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The differences in the concepts of two different parameterizations of cloud microphysics are analyzed. Simulations alternatively applying these parameterizations are performed for a Baltic heat cyclone event. The results of the simulations are compared to each other as well as to observed distributions of cloudiness and snowfall. The main differences between the simulated distributions result from the assumptions on ice, the ice classes, and size distributions of the cloud and precipitating particles. Both schemes succeeded in predicting the position and the main structure of the main cloud and snowfall fields. Nevertheless, the more convective type parameterization overestimates, while the other one underestimates snowfall. / Die Unterschiede in den Konzepten zweier unterschiedlicher Parametrisierungen der Wolkenmikrophysik werden analysiert. Die Ergebnisse der Simulationen werden miteinander und mit den beobachteten Wolken- und Schneeverteilungen für eine Baltische Wärmezyklone verglichen. Die wesentlichen Unterschiede in den berechneten Verteilungen resultieren aus den verschiedenen Annahmen über Wolkeneis, die Eisklassen und die Größenverteilungen der Wolken- und Niederschlagspartikel. Beide Schemata sagen die Position und die wesentlichen Strukturen der Wolken- und Schneeverteilungen erfolgreich vorher. Dennoch überschätzt das eher konvektive Schema den Schneefall, während das andere ihn unterschätzt.

Atmospheric moist convection is one of the main topics discussed on weather and climate. This study purpose is to understand why different and similar cloud microphysics parameterizations produce different patterns of precipitation at the ground through several numerical sensitivity tests with the WRF model in the simulation of a squall line case observed on the Amazon region. Four different bulk microphysics parameterizations (Lin, WSM6, Morrison, and Milbrandt) were tested, and the main results show that statistical errors do not change significantly among each other for the four numerical domains (from 27 km up to 1 km grids). The correlations between radar rainfall data and the simulated precipitation fields show the double-moment parameterization Morrison scheme was the one that displayed better results in the overall: While Morrison scheme show 0.6 correlation in the western box of the 1 km domain, WSM6 and Lin schemes show 0.39 and 0.05, respectively. Nevertheless, because this scheme presents good correlations with the radar rain rates, it also shows a fairly better system lifecycle, evolution, and propagation when compared to the satellite data. Although, the complexity that the way microphysics variables are treated in both one-moment and double-moment schemes in this case study do not highly affect the simulatios results, the tridimensional vertical cross-sections show that the Purdue Lin and Morrison schemes display more intense systems compared to WSM6 and Milbrandt schemes, which may be associated with the different treatments of the ice-phase microphysics. In the specific comparison between double-moment schemes, the ice quantities generated by both Morrison and Milbrandt schemes highly affected thesystem displacement and rainfall intensity. This also affects the vertical velocities intensity which, in its, turn, changes the size of the cold pools. Differences in ice quantities were responsible for distinct quantities of total precipitable water content, which is related with the verticallly integrated ice mixing ratio generated by Morrison. The system moves faster in Milbrandt scheme compared to Morrison because the scheme generated more graupel quantities, which is smaller in size than hail, and it evaporates easier in the processes inside the cloud due to its size. This fact also changed the more intense cold pools intensity for Milbrandt scheme compared to Morrison. / A convecção atmosférica é um dos principais tópicos discutidos no tempo e clima. O objetivo deste estudo é entender por que diferentes e semelhantes parametrizações de microfísica de nuvens produzem diferentes padrões de precipitação no solo através de vários testes numéricos de sensibilidade com o modelo WRF na simulação de um caso de linha de instabilidade observado na região amazônica. Quatro diferentes parametrizações microfísicas de tipo bulk (Lin, WSM6, Morrison e Milbrandt) foram testadas, e os principais resultados mostram que os erros estatísticos não se alteram significativamente entre si para os quatro domínios numéricos (da grade de 27 km até a de 1 km). As correlações entre dados pluviométricos de radar e os campos de precipitação simulados mostram que o esquema Morrison de parametrização de duplo momento foi o que apresentou melhores resultados, no geral: enquanto o esquema de Morrison mostra correlação 0,6 na caixa oeste do domínio de 1 km, os esquemas WSM6 e Lin mostram 0,39 e 0,05, respectivamente. No entanto, como esse esquema apresenta boas correlações com as taxas de chuva do radar, ele também mostra um ciclo de vida, evolução e propagação do sistema relativamente melhores quando comparado aos dados de satélite. Embora a complexidade com que as variáveis microfísicas são tratadas nos esquemas de um momento e de duplo momento neste estudo de caso não afetam muito os resultados simulados, as seções transversais verticais tridimensionais mostram que os esquemas de Purdue Lin e Morrison exibem mais intensos em comparação com os esquemas WSM6 e Milbrandt, que podem estar associados aos diferentes tratamentos da microfísica da fase de gelo. Na comparação específica entre esquemas de momento duplo, as quantidades de gelo geradas pelos esquemas de Morrison e Milbrandt afetaram muito o deslocamento do sistema e a intensidade da chuva. Isso também afeta a intensidade das velocidades verticais que, por sua vez, altera o tamanho das piscinas frias. As diferençaas nas quantidades de gelo foram responsáveis por quantidades distintas de conteúdo total de água, que está relacionado com a razão de mistura de gelo verticalmente integrada gerada por Morrison. O sistema se move mais rápido no esquema de Milbrandt comparado a Morrison porque o esquema gerou mais quantidades de graupel, que é menor em tamanho do que o granizo, e evapora mais facilmente nos processos dentro da nuvem devido ao seu tamanho. Este fato também mudou a intensidade das piscinas frias mais intensas, porém menores em extensão horizontal, para o esquema Milbrandt em comparação com Morrison.

With the launch of CloudSat, direct observations of cloud vertical structure became possible on the global scale. This thesis utilizes over two years of CloudSat data to study large-scale variations of clouds. We compose a global data set of contiguous clouds (echo objects, EOs) and the individual pixels comprising each EO. For each EO many attributes are recorded. EOs are categorized according to cloud type, time of day, season, surface type, and region. From the categorization we first look at gross global climatology of clouds. Maps of cloud cover are subdivided by EO (cloud) type, and results compare well with previous CloudSat work. The seasonality of cloud cover is also examined. Focus topics studied in this thesis include: (1) mid-level clouds, (2) stratocumulus clouds, and (3) clouds across the Madden-Julian Oscillation (MJO). The mid-level cloud work found an unexpected frequency peak in EO top heights between 7-8 km in the tropics, further shown to correspond to a global peak in EO top temperature between -15°C ? -20°C. Hypotheses are discussed regarding cause of this feature. Stratocumulus clouds are defined as low-level (tops < 4.5 km), wide (width > 11 km) EOs. Stratocumulus cloud cover agrees (with understandable differences) with other estimates (ISCCP and CALIPSO). The seasonal cycle of stratocumulus over the main stratocumulus decks is examined. The Peruvian and Namibian decks have increased cloud cover in austral spring in 2007 vs. 2006, corresponding sensibly to sea surface temperature differences and changes in lower static stability. Looking at rain and drizzle statistics, wider EOs are found to drizzle more. Clouds across the MJO are defined relative to temporally filtered OLR data. Cloud cover (volume) doubles (triples) from suppressed to active MJO phases, with some shifts of the relative contributions of different EO types from the front to back of the MJO. Pixel statistics in dBZ-height space correspond to these cloud-type shifts. High anvils and low clouds in front lead deep convection followed by relatively lower anvils in the back.

Arctic clouds are poorly represented in numerical models due to the complex, small-scale interactions which occur within them. Modelled cloud fractions are often significantly less than observed in this region; therefore, the radiative budget is not accurately simulated and forecasts of the melting cryosphere are fraught with uncertainty. Our ability to accurately model Arctic clouds can be improved through observational studies. Recent in situ airborne measurements from the springtime Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign are presented in this thesis to improve our understanding of the cloud microphysical interactions unique to this region. Aerosol-cloud interactions - where aerosol particles act as ice nucleating particles (INPs) or cloud condensation nuclei (CCN) - are integral to the understanding of clouds on a global scale. In the Arctic, uncertainties caused by our poor understanding of these interactions are enhanced by strong feedbacks between clouds, the boundary layer, and the sea ice. In the Arctic spring, aerosol-cloud interactions are affected by the Arctic haze, where a stable boundary layer allows aerosol particles to remain in the atmosphere for long periods of time. This leads to a heightened state of mixing in the aerosol population, which affects the ability of particles to act as INPs or CCN. Aerosol particle compositional data are presented to indicate which particles are present during the ACCACIA campaign, and infer how they may participate in aerosol-cloud interactions. Mineral dusts (known INPs) are identified in all flights considered, and the dominating particle classes in each case vary with changing air mass history. Mixed particles, and an enhanced aerosol loading, are identified in the final case. Evidence is presented which suggests these characteristics may be attributed to biomass burning activities in Siberia and Scandinavia. Additionally, in situ airborne observations are presented to investigate the relationship between the Arctic atmosphere and the mixed-phase clouds - containing both liquid cloud droplets and ice crystals - common to this region. Cloud microphysical structure responds strongly to changing surface conditions, as strong heat and moisture fluxes from the comparatively-warm ocean promote more turbulent motion in the boundary layer than the minimal heat fluxes from the frozen sea ice. Observations over the transition from sea ice to ocean show that the cloud liquid water content increases four-fold, whilst ice crystal number concentrations, N_ice, remain consistent at ~0.5/L. Following from this study, large eddy simulations are used to illustrate the sensitivity of cloud structure, evolution, and lifetime to N_ice. To accurately model mixed-phase conditions over sea ice, marginal ice, and ocean, ice nucleation must occur under water-saturated conditions. Ocean-based clouds are found to be particularly sensitive to N_ice, as small decreases in N_ice allow glaciating clouds to be sustained, with mixed-phase conditions, for longer. Modelled N_ice also influences precipitation development over the ocean, with either snow or rain depleting the liquid phase of the simulated cloud.

The effects of enhanced greenhouse gas concentrations on Earth’s climate are well understood. However, the impacts of anthropogenic aerosol particles, in particular due to the many aerosol-cloud indirect feedback mechanisms are not fully or even explicitly quantified as yet. This PhD seeks to contribute to improve our knowledge and understanding of aerosol – precipitation interactions over the Nepal Himalayas region and their consequences for precipitation patterns there. The research was carried out using the cloud-resolving Weather Research and Forecasting (WRF) model through a series of sensitivity studies and supported by literature reviews of satellite and field observations, although the latter are sparse. To complement the modelling studies, from March to December 2011, aerosols and surface meteorology were also continuously measured at Nagarkot (Lat: 27.7°N, Lon: 85.5° E, Alt: 1900m), Nepal, located in the eastern flank of a bowl shaped Kathmandu valley. The location was chosen to provide a representative vertical profile of aerosol and the impact on topographical flows. Our results showed a unique pattern of diurnal pollution circulation within the valley with a morning and evening peak. The evening peak, which is higher than the morning peak is attributed to the light wind blowing through the valley carrying locally generated fresh evening pollution, further enhanced by re-circulations of aged pollutants through suppression of the mixing layers as suggested by a previous study at a different location. The morning peak is caused by calm wind conditions followed by the transitional growth of the nocturnal boundary layer. It is found that the thermally driven mountain – valley wind circulations are responsible for ventilation of pollutants. The WRF simulations showed that a sophisticated double moment bulk microphysics parameterization scheme performed best, which did not show any statistically significant difference compared to the observed data at 80% confidence interval using a Chi-squared goodness of best fit test. A sensitivity analysis of aerosol and temperature perturbations on the monsoon precipitation was conducted. We found that the model represented the first indirect effect reasonably well however, rainfall was not particularly sensitive to the aerosol perturbations used, due to the poorly documented role of the ice phase processes which assume a greater importance in this region due to the influence of topography and diurnal heating cycle. Further model studies focusing on chemical properties of aerosol and sensitivity of Ice Nuclei (IN) to precipitation in this region are recommended. In contrast, the effects of temperature perturbation were found to be significant, more so than the currently modelled aerosol indirect effects, suggesting that reduced frequency but intense rain events are likely over the Himalayas as the climate warms.

The differences in the concepts of two different parameterizations of cloud microphysics are analyzed. Simulations alternatively applying these parameterizations are performed for a Baltic heat cyclone event. The results of the simulations are compared to each other as well as to observed distributions of cloudiness and snowfall. The main differences between the simulated distributions result from the assumptions on ice, the ice classes, and size distributions of the cloud and precipitating particles. Both schemes succeeded in predicting the position and the main structure of the main cloud and snowfall fields. Nevertheless, the more convective type parameterization overestimates, while the other one underestimates snowfall. / Die Unterschiede in den Konzepten zweier unterschiedlicher Parametrisierungen der Wolkenmikrophysik werden analysiert. Die Ergebnisse der Simulationen werden miteinander und mit den beobachteten Wolken- und Schneeverteilungen für eine Baltische Wärmezyklone verglichen. Die wesentlichen Unterschiede in den berechneten Verteilungen resultieren aus den verschiedenen Annahmen über Wolkeneis, die Eisklassen und die Größenverteilungen der Wolken- und Niederschlagspartikel. Beide Schemata sagen die Position und die wesentlichen Strukturen der Wolken- und Schneeverteilungen erfolgreich vorher. Dennoch überschätzt das eher konvektive Schema den Schneefall, während das andere ihn unterschätzt.

Esta dissertação analisou tempestades que produziram granizo na Região Metropolitana de Campinas com o objetivo de identificar fatores determinantes para a produção e precipitação de granizo. De forma inédita, uma rede de detecção de granizo instalada na região permitiu a identificação e determinação das intensidades das tempestades entre 2016 e 2017. O ciclo de vida, estrutura microfísica e cinemática de casos específicos foram estudados usando três radares meteorológicos instalados no estado de São Paulo e uma rede de detecção de raios, usando ferramentas como rastreamento de sistemas convectivos, identificação de hidrometeoros e recuperação de vento tridimensional por Multi-Doppler. Comparando com escalas de intensidade de granizo aplicadas ao continente europeu, os casos analisados apresentaram intensidade baixa, com granizo de no máximo 22,4 mm de diâmetro. O caso de 2017-03-14 apresentou o tempo de vida mais longo (6,2 h), queda de granizo em duas localidades (com diâmetro máximo de 11,8 mm) e atividade elétrica mais intensa (taxa máxima de 107 (31) flashes/min IC (CG)), enquanto que o caso de 2017-11-15, com tempo de vida mais curto (2,2 h), apresentou baixa atividade elétrica (total de 46 (20) flashes IC (CG)) porém com queda de granizo mais intensa (com diâmetro máximo de 22,4 mm). Todas as quedas de granizo dos casos específicos citados anteriormente estão associadas a uma extensa coluna de granizo identificada pelo radar polarimétrico e correntes ascendentes de até 30 m/s antes do evento; o granizo maior no caso de 2017-11-15 possivelmente tem contribuição da precipitação na forma líquida (associada à correntes descendentes mais intensas) que previne a diminuição de tamanho do granizo ao mesmo tempo que contribui para o seu crescimento mesmo abaixo da base da nuvem. Alguns fatores determinantes encontrados em ambos os casos foram o aumento da atividade elétrica antes ou depois da queda de granizo, a presença de granizo em uma camada extensa dentro da nuvem e a atuação da corrente ascendente dentro da região de fase mista contribuindo para a formação e crescimento do granizo. / This dissertation analyzed hail producing storms on the Metropolitan Region of Campinas to identify key factors for hailfall occurrence. For the first time, a hail detection network installed in the region allowed the identification and determination of thunderstorm intensity in the 2016-2017 period. The life cycle, microphysical structure and kinematics of specific cases were studied using three meteorological radars installed in São Paulo state and a lightning detection network, with tools such as tracking of convective systems, hydrometeor identification and Multi-Doppler 3D wind retrieval. The analyzed cases had low hailfall intensity when compared with scales applied in Europe, with 22.4 mm maximum hail diameter. The 2017-03-14 case presented the longest lifetime (6.2 h), hailfall in two locations (11.8 mm maximum hail diameter) and the most intense lightning activity (107 (31) flashes/min IC (CG) maximum rate), while the 2017-11-15 case, with a shorter lifetime (2.2 h), presented low electrical activity (46 (20) flashes IC (CG) total) with the most intense hailfall (22.4 mm maximum hail diameter). All hailfall cases of the specific cases mentioned earlier are associated with a extensive hail column identified by the polarimetric radar and up to $30 m/s updrafts before the events; the bigger hail in the 2017-11-15 case possibly had the contribution of liquid precipitation (associated with larger downdrafts) which prevents hail size decrease as well as contributes to its growth below the cloud base. Some key factors found in both cases were the increase in electrical activity before or after hailfall, the presence of hail in a extensive layer within cloud and the updrafts within mixed phase layer contributing to hail formation and growth.

Marine stratocumulus cloud regimes exert a strong climatic influence through their high solar reflectivity. Human-induced changes in stratocumulus clouds, attributed to an increase of the aerosol burden (indirect effects), can be significant given the cloud decks proximity to the continents; nevertheless, the magnitude and the final climatic consequences of these changes are uncertain. This thesis investigates further the interactions between aerosols, cloud microphysics, regional circulation, and radiative response in the Southeast Pacific stratocumulus cloud deck, one of the largest and most persistent cloud regimes in the planet. Specifically, three different aspects are addressed by this thesis: The importance of the synoptic atmospheric variability in controlling cloud microphysical and radiative changes, a validation analysis of satellite retrievals of cloud microphysics from MOderate Resolution Imaging Spectroradiometer (MODIS), and the quantitative assessments of cloud aerosol interactions along with their associated radiative forcing using primarily aircraft remote sensing data. Synoptic and satellite-derived cloud property variations for the Southeast Pacific region associated with changes in coastal satellite-derived cloud droplet number concentration (Nd) are analyzed through a composite technique. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd, for the five October months of 2001, 2005, 2006, 2007, and 2008. The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds than the MIN-Nd composite. Additionally, the MAX-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres. At 85˚W, the top-of-atmosphere shortwave fluxes are significantly higher (50%) for the MAX-Nd, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd. The change in Nd at this location is small, suggesting that the MAX-MIN Nd composite differences in radiative properties primarily reflects synoptic changes. The ability of MODIS level 2 retrievals to represent the cloud microphysics is assessed with in-situ measurements of droplet size distributions, collected during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The MODIS cloud optical thickness (t) correlates well with the in-situ values with a positive bias (1.42). In contrast, the standard 2.1 micron-derived MODIS cloud effective radius (r_e) is found to systematically exceed the in-situ cloud-top r_e, with a mean bias of 2.08 um. Three sources of errors that could contribute to the MODIS r_e positive bias are investigated further: the spread of the cloud droplet size distribution, the presence of a separate drizzle mode, and the sensor viewing angles. The sensor zenith viewing angles were found to have little impact, while the algorithm assumption about the cloud droplet spectra and presence of a precipitation mode could affect the retrievals but not by enough to fully explain the positive MODIS r_e bias. The droplet spectra effects account for r_e offsets smaller than 0.6 um, 0.9 um, and 1.6 um for non-drizzling, light-drizzling, and heavy-drizzling clouds respectively. An explanation for the observed MODIS bias is lacking although three-dimensional radiative effects were not considered. This investigation supports earlier studies documenting a similar bias, this time using data from newer probes. MODIS r_e and t were also combined to estimate a liquid water path (LWP) and Nd. A positive bias was also apparent in LWP, and attributed to r_e. However, when selected appropriate parameters a priori, the MODIS Nd estimate was found to agree the best with the insitu aircraft observations of the four MODIS variables. Lastly, the first aerosol indirect effect (Twomey effect) is explicitly investigated with VOCALS-REx observations, collected during three daytime research flights (Nov 9, 11, and 13), utilizing an aerosol-cloud interactions metric, and defined as ACI=dln(t)/dln(Na), with Na corresponding to the accumulation mode aerosol concentration, t derived from a broadband pyranometer, and ACI binned by cloud LWP derived from a millimeter-wavelength radiometer. Aircraft remote sensing estimates of the ACI, during sub-cloud transects, show that the cloud aerosol-interactions are strong and close to the maximum theoretical value for thin clouds, with a decrease of ACI with LWP. Although an explanation for the dependence of ACI on LWP is lacking, we found that a decrease in ACI with LWP is associated with decreases in both surface meridional winds and Nd. Similar to ACI, albedo fractional changes due to Nd fractional changes also tended to be smaller for higher LWPs, but with an overall radiative forcing larger than conservative global estimates obtained in global circulation models. The findings of this thesis emphasize the strong stratocumulus albedo response to an aerosol perturbation and its dependence on the regional scale atmospheric configuration. The results presented here can be used as a benchmark for testing regional and climate models, as well as helping to improve the current parameterizations of the first aerosol indirect effect.

The Second Aerosol Characterization Experiment (ACE-2) data set along with ECMWF reanalysis meteorological fields provided the basis for the single column model (SCM) simulations, performed as part of the PACE (Parameterization of the Aerosol Indirect Climatic Effect) project. Six different SCMs were used to simulate ACE-2 case studies of clean and polluted cloudy boundary layers, with the objective being to identify limitations of the aerosol/cloud/radiation interaction schemes within the range of uncertainty in in situ, reanalysis and satellite retrieved data. The exercise proceeds in three steps. First, SCMs are configured with the same fine vertical resolution as the ACE-2 in situ data base to evaluate the numerical schemes for prediction of aerosol activation, radiative transfer and precipitation formation. Second, the same test is performed at the coarser vertical resolution of GCMs to evaluate its impact on the performance of the parameterizations. Finally, SCMs are run for a 24–48 hr period to examine predictions of boundary layer clouds when initialized with large-scale meteorological fields. Several schemes were tested for the prediction of cloud droplet number concentration (N). Physically based activation schemes using vertical velocity show noticeable discrepancies compared to empirical schemes due to biases in the diagnosed cloud base vertical velocity. Prognostic schemes exhibit a larger variability than the diagnostic ones, due to a coupling between aerosol activation and drizzle scavenging in the calculation of N. When SCMs are initialized at a fine vertical resolution with locally observed vertical profiles of liquid water, predicted optical properties are comparable to observations. Predictions however degrade at coarser vertical resolution and are more sensitive to the mean liquid water path than to its spatial heterogeneity. Predicted precipitation fluxes are severely underestimated and improve when accounting for sub-grid liquid water variability. Results from the 24–48 hr runs suggest that most models have problems in simulating boundary layer cloud morphology, since the large-scale initialization fields do not accurately reproduce observed meteorological conditions. As a result, models significantly overestimate optical properties. Improved cloud morphologies were obtained for models with subgrid inversions and subgrid cloud thickness schemes. This may be a result of representing subgrid scale effects though we do not rule out the possibility that better large-forcing data may also improve cloud morphology predictions.