Thermally-driven Mesoscale Flows and their Interaction with Atmospheric Boundary Layer Turbulence by Jon Ander Arrillaga Mitxelena

Author:Jon Ander Arrillaga Mitxelena
Language: eng
Format: epub
ISBN: 9783030485795
Publisher: Springer International Publishing

3.5 Summary and Conclusions

This work analyses SB characteristics in the Basque coast, a region with complex topography washed by the Cantabrian Sea in the North of Spain. Observations from two contrasting locations are employed: Sondika, located in a valley facing the sea; and Azpeitia, in a closed valley. Besides, simulations from the WRF mesoscale model complement the observations.

Two offshore-wind days in Sondika are selected by the SB selection method incorrectly. For that reason, the selection method should be modified for future studies: on the one hand, including an extra filter to assure that the final wind direction is onshore; and on the other, adapting it to reject situations dominated by flows induced by the complex topography or by a strong synoptic forcing.

A SB day selected in both locations is analysed from observations and a simulation with the WRF model. It reproduces the onset of the SB in Sondika, although it does not represent correctly the interaction between the existing two SB systems in the region. In Azpeitia, the model reproduces a SB with a stronger wind speed and a different direction. The main reason for the worse simulation in this location is linked to the misrepresentation of the topography. In fact, the observed closed-valley circulation, a mesoscale phenomena with a smaller length scale, and its interaction with the SB flow are not reproduced. Besides, the comparison of the simulated and observed wind profiles shows that surface heating and its influence on the lower atmosphere are not properly reproduced by the model.

The WRF model is evaluated using four sensitivity experiments for one of the offshore-wind days that passed the selection method incorrectly, which proves to be a LSB day. In general, the temperature evolution is smoothed and wind speed overestimated. The use of the grid analysis nudging reduces the overestimation of the wind speed, but in return gives rise to an unreal onset of the SB. This reveals that the threshold value of the ambient offshore wind that blocks the inland propagation of the SB is higher in the model than in reality. Regarding the ABL scheme and the corresponding surface-layer schemes, YSU MM5 provides the best results and MYJ + Eta, which uses a TKE closure, fails to reproduce correctly the onset time of the observed LSB during the ET of the ABL.

By comparing SB and LSB situations, we find that the generated updraughts and production of turbulence due to the passage of the SBF are greater when the onset of the SB occurs during diurnal convective conditions. In a LSB event, i.e. when the onset takes place during the ET of the ABL, the stable stratification of the SBL inhibits vertical updraughts and the increase of turbulence. These relationships between the mesoscale flow and turbulence are studied in more detail by exploring a greater number of events over flat terrain in Chap. 4.

To sum up, in this analysis we have learnt about the characteristics of the SB over complex terrain, and their interaction with the MBs induced by that complex orography.

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