Atypical Occlusion Process Caused by the Merger of a Sea-breeze Front and Gust Front

An atypical occlusion process that occurred in North China on 14 July 2011 is studied based on both observations and a real-data Weather Research and Forecasting （WRF） model simulation. The results show that this atypical occlusion process was significantly different from the traditional, synoptic-scale occlusion process that occurs within extratropical cyclones. It was caused by the merger of two cold-type mesoscale fronts. One of the fronts developed from the gust front of convective storms, while the other was a sea-breeze front. As the two fronts moved towards each other, the warm air between them was squeezed and separated from the surface. An atypical occluded front was formed when the two fronts merged, with the warm air forced aloft. This kind of occlusion is termed a ＂merger＂ process, different from the well-known ＂catch-up＂ and ＂wrap-up＂ processes. Moreover, local convection was found to be enhanced during the merger process, with severe convective weather produced in the merger area.

Numerical Study of the Evolution of a Sea-Breeze Front Under Two Environmental Flows

The evolution of a sea-breeze front （SBF） in parallel and offshore environmental flows was investigated by using high-resolution simulations of two SBF cases from the Bohai Bay region, China. The results show that the combination of a distinct vertical wind shear caused by the sea-breeze circulation with a neutral or slightly stable atmospheric stratification associated with the thermal inner boundary layer promoted the occurrence and maintenance of a Kelvin-Helmholtz billow （KHB）. In a parallel environmental flow, the SBF evolved into a few connected segments because of the inhomogeneity of the sea-breeze direction and intensity as it penetrated inland. A significant upward vertical motion occurred at the two ends of the SBF segment owing to the sea-breeze convergence and was accelerated by the KHB. The KHB made a notable contribution to the intensity at the ends of the segment, whereas the intensity at the middle segment was primarily attributed to the convergence between the sea breeze and the parallel flow. In the offshore environmental flow, the clockwise rotation of the offshore flow varying with time increased the downstream convergence of the interface between the sea breeze and the offshore flow and pushed the downstream convergence zone to an orientation consistent with the offshore flow. The air parcels ascending from the downstream part of the SBF were continuously lifted by the downstream convergence zone during their advection, leading to a significant downstream development of the SBF. The significant upward vertical motion caused by the sea-breeze convergence behind the upstream end of the SBF was shifted to the upstream end of the SBF by the KHB, which enhanced the intensity of the upstream end of the SBF.

Seasonal Behavior of Aerosol Vertical Concentration in Dakar and Role Played by the Sea-Breeze

The Westward transport of mineral dust from the North Africa continent to Atlantic Ocean can produce poor air quality, low visibilities, and negatively impacting respiratory and cardiac health due to the optical and physical properties of aerosols. The dynamical impact of the sea-breeze on the dust vertical distribution in West Africa remains unknown. To investigate this issue, we have used in-situ measurements from lidar. We have focused on the attenuated backscatter of aerosols to study the effect of the local circulation on the vertical profile of mineral dust at land-sea transition. The results highlight a strong diurnal cycle of mineral dust associated with the nocturnal low-level jet (NLLJ). The jet is located between 500 m and 1000 m and crucially affected by the dynamic of the sea-breeze circulation.

Evaluation of Weather Research and Forecasting Model Parameterizations under Sea-Breeze Conditions in a North Sea Coastal Environment

Three atmospheric boundary layer （ABL） schemes and two land surface models that are used in the Weather Research and Forecasting （WRF） model, version 3.4.1, were evaluated with numerical simulations by using data from the north coast of France （Dunkerque）. The ABL schemes YSU （Yonsei University）, ACM2 （Asymmetric Convective Model version 2）, and MYJ （Mellor-Yamada-Janjic） were combined with two land surface models, Noah and RUC （Rapid Update Cycle）, in order to determine the performances under sea-breeze conditions. Particular attention is given in the determination of the thermal internal boundary layer （TIBL）, which is very important in air pollution scenarios. The other physics parameterizations used in the model were consistent for all simulations. The predictions of the sea-breeze dynamics output from the WRF model were compared with observations taken from sonic detection and ranging, light detection and ranging systems and a meteorological surface station to verify that the model had reasonable accuracy in predicting the behavior of local circulations. The temporal comparisons of the vertical and horizontal wind speeds and wind directions predicted by the WRF model showed that all runs detected the passage of the sea-breeze front. However, except for the combination of MYJ and Noah, all runs had a time delay compared with the frontal passage measured by the instruments. The proposed study shows that the synoptic wind attenuated the intensity and penetration of the sea breeze. This provided changes in the vertical mixing in a short period of time and on soil temperature that could not be detected by the WRF model simulations with the computational grid used. Additionally, among the tested schemes, the combination of the local- closure MYJ scheme with the land surface Noah scheme was able to produce the most accurate ABL height compared with observations, and it was also able to capture the TIBL.