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 1 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.