A Modelling Study of the Interaction between Cold Air Pool and Urban Structure. the Madrid Case.

A lot of attention has been paid, historically, to study the behaviour of the urban boundary layer (UBL) during summer periods. This has been motivated by the need to understand the causes of the urban overheating and develop tools that can be used to evaluate mitigation strategies. Therefore, “urbanized” mesoscale atmospheric models have been extensively tested to reproduce at best the behaviour of UBL during summer. On the other hand, comparatively less attention has been dedicated to investigate the UBL behaviour during winter, in particular during the presence of the so called persistent Cold Air Pools (CAP), an atmospheric phenomena charactertized by the presence of a stagnant layer of air close to the ground that is colder than the air above. A CAP over a city suppresses the dispersion of the pollutants, and can lead to very poor air quality levels. A correct simulation of these situations is crucial if we want to use numerical models to evalute, for example, air pollution abatement staregies, both at long and short terms.

The simulation of a CAP is recognized as one of the most challenging tasks for a mesoscale model, because it requires a fine interplaying between the surface-atmosphere scheme, the PBL scheme and the numerics.

The main scientifc questions that motivate this contribution are:

• How good are the current mesoscale models with urban canopy paramterizations in reproducing CAP over urban areas? Which are their strengths and weaknesses?
• What can be learnt, from model results, about the interactions between urban morphology, building thermal behaviour and CAP?

To answer these questions, the mesoscale model WRF, coupled with the multilayer urban canopy parameterization BEP-BEM, has been used to simulate a 10 day CAP episode that occurred over the region of Madrid (Spain), at the end of December 2016, at 1km resolution. The simulation took advantage of a detailed urban morphology derived from LIDAR data, that provided gridded information on plan area building density, density of vertical building surfaces, and building heigth distribution. To investigate the impact on pollutant dispersion a passive tracer, representing CO, has been also simulated. The model results were compared against 20 surface stations of the Spanish meterological service, the vertical sounding of the Barajas airport, the land surface temperatures derived from MODIS satellite, and the CO measurements recorded by the city Council of Madrid. The comparison shows that model’s skills in simulating this type of epidodes are considerably poorer compared to the summer situations. While the model is able to reproduce the presence of the CAP, and the strong atmospheric inversion, the temperature field has a very high spatial variability, in particular at night, and the model fails in reproducing, in some cases, the correct intensity of the inversion at the right place. The study indicates that to improve the modelling of CAP over urban areas we need 1) a very dense metereological network within the city to assess if the strong spatial vartiability modelled is correct, 2) very detailed information on surface parameters (not only urban morphology, and building thermal properties, but also vegetation type, soil moisture and initial and deep temperature for the pervious areas of the city), 3) a detailed evaluation of the PBL scheme in such conditions.