Numerical Simulation of Late Wintertime Local Flows in Kathmandu Valley, Nepal

Implication for Air Pollution Transport

抄録

Air pollution transport in Kathmandu valley has been investigated in reference to the observed concentration distribution of NO2 and SO2 and numerical simulation of local flow fields. The spatial distributions of NO2 and SO2 were measured in two ways: one is a 3-weeks average during 18 February to 11 March and the 1-day average on 12 April 2001 at identical sites with passive samplers. Using the PSU-NCAR Mesoscale Model MM5, initializing with ECMWF meteorological fields, meteorological conditions over Kathmandu valley has been simulated.
The simulation reasonably reproduced the characteristic late wintertime meteorological conditions of the valley as inferred from the surface observations at the Airport as well as Sodar observation. The simulation results suggest that intrusion of the cooler air masses into the valley in the afternoon resulted in shallow stablelayer, in the lower part of which thermal internal boundary layer developed over Kathmandu valley. Pictures of smoke taken during the observation period confirmed that the shallow stable layer suppressed the vertical dispersion and trapped the air pollutants below 200 meters. The observed spatial distribution of NO2 and SO2 well coincided with the spatial patterns of the shallow southwesterly and northwesterly afternoon winds, intruding and merging into the westerly wind in the Basin. This suggested their major role in valley's air pollution transport thereby causing relatively high concentration in the downwind eastern area of the valley. These southwesterly and northwesterly winds are thought to be the valley winds from the southern plain area and western valley effectively channeled by underlying river gorge and low-mountain passes respectively.