<p> Tropical East Africa is influenced by two main rainy seasons, during autumn and spring. During autumn, tropical East African precipitation is clearly influenced by Indian Ocean Dipole (IOD) and/or ENSO. During spring, there is no clear SST pattern in the Indian Ocean. The association between El Ni&ntilde;o and positive IOD phases is much stronger than the association between La Ni&ntilde;a and negative IOD during October and November. During October, the association between El Ni&ntilde;o and wet condition over tropical eastern Africa is stronger than association between La Ni&ntilde;a and dry conditions. During November, the association between positive IOD and eastern African precipitation is stronger than the association between La Ni&ntilde;a and dry conditions. </p><p> During short wet phases (such as autumn) over eastern Africa, two anticyclones form in the lower troposphere with upper baroclinic structure. These anticyclones decay rapidly by December. These anticyclones are responsible for supplying East Africa with increased moisture. </p><p> Most strong positive IOD events are associated with wet outcomes over eastern Africa. Not all strong El Ni&ntilde;o events lead to wet outcomes. </p><p> It is well known that during northern spring, precipitation over eastern Africa is not connected to any inter-annual SST modes of variability. During northern spring, SST in Indian Ocean is nearly always sufficiently high to sustain convection, however, convection is not always active. We found that precipitation over eastern Africa during spring is associated with a dipole pattern of outgoing longwave radiation anomaly (OLRA) not associated with SST variability.</p>

<p> The Southern Ocean is an important part of the global carbon cycle, responsible for roughly half of the carbon dioxide (CO₂) absorbed by the global ocean. The air-sea CO₂ flux (<i>F_c</i>) can be expressed as the product of the water-air CO₂ partial pressure difference (&Delta;pCO₂) and the gas transfer velocity (<i> k</i>), an exchange coefficient which represents the efficiency of gas exchange. Generally, <i>F_c</i> is negative (a sink) throughout the Southern Ocean and Antarctic sea ice zone (SIZ), but uncertainty in <i> k</i> has made it difficult to develop an accurate regional carbon budget. Constraining the functional dependence of k on wind speed in open water environments, and quantifying the effect of sea ice on <i>k,</i> will reduce uncertainty in the estimated contribution of the Southern Ocean and Antarctic SIZ to the global carbon cycle. </p><p> To investigate <i>F_c</i> in the Southern Ocean, a ruggedized, unattended, closed-path eddy covariance (EC) system was deployed on the Antarctic research vessel <i>Nathaniel B. Palmer</i> for nine cruises during 18 months from January 2013 to June 2014 in the Southern Ocean and coastal Antarctica. The methods are described and results are shown for two cruises chosen for their latitudinal range, inclusion of open water and sea ice cover, and large &Delta;pCO₂. The results indicated that ship-based unattended EC measurements in high latitudes are feasible, and recommendations for deployments in such environments were provided. </p><p> Measurements of <i>F_c</i> and &Delta;pCO₂ were used to compute <i>k.</i> The open water data showed a quadratic relationship between <i>k</i> (cm hr^&ndash;1) and the neutral 10-m wind speed (<i>U</i>_10n, m s^&ndash;1), <i>k</i>=0.245 <i>U</i>_10n²+1.3, in close agreement with tracer-based results and much lower than previous EC studies. In the SIZ, it was found that <i>k</i> decreased in proportion to sea ice cover. This contrasted findings of enhanced <i> F_c</i> in the SIZ by previous open-path EC campaigns. Using the NBP results a net annual Southern Ocean (ocean south of 30&deg;S) carbon flux of &ndash;1.1 PgC yr^&ndash;1 was calculated. </p>

Ice clouds are crucial to the Earth's radiation balance. They cool the Earth-atmosphere system by reflecting solar radiation back to space and warm it by blocking outgoing thermal radiation. However, there is a lack of an observation-based climatology of ice cloud properties and their radiative effects. Two active sensors, the CloudSat radar and the CALIPSO lidar, for the first time provide vertically resolved ice cloud data on a global scale. Using synergistic signals of these two sensors, it is possible to obtain both optically thin and thick ice clouds as the radar excels in probing thick clouds while the lidar is better to detect the thin ones. First, based on the CloudSat radar and CALIPSO lidar measurements, we have derived a climatology of ice cloud properties. Ice clouds cover around 50% of the Earth surface, and their global-mean optical depth, ice water path, and effective radius are approximately 2 (unitless), 109 g m⁻² and 48 μm, respectively. Ice cloud occurrence frequency not only depends on regions and seasons, but also on the types of ice clouds as defined by optical depth (τ) values. Optically thin ice clouds (τ < 3) are most frequently observed in the tropics around 15 km and in the midlatitudes below 5 km, while the thicker clouds (τ > 3) occur frequently in the tropical convective areas and along the midlatitude storm tracks. Using ice retrievals derived from combined radar-lidar measurements, we conducted radiative transfer modeling to study ice cloud radiative effects. The combined effects of ice clouds warm the earth-atmosphere system by approximately 5 W m⁻², contributed by a longwave warming effect of about 21.8 W m⁻² and a shortwave cooling effect of approximately -16.7 W m⁻². Seasonal variations of ice cloud radiative effects are evident in the midlatitudes where the net effect changes from warming during winter to cooling during summer, and the net warming effect occurs year-round in the tropics (∼ 10 W m⁻² ). Ice cloud optical depth is shown to be an important factor in determining the sign and magnitude of the net radiative effect. On a global average, ice clouds with τ < 4.6 display a warming effect with the largest contributions from those with τ ~ 1.0. Optically thin and high ice clouds cause strong heating in the tropical upper troposphere, while outside the tropics, mixed-phase clouds cause strong cooling at lower altitudes (> 5 km). In addition, ice clouds occurring with liquid clouds in the same profile account for about 30% of all observations. These liquid clouds reduce longwave heating rates in ice cloud layers by 0-1 K/day depending on the values of ice cloud optical depth and regions. This research for the first time provides a clear picture on the global distribution of ice clouds with a wide range of optical depth. Through radiative transfer modeling, we have gained better knowledge on ice cloud radiative effects and their dependence on ice cloud properties. These results not only improve our understanding of the interaction between clouds and climate, but also provide observational basis to evaluate climate models. / A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / April 28, 2017. / Includes bibliographical references. / Guosheng Liu, Professor Directing Dissertation; Eric Chicken, University Representative; Robert Ellingson, Committee Member; Ming Cai, Committee Member; Zhaohua Wu, Committee Member.

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In this thesis, we introduce a new approach to classify rain clouds based on the relationship between the emission signal and scattering signal derived from microwave brightness temperature data. Two parameters are used as indicators of emission signal and scattering signal respectively: one is the polarization difference (D) at 19 GHz, and the other one is the polarization-corrected temperature (PCT) at high-frequencies channels. D is related to the emission of liquid hydrometeors, and PCT mainly reflects the brightness temperature depression due to the scattering by ice particles. Both D and PCT decrease with increasing precipitation rate. Therefore, certain combinations of D and PCT can be regarded as the representatives of cloud hydrometeor structures. Based on the D-PCT relationship investigated in this study, we classified the observed rain clouds into five categories—non-precipitating, light-precipitating, liquid-dominant precipitating, well-mixed precipitating, and ice-dominant precipitating clouds. We verified the results of the classification of different precipitation cases over tropical regions. For both the hurricane and front cases, the results show that the distributions of categorized cloud pixels can reflect the horizontal structure of the weather systems. The monthly gridded mean frequencies of categorized precipitating clouds are used to analyze the relationship between the seasonal and interannual cycles of tropical precipitation and clouds’ hydrometeor components. Moreover, the results indicated that in an annual cycle or an ENSO cycle, when the local precipitation frequencies increase, the occurrence frequencies of all kinds of rain clouds will increase. However, among those precipitating systems, the proportions of ice-dominant and well-mixed clouds increases while that of water-dominant clouds decrease as the local precipitation increases. Anomalies of the opposite sign tend to accompany the decreasing precipitations situations. Overall, the classification method proves to be useful to extract objective information from observed emission and scattering signals. Since clouds have always been signs of the weather systems, the long-term variances of global distribution and characteristics of rain clouds are as an aspect of cloud climatology. Moreover, the categorization of precipitation types can be useful in developing the best retrieval algorithm of rain rate for a specific cloud type. Additionally, the information about cloud types can be used to improve our understanding of cloud processes and to increase the accuracy of weather and climate models. / A Thesis submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester 2019. / March 5, 2019. / Classification, Emission, Microwave, Rain clouds, Scattering / Includes bibliographical references. / Guosheng Liu, Professor Directing Thesis; Vasubandhu Misra, Committee Member; Allison Wing, Committee Member.

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