Numerical Simulation of Late Wintertime Local Flows in Kathmandu Valley, Nepal: Implication for Air Pollution Transport

Abstract

Air pollution transport in the Kathmandu valley/basin has been investigated by numerical simulation of local flows and the observation of NO 2 and SO 2 . The observation was performed at 22 sites with passive samplers from February to April 2001, and the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) was utilized for the flow simulation. The calculation reproduced reasonably well the surface wind and temperature at the Tribhuvan International Airport (TIA) as well as the vertical wind profile taken at the center of the valley by sodar observation. The calculation showed that two characteristic local flows tend to intrude into the valley/basin in the afternoon through the mountain gaps surrounding Kathmandu, that is, the southwesterly from the Indian Plain and the northwesterly from the valley west to Kathmandu. These cool wind layers meet at the center of the Kathmandu basin and form a double-layering structure there. The lower layer is shallow with a depth of about 250 m, being composed of the cooler southwesterly air mass from the Indian Plain. It was concluded that this local flow structure suppresses vertical mixing and leads to high air pollution by decreasing the daytime ventilation of air mass over the valley. The observations performed during the period confirmed it.

Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

“…Because Dhulikhel is downwind of Kathmandu Valley, the influence of pollutants from the city is expected, and measurements are likely to represent a combination of well mixed aerosols from Kathmandu and the broader regional background. Regmi et al (2003) …”

An examination of boundary layer structure under the influence of the gap winds in Urumqi, China, during air pollution episode in winter

“…Compared comprehensively, the ESEG flow properties in Urumqi and its influence on local air pollution are different from the gap winds in Mexico City (Doran and Zhong, 2000;Fast et al, 2007;Foy et al, 2006) and Kathmandu Valley (Regmi et al, 2003). First, the formation mechanism of the gap winds in Mexico City and Kathmandu valley are thermally driven, so the gap winds there exhibit significant alternation features during sunrise and sunset periods (Doran and Zhong, 2000;Regmi et al, 2003). However, the gap winds of the ESEG in Urumqi retain their southeasterly direction all day because their formation is mainly caused by the pressure gradient between the two sides of the Tianshan Mountain range.…”

“…Depending upon the topography of the mountain valley and the surrounding environment, different mechanisms govern the local air flows in the different basins (Regmi et al, 2003). Compared comprehensively, the ESEG flow properties in Urumqi and its influence on local air pollution are different from the gap winds in Mexico City (Doran and Zhong, 2000;Fast et al, 2007;Foy et al, 2006) and Kathmandu Valley (Regmi et al, 2003).…”

“…Gap winds also occur in Kathmandu, Nepal. Two different gap flows meeting at the center of the city form a double-layered structure that suppresses vertical mixing and results in high air pollution (Regmi et al 2003). Urumqi was evaluated as one of the 10 most heavily air-polluted cities in the world in 1998 (Mamtimin and Meixner, 2007).…”

“…The famous foehn in Alps results in Central Europe enjoying a warmer climate (Mcknight et al, 2000). Here, the so-called variation of the foehn could intrudes over the Urumqi above $400 m in winter, which differs from the other gap winds in Mexico City and Kathmadu (Doran and Zhong, 2000;Foy et al, 2006;Regmi et al, 2003).…”