Mesoscale Motions in Oceanic Stratus as Revealed by Satellite Data

Over maritime areas, meteorological satellite data generally havc been used only in the study of synoptic scalc systems. Howcvcr, TIROS 7 pictures of summcrtimc stratus ovcr thc northcrn Pacific, whcn rcctified and combiiicd with mesosealc radiation analyses, give valuablc clucs regarding the mcsoscalc dynamics of thc lower atmosphcrc. Radiation data distinguish stratus from middlc lcvcl cloud laycrs. Stratus bands, averaging 13 km. widc and oricntcd 16' t o the left of thc geostrophic mind, arc thought to bc related to horizontal roll vorticcs. Rcflectancc cross scctions across thcsc bands iiidicatc sharp variations in thc thiekncss of thc stratus. Precisely gridded photographs show thc relation of marked clear areas in thc stratus to thc topography of lhc Aleutian Islands. Only wlicii cxcccding a critical sizc do isolated mountainous islands producc clcar wakes. Oiic island appears to bc shedding a possiblc vortex strcct. A derived modcl for the flow of thc stable air ovcr long mountain ridges in the Alcutians includcs such phcnomcna as upstrcam blocking, windward slopc bolstcr cddics, and cxtrcmc turbulent mixing on the lcc slopes causing clcar areas to cxtend ovcr 100 km. downstream. Radiosonde data indicatc that the mixed laycr downstream is approxirnatcly lhc depth of thc obstructing ridgc.