Wednesday, 13 June 2018: 2:00 PM
Ballroom D (Renaissance Oklahoma City Convention Center Hotel)
We investigate how sea-breeze (SB) fronts disrupt the atmospheric boundary layer (ABL) turbulence, placing particular emphasis on their role during the afternoon and evening transition. Our analysis combines a comprehensive 10-yr observational database from the 213-m tower at CESAR (Cabauw, The Netherlands), and numerical simulations from the mesoscale WRF model spanning the same 10-year period. Relevant for the study is the high horizontal resolution (2 km) in the WRF numerical experiments. A systematic SB-criteria selection algorithm is applied to both the observations and the model results in order to obtain objective SB databases. By comparing the selected events from the application of the SB algorithm to both observations and WRF simulations, we are able to determine how sensitive is the SB prediction to synoptic, regional and local conditions.
Based on the observational analysis, we discuss the impact of SB on local-turbulence conditions. SB events are then classified into three different ABL regimes according to the surface sensible heat (SH) flux at the moment of the SB arrival: convective, transition and stable regimes. In the convective regime, the arrival of the SB front has a minimal impact on the thermally buoyant ABL conditions, whereas in the transition regime there is a sudden increase in wind speed (> 2 m s-1) and an abrupt decrease of the ABL depth (~250 m h-1). These changes are accompanied by an acceleration of the afternoon transition, quantified through an enhancement of the wind shear and stable thermal stratification. Under the stable regime, we found that the surface-layer wind profile is disrupted.
In applying a similar analysis to the WRF results, we found that the model reproduces the main features of the SB events and the mesoscale-microscale interactions in the convective and transition regimes. However, the surface SH flux is largely overestimated and consequently the modelled vertical mixing is larger compared to observations. The disruption of the surface-layer wind profile in the stable regime is not correctly represented by WRF, probably due to a failure of the Monin-Obukhov Similarity Theory (MOST) in reproducing the wind profile just after the SB onset, as concluded from the observational analysis. Finally, the simultaneous analysis of observations and WRF results enables us to study several processes acting at different scales on the SB-ABL interaction: at larger scales the role of Coriolis in veering the SB front, and at smaller scales the role of ABL convection in weakening and decelerating the SB front.
Based on the observational analysis, we discuss the impact of SB on local-turbulence conditions. SB events are then classified into three different ABL regimes according to the surface sensible heat (SH) flux at the moment of the SB arrival: convective, transition and stable regimes. In the convective regime, the arrival of the SB front has a minimal impact on the thermally buoyant ABL conditions, whereas in the transition regime there is a sudden increase in wind speed (> 2 m s-1) and an abrupt decrease of the ABL depth (~250 m h-1). These changes are accompanied by an acceleration of the afternoon transition, quantified through an enhancement of the wind shear and stable thermal stratification. Under the stable regime, we found that the surface-layer wind profile is disrupted.
In applying a similar analysis to the WRF results, we found that the model reproduces the main features of the SB events and the mesoscale-microscale interactions in the convective and transition regimes. However, the surface SH flux is largely overestimated and consequently the modelled vertical mixing is larger compared to observations. The disruption of the surface-layer wind profile in the stable regime is not correctly represented by WRF, probably due to a failure of the Monin-Obukhov Similarity Theory (MOST) in reproducing the wind profile just after the SB onset, as concluded from the observational analysis. Finally, the simultaneous analysis of observations and WRF results enables us to study several processes acting at different scales on the SB-ABL interaction: at larger scales the role of Coriolis in veering the SB front, and at smaller scales the role of ABL convection in weakening and decelerating the SB front.
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