Venus: Cloud level circulation during 1982 as determined from Pioneer cloud photopolarimeter images: II. Solar longitude dependent circulation

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A strong solar-locked dependence of cloud level circulation is observed in the averaged cloud motions as well as in the ultraviolet reflectivity of the Venus cloud layer as determined from Pioneer Venus Orbiter images acquired during 1982 ( S. S. Limaye, C. Grassotti, and M. J. Kuetemeyer, 1988, Icarus 73, 193–211). The cloud motions were determined by manually tracking cloud features while the ultraviolet reflectivity was determined by removing the scattering geometry dependence of the observed intensities in ultraviolet images of Venus via a Minnaert scattering model. In low latitudes the average zonal component is slowest at about 10:40 AM local solar time, while the maximum poleward flow occurs at noon at most latitudes. The cloud motions are divergent with the maximum (horizontal) divergence (5 × 10 −6 sec −1) occurring in equatorial latitudes just past noon and become convergent at about 30 (morning) to 40° (afternoon) lat on both sides of the equator. The lowest ultraviolet reflectivity is found near the equator, at local noon. Peak reflectivity is found in broad, tilted (with respect to latitude circles) bands near 45° lat in the north and south hemispheres. An apparent relationship between the horizontal divergence and ultraviolet is indicated such that the highest reflectivities are found in regions of convergence in high latitudes and lower values where the motions are divergent (in equatorial latitudes). In solar-locked coordinates the root mean square deviation of the normalized ultraviolet brightness is not uniformly distributed about the subsolar point, but tends to be higher around 45° lat than in equatorial regions, and over periods of about 2 weeks shows substantial small-scale structure as well.