Sea Breeze Circulation in the Auckland Region:Observational Data Analysis and NumericalModelling (2010)
AuthorsKhan, Basit Alishow all
The main aim of this research is to improve our knowledge of the sea breeze circulation in the complex coastal environments, where more than one mesoscale circulations occur. Interaction of these circulations with each other and with external factors such as topographical features and large scale winds leads to pronounced changes in the thermodynamic structure of the boundary layer. The variations in sea breeze circulation also have distinct effect on the pollutant transport and dispersion mechanisms in the coastal urban areas. In this research, dynamic and thermodynamic characteristics of the sea breeze circulation and their associated air pollution potential have been investigated by utilizing observational data for two summer periods and numerical modelling techniques. Effect of some external factors such as gradient flow and terrain elevation has also been examined. Observed meteorological and air quality data was obtained from a number of monitoring sites within and around Auckland while Advanced Weather Research & Forecasting (WRF) and ‘The Air Pollution Model’ (TAPM) were employed to simulate meteorology and pollutant dispersion in Auckland. WRF is used to investigate the thermally induced mesoscale circulation while TAPM has been employed to examine the pollutant dispersion in the region. Both models were validated against observed data from six different sites within Auckland. Validation results of WRF and TAPM are also compared with surface meteorology. Validation and inter-comparison of the two models show that WRF performed better than TAPM for all the surface meteorology variables. WRF showed a positive bias in predicted winds speed and relative humidity and a cold bias in the near surface Temperature. TAPM on the other hand under-predicted surface winds, while near surface temperature and relative humidity are similar to WRF.
Results show that the sea breeze occurred around 20% of the two summer periods of 2006 and 2007. Both observed data analysis and the numerical modelling results confirmed the existence of two thermally induced systems in the Auckland region. Bay breezes are initiated in the morning hours (0800 – 1000 hours) from small bodies of water (Manukau, Waitemata, and Kaipara Harbour, and along the Hauraki Gulf coastline), followed by mature sea breezes from the main bodies of water (Tasman Sea and larger Hauraki Gulf area) in the late morning. The cessation of sea breezes started after 1600 hours. Frequency of sea breeze days was the highest under coast-parallel gradient winds (southeast and northwest), with speeds < 6 m s-1. The predicted depth of the sea breeze inflow ranged between 200 and 600 m, while the depth of the return flow was in the range of 200 – 500 m. Sensible heat flux is an important control in the development of sea breeze over the region. Coastal mountain ranges helped early onset of the sea breeze, but also inhibited inland propagation. Strong jet-like westerly winds along the coastline near the Manukau Harbour are due partly to the narrow opening at the Manukau Head, reduced friction over the harbour water, and divergence of wind due to coastline shape. Gradient winds significantly affect the evolution of the sea breeze and modify many of its dynamics, such as the sea breeze inflow layer, return flow, inland penetration, sea breeze head, etc. Under northerly gradient flow northeast sea breeze lasts longer while under southerly gradient flow cessation of the westerly sea breeze was delayed.
Over both east and west coasts, WRF predicted anticlockwise rotation, especially under easterly gradient wind conditions. However, inland stations near Manukau Harbour show partial and complete clockwise rotation, which is primarily due to orographic features of the region. The diurnal rotation of the sea breeze system may contribute to recirculation of pollutants in the morning hours under coast-parallel gradient wind conditions. Pollutants that are emitted during morning peak traffic hours and advected towards Manukau Harbour by the remnants of the land breeze may be returned by bay breezes in the mid morning hours.
Mixed layer height over land before arrival of the sea breeze also varied a lot and ranged between 600 to 1400 m. A convective internal boundary layer (CIBL) forms in the surface layer after arrival of the sea breeze. The CIBL under coast parallel gradient winds was relatively shallow (200 – 400 m), while under coasts-normal gradient winds (southwest and northeast), the predicted depth was in the range of 400 to 500 m. However, the inland extent of the CIBL was greater under coast-normal winds, especially under south-westerly gradient winds. The ground level concentration of air pollutants thus can be increased during sea breeze inflow over the region.
Both bay breeze and mature sea breeze contribute towards development, extent and strength of the sea breeze convergence zones (SBCZs). Gradient winds and terrain play an important role in the position and strength of SBCZs. Under strong south-westerly gradient flow, a SBCZ is formed along the eastern coastline, while under north-easterly gradient winds a SBCZ is formed along the west coastline. During coast-parallel gradient winds the SBCZ is formed in the middle of landmass, and is then gradually displaced eastward or westward depending on the balance between large scale PGF and surface friction effect. In addition to SBCZs, terrain and coastline-induced convergences were also evident. Higher ground level concentrations of pollutants are expected under coast-normal gradient winds, when SBCZs are formed in the middle of the land mass and the wind speed of the sea breeze inflow and the sea breeze front is relatively low. This may increase pollution concentration, especially in the evening hours, to unacceptable levels. Results of this research suggest that given the size, synoptic meteorology and specific geography of the region, significant recirculation of pollutants is not likely to happen to contribute to next day’s pollution. The pollutant concentration may increase in the SBCZs, but their ability to recirculate the pollutants requires more extensive research.
A closed sea breeze circulation cell is unlikely to form in this region due to topographical influences and a strong gradient wind effect. The pollutant plume is expected to be advected in the return flow over the peaks of higher terrain and via the top of the convergence zones, but its remixing in the onshore flow is subject to many factors such as gradient wind speed and direction, direction of the return flow and nature (size and state) of the pollutant. In appropriate conditions, pollution levels may reach to unhealthy levels under coast-parallel gradient wind condition.