http://hdl.handle.net/1957/1310 Search the Channel search http://ir.library.oregonstate.edu/dspace/simple-search http://hdl.handle.net/1957/6546 Title: Buoy and satellite observation of wind induced surface heat exchange in the intraseasonal oscillation over West Pacific and Indian Ocean <br/> <br/>Abstract: The importance of wind-driven latent heat fluxes for supporting tropical intraseasonal precipitation variability is analyzed. Tropical intraseasonal variability in the west pacific and Indian ocean is analyzed for northern and southern hemisphere summer during 1999-2005 using satellite and buoy observations. A composite analysis of QuikSCAT ocean vector winds and TRMM precipitation for the intraseasonal oscillation (ISO) indicate that enhanced precipitation is associated with anomalous surface westerly flow over the west Pacific and the Indian oceans. Anomalous westerly flow associated with the ISO is also accompanied by enhanced wind speed over the west Pacific and Indian ocean. Enhanced wind speed during westerly phases in the west Pacific and Indian oceans suggests an increase in the wind-driven component of latent heat flux. An analysis using TAO buoys and TRMM precipitation shows a significant correlation between intraseasonal (30-100 day) latent heat flux and TRMM precipitation in the region where enhanced precipitation occurs during the ISO westerly phase. Latent heat flux anomalies are about 20% of precipitation anomalies, a magnitude approximately sufficient as suggested by previous studies to support a flux-driven convective instability. The correlation of QuikSCAT wind speed and TRMM precipitation was analyzed over the west Pacific and it also showed a correlation of similar magnitude as the correlation of latent heat flux and precipitation. As a sensitivity test to determine how much of the latent heat flux anomalies were wind-driven, latent heat flux anomalies were recalculated at the location of strong correlation by fixing SST, boundary layer relative humidity, and boundary layer temperature to their sixty days running averages. This sensitivity analysis shows that most of the latent heat flux anomaly is wind-driven, indicating that intraseasonal precipitation may be supported by a wind-evaporation feedback mechanism. Wind speed and precipitation anomalies also show a significant correlation in the Indian ocean. This result suggests that if latent heat flux anomalies in the Indian ocean are primarily wind-driven, then a wind-evaporation feedback mechanism may also be supported there. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/6391 Title: Surface wind modification near mid-latitude ocean fronts : observational and dynamical analysis <br/> <br/>Abstract: Interactions between surface winds and meanders in mid-latitude sea surface temperature (SST) fronts with horizontal length scales of 100-1000 km are investigated from satellite observations and numerical simulations. Observations from the Sea- Winds scatterometer on the QuikSCAT satellite show that the magnitude, direction, curl, and divergence of the surface wind stress and 10-m winds are well correlated with small-scale SST structures associated with large-scale ocean currents. Detailed analysis of the response of the surface winds to SST fronts from these satellite observations exposed shortcomings in previous conceptual hypotheses governing the relationships between surface winds and SST. To gain understanding of the physical mechanisms needed to explain the satellite wind observations, we performed a numerical experiment simulating the atmospheric flow over meandering SST fronts. Based on these results, a new conceptual model is constructed to explain the dynamical response of the surface winds consistent with the satellite observations and numerical simulation analysis. Of particular importance was the finding that the wind stress curl and divergence fields observed from QuikSCAT are linearly related to the crosswind and downwind components of the SST gradient, respectively. This relationship was generally thought to result from modification of the vertical turbulent mixing of momentum within the atmospheric boundary layer (ABL). We show that this mechanism is overly simplistic; nearly all of the terms in the momentum budget are needed to explain these observed statistical relationships, consistent with recent work. SST-induced surface wind changes are a manifestation of more complicated changes to the vertical structure of the dynamic forces within the ABL. Among the most significant of several new findings presented here concerns the influence of SST on the meridional wind field. Since winds are generally westerly at mid-latitudes, SST-induced changes in meridional wind cause changes in the surface wind direction that significantly influence the wind stress curl and divergence fields through modification of streamline curvature and diffluence. From numerical and analytical results, these meridional wind perturbations are shown to result from a baroclinic Ekman adjustment mechanism modified by horizontal advection. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/5710 Title: Overestimation of cloud cover in the MODIS cloud product <br/> <br/>Abstract: The operational processing of MODIS imagery to produce the MOD06 cloud product is based on the assumption that cloudy 1-km pixels are overcast. This assumption is examined using a partly cloudy pixel retrieval scheme, which allows for fractional cloud cover within the 1-km pixels. Cloud flags attached to 250-m MODIS imagery data were aggregated to 1-km resolution to compare with fractional cloud cover retrieved using the partly cloudy pixel scheme. For regions containing only single-layered, low-level marine stratocumulus, the fractional cloud cover derived from the 250-m flags was substantially greater than that obtained with the partly cloudy pixel retrievals. When 1-km visible reflectances and 11-μm radiances were interpreted according to the pixel-scale cloud cover fraction, the interpretations obtained with the partly cloudy pixel retrieval scheme were those expected based on radiative transfer theory. Those obtained from the 250-m flags, however, suggested that many of the 1-km pixels that were identified as overcast were only partially cloud covered. These findings were confirmed using 500-m MODIS imagery to identify pixels that were overcast by marine stratocumulus. Based on the spatial uniformity of 1.6- and 2.1-μm reflectances for overcast and cloud-free pixels, reflectances at these wavelengths were used to distinguish between overcast and partly cloudy 500-m pixels. The 250-m cloud flags obtained from the MOD06 cloud product, however, identified many of the partly cloudy pixels as overcast. The overcast assumption made by the MOD06 cloud product also leads to biases in cloud properties such as cloud optical depth and droplet effective radius. Comparisons of cloud layer temperature, optical depth, and droplet effective radius derived from 2.1-μm reflectances show that overcast results agree with what is expected from cloud parcel models. Pixels with partial cloud cover, however, underestimate optical depth and overestimate droplet effective radius and the biases produce trends that counter expectations based on cloud parcel models. These biases become more evident as cloud cover within a pixel decreases. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/5500 Title: Properties of marine stratus and stratocumulus derived using collocated MODIS and CALIPSO observations <br/> <br/>Abstract: Adiabatic parcel models suggest specific relationships between cloud thickness and cloud properties. Such relationships govern cloud radiative forcing and thus cloud feedbacks in the climate system. Current remote sensing techniques work well at measuring these properties in low-level cloud systems that are overcast. Significant biases exist, however, when measuring cloud properties in regions of broken clouds. The operational MODIS cloud product (MOD06) assumes each cloudy 1-km imager pixel is overcast. The failure to account for partial cloudiness can result in substantially greater cloud top temperatures than in overcast regions and a bias in the subsequent optical properties. Such discrepancies are accounted for in a partly cloudy pixel retrieval scheme that takes into account the fractional cloud cover in pixels that are only partly cloud covered. To assess the cloud properties in the MOD06 product and those obtained with the partly cloudy pixel retrievals, CALIPSO lidar returns were used to identify low-level, single-layered cloud systems over oceans collocated with 1-km MODIS Aqua imagery. The MODIS cloud product, CALIPSO, and the partly cloudy pixel retrieval scheme all show similar relationships between cloud thickness and cloud properties when MODIS pixels are overcast. Cloud altitudes in overcast regions show relatively little variation within 50-km scales. When marine stratus cloud layers are broken, cloud top temperatures from the MODIS cloud product are substantially greater for partly cloudy pixels than overcast pixels within the same region. The cloud product suggests that clouds in partly cloudy pixels are at significantly lower altitudes. Such clouds exhibit biases in their cloud properties when compared with those derived using the partly cloudy pixel retrieval scheme. CALIPSO and the partly cloudy retrievals show similar relationships for cloud altitudes and cloud properties retrieved in both broken and overcast regions. The results suggest that the MODIS cloud products should be used to assess relationships between cloud altitude and cloud properties only for regions that are extensively overcast by clouds in a single layer. <br/> <br/>Description: Graduation date: 2007