Influence of gravity waves on fog structure revealed by a millimeter-wave scanning Doppler radar

Abstract

Multiscale band structure in fog influenced by gravity waves and Kelvin-Helmholtz instability (KHI) is studied based on observations with a millimeter-wave scanning Doppler radar on August 2000 in Kushiro District, Hokkaido Prefecture, Japan. Band structure in fog having radar reflectivity >-20 dBZ and a scale of ∼1.5 km was observed around 50 m altitude. The band structure was also observed in Doppler velocity. The band structure in both radar reflectivity and Doppler velocity propagated northwestward with a speed of 4.17 m s -1. From linear gravity wave theory it is estimated that the observed gravity waves had a horizontal wavelength of 1.5 km and period of 6 min. The updraft induced by gravity waves is estimated to be 7.8 cm s -1. The increase in liquid water content (LWC) caused by the updraft is 0.035 g m -1, and the increase of radar reflectivity is 2 dBZ. Within the band structure induced by the gravity waves (horizontal scale ∼1.5 km), roll structure with a smaller horizontal scale (∼300-600 m) was also observed in radar reflectivity. A previous study has shown that this smaller-scale roll structure is caused by shear-induced KHI. This study shows that both gravity waves and KHI modulated the structure of fog and resulted in the existence of multiscale (1.5 km and 300-600 m scale) fog structure. Copyright 2007 by the American Geophysical Union.