia. 4. As in Fig. 2, but for resolved TKE.
Fia. 4. As in Fig. 2, but for resolved TKE.
FIG. 6. Heights of the bottom (dotted line) and top (solid line) of the inversion layer in the center of the valley for different urban area fractions (ranging from 0% to 100%).
FIG. 6. Heights of the bottom (dotted line) and top (solid line) of the inversion layer in the center of the valley for different urban area fractions (ranging from 0% to 100%).
FIG. 7. Idealized sketch of the evolution in time of the BTI process and the mean cross-valley circulation depicting the inversion layer (dashed lines), the main wind flows (black arrows), and the resolved TKE field (shading), for the (left) rural, (center) 40% urbanized, and (right) 80% urbanized valleys. Isentropes (solid lines) are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.
FIG. 7. Idealized sketch of the evolution in time of the BTI process and the mean cross-valley circulation depicting the inversion layer (dashed lines), the main wind flows (black arrows), and the resolved TKE field (shading), for the (left) rural, (center) 40% urbanized, and (right) 80% urbanized valleys. Isentropes (solid lines) are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.
FIG. 8. Consecutive longitudinal mean fields of the (left) cross-valley w and (center) u wind speed components, and (right) the passive tracer mixing ratio (mg kg ') in the 20% urbanized valley. Isentropes are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.
FIG. 8. Consecutive longitudinal mean fields of the (left) cross-valley w and (center) u wind speed components, and (right) the passive tracer mixing ratio (mg kg ') in the 20% urbanized valley. Isentropes are shown every 1 K. The thick (thin) outline of the orography marks the urban (rural) area.
Related topics:
Environmental ScienceAir QualityAtmospheric sciencesHeat IslandsUrban ClimateUrban heat islandAir Quality Index
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