Abstract : B.3
KLAM_21 drainage wind modelling of wintertime air pollution events in Christchurch, New Zealand

Meinolf Kossmann, Uwe Sievers
Deutscher Wetterdienst

High emissions, mainly from home heating, and poor dispersion conditions during stagnant synoptic situations lead to the regular occurrence of high air pollution levels during wintertime in Christchurch, a city of about 330000 inhabitants, located on the east coast of New Zealandís South Island. The 50 microgrammes per cubicmeter 24h mean PM10 concentration health guideline is exceeded about 30 times each winter. Because the daytime urban boundary layer is usually well ventilated, the main contribution to high 24h mean concentration values originates from nighttime hours when strong surface inversions and flow convergence of local winds favour the accumulation of emitted pollutants.

The cold air drainage model KLAM_21 of the Deutscher Wetterdienst (Sievers, 2005) is used to simulate the local winds for typical winter smog nights in the Christchurch area. The model calculates the depth of a surface based stable layer that evolves from a neutrally stratified dry atmosphere during nighttime. The velocity and direction of the cold air drainage is calculated from vertically integrated momentum equations in which momentum tendency results from imbalances between gravitational forcing and friction. Nocturnal energy loss in the cold air layer is caused by local turbulent and radiative energy loss and advection. The turbulent and radiative energy loss in the cold air layer is prescribed as constant values depending on surface characteristics. The modelling domain is 150 km x 140 km with a grid resolution of 100 m in the nested inner grid and 500 m in the outer domain. A set of model runs is used to study the effects of complex topography and ambient wind conditions on the nocturnal surface wind field. Model results are compared with detailed observations collected during a field campaign.

Sievers, U., 2005: Das Kaltluft-Abfluss-Modell KLAM_21. Theoretische Grundlagen, Anwendungen und Handhabung des PC-Modells. Berichte des Deutschen Wetterdienstes 227.