433 research outputs found

    Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

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    The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

    Characteristics of precipitation and extreme precipitation events over the Drakensberg Mountain range in past and future climates

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    The Drakensberg Mountains is one of the most valuable natural resources in Southern Africa because precipitation over the mountains is the source of rivers that support socio-economic activities in Lesotho, South Africa, and Namibia. Meanwhile, extreme precipitation events over the Drakensberg are a threat to the communities around the mountains. While several studies have shown that mountains are among the most sensitive regions to climate variability and change, the potential impacts of global warming on precipitation and extreme precipitation occurrences over the Drakensberg are poorly understood. This thesis examines the characteristics of precipitation and extreme precipitation events over the Drakensberg in past climate and investigates how the characteristics might change in future climate at various global warming levels under RCP8.5 future climate scenario. Series of climate datasets were analysed for the study. These include observed precipitation datasets from eight satellite products, reanalysis datasets from National Centre for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and climate simulation datasets from the Model for Prediction Across Scales (MPAS), Weather Research and Forecasting Model (WRF), and the National Aeronautics and Space Administration (NASA) Earth Exchange Global Daily Downscaled Projections (NEX), and the Coordinated Regional Climate Downscaling Experiment (CORDEX). All the simulation datasets were evaluated against the observation datasets. Precipitation indices were used to characterize precipitation and extreme precipitation events over the Drakensberg Mountains, with emphasis on widespread extreme events (WEREs). Self-Organizing Map (SOM) technique was employed to group the synoptic patterns over southern Africa, WERE patterns over DMR, and the future climate change projections over the Drakensberg Mountains. Results of this dissertation reveal that the weak synoptic days, which are associated with highpressure systems or the ridging of highs, account for 16 − 20% of weather conditions in MarchAugust and 5% of annual rainfall over the Drakensberg. Wet weak synoptic days can induce widespread extreme rainfall (up to 20mm day−1 ) over the Drakensberg. CFSR underestimates the magnitude of the weak-synoptic-day rainfall but the WRF downscaling of the CFSR dataset enhances the quality of the simulated rainfall. All of the climate simulation datasets (WRF, MPAS, CORDEX, NEX) give realistic simulations of the precipitation indices over Southern Africa, especially over South Africa and DMR. In most cases, the biases in the simulations are within the observation uncertainties. SOM analysis reveals four major patterns of WERE patterns over the Drakensberg. The most prevalent WERE pattern usually occurs on the eastern side of the mountain, stretching from north-east to the south-west along the coastline, and it is usually induced by tropical temperate troughs, cold fronts, and the ridging highs. There is no agreement among simulations ensemble means on the annual precipitation projection over DMR. However, the ensemble means agreed on an increase in the intensity of normal precipitation and a decrease in the number of precipitation days and the number of continuous wet days. They also agreed on a future increase in frequency and intensity of extreme precipitation and widespread extreme events over DMR. SOM analysis, which elucidates the range of projection patterns that lie beneath the simulation ensemble means of the simulations, shows the most probable combination of projected changes in the annual precipitation and extreme precipitation events (intensity and frequency) over DMR: (i) an increase in both annual precipitation and extreme precipitation events; (ii) a decrease in both annual precipitation and extreme precipitation events; (iii) a decrease in annual precipitation but increase in extreme precipitation events. Results of this study can provide a basis for developing climate change adaptation and mitigating strategies over the Drakensberg

    Understanding and forecasting tropical cyclone intensity change

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    Includes bibliographical references.This research investigates several issues pertaining to tropical cyclone intensity change. Previous research on tropical cyclone intensity change is reviewed in great detail. The applicability of upper-level forcing theories is questioned. Inner-core processes related to intensity change are studied, with particular attention on the relationship between the vertical profile of the tangential wind (vt) field in the eyewall region and future pressure changes. For cases under minimal wind shear and warm SSTs such that vigorous inner-core updrafts exist, the cyclonic circulation should be mostly conserved almost to the upper-troposphere, with the largest vertical vt variation confined near the tropopause. It is hypothesized that a vertically conserved wind profile is conducive to fast intensification. Observations support this theory. By stratifying inner-core data into fast and slow developers, it is shown that fast developing tropical cyclones contain a more vertically stacked inner-core vortex than slow developers. It is also shown that a direct correlation exists between inner-core upper-level winds and tropical cyclone intensification, with the rate of intensification proportional to the magnitude and symmetry of upper-level vt. Diagnostic calculations using the Balanced Vortex equations also support this assertion. An alternative air-sea interaction theory is presented which incorporates boundary layer cooling. The buoyancy calculations include partial water-loading and ice microphysics, and their relevance to CAPE calculations in the tropics is discussed. It is shown that the lateral extension of the eye, above a sloping eyewall, is the crucial component in maintaining the air-sea interaction despite boundary layer cooling. Implications on the maximum intensity a storm may achieve are discussed. A multiple regression scheme with intensity change as the dependent variable has been developed. The new scheme is titled the Typhoon Intensity Prediction Scheme (TIPS), and is similar to one used operationally at the National Hurricane Center. However, TIPS contains two major differences: it is developed for the western North Pacific Ocean, and utilizes digitized satellite data. It is shown that the satellite data can distinguish between fast and slow developing tropical cyclones. The importance of other statistical predictors (such as SSTs, wind shear, persistence, and climatology) to intensity change are also clarified. The statistics reveal threshold values useful to forecasters. It is shown that TIPS is competitive with the Joint Typhoon Warning Center.DOD-USAF-OSR: F49620-93-1-0415

    Scientific opportunities using satellite surface wind stress measurements over the ocean

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    Scientific opportunities that would be possible with the ability to collect wind data from space are highlighted. Minimum requirements for the space platform and ground data reduction system are assessed. The operational uses that may develop in government and commercial applications of these data are reviewed. The opportunity to predict the large-scale ocean anomaly called El Nino is highlighted

    CIRA annual report 2007-2008

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