No description available.

No description available.

Cloud Resolving Models use microphysics parameterisation schemes for the simulation of clouds. The thesis reports on the introduction of two single-moment Bulk Microphysics Parameterisation (BMP) schemes in the Nonhydrostatic - coordinate Model (NSM). The rst BMP is known as the PURDUE-LIN scheme, and can be used with ve (excluding graupel) or six (including graupel) classes of the water substance. The second scheme was developed using the PURDUELIN scheme as a starting point, and is known as SBU-YLIN. Graupel and snow share a category and processes in the latter scheme. Simulations of two hours in length are made, with convection initiated through inserting a warm thermal into a cooler environment, using a six-class and ve-class PURDUE-LIN and the SBU-YLIN BMPs. The simulations are performed at various horizontal resolutions of 500 m, 1 km and 2 km. The six-class PURDUE-LIN scheme simulates more rainfall than the ve-class PURDUE-LIN and the SBU-YLIN schemes. The SBU-YLIN scheme generally rains the least, looses the least water vapour to hydrometeors and warms up the least. The PURDUE-LIN schemes simulate two convective cells in a no shear environment. The maximum updrafts associated with the rst cell (triggered by the warm perturbation) are similar in all the simulations. The second cell is triggered by a cold pool. While the cold pool is stronger in the six-class PURDUE-LIN scheme simulations, the updrafts in the second cell are stronger in the ve-class PURDUE-LIN simulation. The SBU-YLIN scheme generally simulates just one cell because of a weak cold pool. Simulations were also made for three di erent periods dominated by suppressed convection with deep convection at the beginning and end of the three periods, forced with large scale tendencies observed during the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The NSM is able to capture di erences in the suppressed and deep convection periods. Qualitatively, the simulations provide new insight into the interplay between cloud microphysics and cloud dynamics, and points out the potential for better describing the uncertainty range associated with projections of future climate change, through the improvement and stochastic application of cloud microphysics schemes. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Geography, Geoinformatics and Meteorology / unrestricted

This thesis forms a NERC funded CASE studentship with the Met Office, whose aim is to investigate the treatment of cloud microphysical processes in numerical models, with a particular focus on exploring the impacts and possible benefits of microphysical complexity for the purpose of simulating clouds and precipitation. The issue of complexity is an important one in numerical modelling in order to maintain computational efficiency, particularly in the case of operational models. The latest numerical modelling tools are utilised to perform simulations of cloud types including idealised trade wind cumulus, orographic wave cloud and wintertime shallow convective cloud. Where appropriate, the modelling results are also validated against observations from recent field campaigns. The Factorial Method is employed as the main analysis tool to quantify the effect of microphysical variables in terms of their impact on a chosen metric. Ultimately it is expected that the techniques and results from this thesis will be used to help inform the future development of cloud microphysics schemes for use in both cloud resolving and operational models. This is timely given the current plans to upgrade the microphysics options available for use within the Met Office Unified Model. For an idealised warm cloud, it is shown that different bin microphysics schemes can produce different results, and therefore additional microphysical complexity does not necessarily ensure a more consistent simulation. An intercomparison of bin microphysics schemes in a 1-D column framework is recommended to isolate the origin of the discrepancies. In relation to the mixed-phase wave cloud, model simulations based on an adaptive treatment of ice density and habit struggled to reproduce the observed ice crystal growth rates, highlighting the need for further laboratory work to improve the parameterization of ice growth by diffusion within the sampled temperature regime. The simulations were also found to be largely insensitive to values of the deposition coefficient within the range of 0.1 to 1.0. Results from a mesoscale modelling study of shallow wintertime convection demonstrate the importance of the representation of dynamical factors that control cloud macrostructure, and how this has the potential to overshadow any concerns of microphysical complexity. Collectively, the results of this thesis place emphasis on the need to encourage more synergy between the dynamics and microphysics research communities in order to improve the future performance of numerical models, and to help optimise the balance between model complexity and computational efficiency.

We report observations of Martian mesospheric ice clouds and thermospheric scale heights by the Imaging Ultraviolet Spectrograph on NASA's Mars Atmosphere and Volatile Evolution mission. The clouds are observed between 6A.M. and 8A.M. local time using mid-UV limb observations between 60 and 80km tangent altitude where ice particles that scatter sunlight can appear as detached layers near the equator. The equatorial longitudinal distribution shows populations of clouds near -110 degrees E and -10 degrees E as well as a population near 90 degrees E, which does not have a clear precedent. The cloud populations indicate a wave 3 pattern near 70km, which is confirmed by independent mesospheric temperature observations. Scale heights 100km above the clouds derived from concurrent Imaging Ultraviolet Spectrograph (IUVS) observations also reveal a wave 3 longitudinal structure, suggesting that the temperature oscillations enabling the formation of mesospheric clouds couple to the upper atmosphere. Plain Language Summary The manuscript describes the observation of Martian mesospheric clouds between 60 and 80km altitude by the Imaging Ultraviolet Spectrograph (IUVS) on NASA's MAVEN spacecraft. The cloud observations are uniquely obtained at early morning local times, which complement previous observations obtained primarily later in the diurnal cycle. Differences in the geographic distribution of the clouds from IUVS observations indicate that the local time is crucial for the interpretation of mesospheric cloud formation. We also report concurrent observations of upper atmospheric scale heights near 170 km altitude, which are diagnostic of temperature. These observations suggest that the dynamics enabling the formation of mesospheric clouds propagate all the way to the upper atmosphere.

Anthropogenic aerosol emissions lead to an increase in the amount of cloud condensation nuclei and consequently an increase in cloud droplet number concentration and cloud albedo. The corresponding negative radiative forcing due to aerosol cloud interactions (RFaci) is one of the most uncertain radiative forcing terms as reported in the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Here we show that previous observation-based studies underestimate aerosol-cloud interactions because they used measurements of aerosol optical properties that are not directly related to cloud formation and are hampered by measurement uncertainties. We have overcome this problem by the use of new polarimetric satellite retrievals of the relevant aerosol properties (aerosol number, size, shape). The resulting estimate of RFaci = −1.14 Wm 2 (range between −0.84 and −1.72 Wm 2) is more than a factor 2 stronger than the IPCC estimate that includes also other aerosol induced changes in cloud properties.

<p> The effect of clouds on the performance of the McMaster Solar Radiation Model was analyzed using global irradiance data from five Australian stations for the period 1978 to 1982. Ten cloud types were examined. Using values of RMSE and MBE, the model was found to perform well in 15 of the 24 years of analyzed data. The results, on average, parallel those found in previous Canadian studies. The dominance of low layer clouds coincided with all years displaying bad performance but a l so for some years displaying good performance. The analysis of specific cloud effects revealed that the model underestimates in the presence o f low layer clouds, is not well represented with middle layer clouds and overestimates with high layer clouds. These cloud effects can be used to explain some of the error found in the model's performance, much of the error present is random and cannot be explained by the effects of clouds. </p> / Thesis / Bachelor of Arts (BA)

No description available.

<p>The entry of satellite-derived decadal cloud datasets with homogeneous coverage in time and space enables studies not possible before. This thesis presents two such applications. The first study deals with cloud-aerosol processes and the second with an evaluation of cloud simulations from a regional climate model.</p><p>The first part of the thesis describes the used satellite-derived dataset based on imagery from the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting NOAA satellites. A method for cloud retrieval and the compilation of a 1991-2000 Scandinavian cloud climatology are described.</p><p>The second part reveals an intriguing anti-correlation between monthly mean satellite-derived cloudiness and the concentration of the cosmogenetic isotope Beryllium-7 in near-surface aerosol samples for three measurement sites in Sweden. Large-scale transport processes are suggested as the most likely physical mechanism for this behaviour but more complex relations to cloud microphysical processes are not ruled out.</p><p>The final part presents a thorough evaluation of cloud simulations of the SMHI Rossby Centre regional atmospheric model (RCA3). Several model-to-satellite adaptations are applied to avoid artificial biases of results. The study stresses the necessity to account for initial differences between observed and modelled clouds caused by satellite cloud detection limitations. Results show good agreement of modelled and observed cloud amounts while the vertical distribution of clouds appears largely different. RCA3 underestimates medium-level clouds while overestimating low- and high-level clouds. Also, the current use of the Maximum cloud overlap approach in the radiation scheme and an indicated excess of cloud condensate in modelled clouds appear to create excessive cloud optical thicknesses with serious implications for the surface radiation budget.</p><p>Future applications are outlined based on greatly enhanced satellite-derived cloud and radiation budget datasets.</p>

The entry of satellite-derived decadal cloud datasets with homogeneous coverage in time and space enables studies not possible before. This thesis presents two such applications. The first study deals with cloud-aerosol processes and the second with an evaluation of cloud simulations from a regional climate model. The first part of the thesis describes the used satellite-derived dataset based on imagery from the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting NOAA satellites. A method for cloud retrieval and the compilation of a 1991-2000 Scandinavian cloud climatology are described. The second part reveals an intriguing anti-correlation between monthly mean satellite-derived cloudiness and the concentration of the cosmogenetic isotope Beryllium-7 in near-surface aerosol samples for three measurement sites in Sweden. Large-scale transport processes are suggested as the most likely physical mechanism for this behaviour but more complex relations to cloud microphysical processes are not ruled out. The final part presents a thorough evaluation of cloud simulations of the SMHI Rossby Centre regional atmospheric model (RCA3). Several model-to-satellite adaptations are applied to avoid artificial biases of results. The study stresses the necessity to account for initial differences between observed and modelled clouds caused by satellite cloud detection limitations. Results show good agreement of modelled and observed cloud amounts while the vertical distribution of clouds appears largely different. RCA3 underestimates medium-level clouds while overestimating low- and high-level clouds. Also, the current use of the Maximum cloud overlap approach in the radiation scheme and an indicated excess of cloud condensate in modelled clouds appear to create excessive cloud optical thicknesses with serious implications for the surface radiation budget. Future applications are outlined based on greatly enhanced satellite-derived cloud and radiation budget datasets.