Marine stratocumulus clouds cover extensive areas of the subtropical oceans and greatly influence Earth?s radiation by strongly reflecting the incoming solar radiation. The most climatologically pronounced stratus regime is located in the South-East Pacific. Drizzle is one of the several physical processes that affects the lifecycle and evolution of marine stratus by depleting the cloud liquid water and by stabilizing the marine boundary layer through evaporative cooling. In this study we use ship-borne radar observations from two innovative research cloud radars ? a Millimeter Cloud Radar (MMCR) (lambda=8 mm) and Frequency Modulated Continuous Wave (FMCW) radar (lambda=3 mm) to study the fallout of drizzle in the sub-cloud layer. Radar inter-comparison is used to perform calibration and quality control of the FMCW radar. The FMCW observations suffer no saturation and provide profiles of radar Doppler moments from the ship level to the cloud base. A lognormal drizzle drop size distribution is assumed and the parameters (N0, r0 and sigma x) are retrieved using the observed radar reflectivity and mean Doppler velocity profiles. The retrieved parameters are used to extract bulk parameters of the drizzle size distribution such as liquid water content, total number of droplets and rainfall rates at various heights within the sub-cloud layer (typically from 50-500 m). It is demonstrated that a simple evaporation model can be used to constrain the inversion from radar observables to drizzle size distribution parameters. The model output showed that the drizzle DSD is truncated at lower end due to the rapid evaporation of smaller drops and the logarithmic width of drizzle DSD is lower than the typically prescribed value of 0.35.

Clouds are one of the major uncertainties in climate change predictions caused by their complex structure and dynamics. Numerous cloud processes are acting from cloud-scale down to mm-scale and interplay with each other as well as with atmospheric processes. This complexity on the one hand and the high spatial resolution required to analyse the small scale processes on the other hand cause difficulties in cloud research. One important and until now insufficiently understood process in cloud microphysics is the entrainment process. It defines the turbulent transport of sub-saturated environmental air into the cloud region. Subsequent mixing leads to the evaporation of cloud droplets resulting in negatively buoyant air at cloud edge. One distinguishes between two types of entrainment processes: cloud top and lateral entrainment. While the first type is mostly detected at the top of stratiform clouds, lateral entrainment plays an important role for the dynamics of cumulus clouds. Within in this thesis, highly-resolved measurements with a resolution down to the centimeter scale performed with the helicopter-borne measurement payload ACTOS (Airborne Cloud Turbulence Observation System) are used to study both types of entrainment processes. Shear-induced cloud top entrainment leads to a turbulent inversion layer (TIL) atop of a stratocumulus layer consisting of clear air. The TIL seems to be coupled with the underlying cloud layer due to the turbulence intensity. With increasing thickness of the TIL the turbulence inside is damped monotonically leading to a maximum layer thickness and inhibiting direct mixing between cloud top and free troposphere. At the edges of shallow trade wind cumuli, shear-induced lateral entrainment generates a subsiding shell. Its evolution is analysed based on detailed measurements in continuously developing shallow cumuli. With the cloud evolution, the subsiding shell grows at the expense of the cloud core region and an increasing downdraft velocity is observed within this region. These observations are confirmed with the simulation of an idealised subsiding shell. The results present unique observations at the edges of clouds and are an appreciable progress in cloud research which decisively influence future research.

Boundary layer (BL) stratocumulus clouds are an important factor in the earth's radiation budget due to their high albedo and low cloud top heights. Continental BL stratocumulus clouds are closely coupled to the diurnal cycle and the turbulence in the BL affecting the surface energy and moisture budgets. In this study the turbulence and mass-transport structures in continental BL stratocumulus clouds are studied using data from the Atmospheric Radiation Measurements (ARM)'s Southern Great Plains (SGP) observing facility located at Lamont, Oklahoma. High temporal (4 sec) and spatial (45 m) resolution observations from a vertically pointing 35 GHz cloud Doppler radar were used to obtain the in-cloud vertical velocity probability density function (pdf) in the absence of precipitation size hydrometeors. A total of 70 hours of radar data were analyzed to report halfhourly statistics of vertical velocity variance, skewness, updraft fraction, downdraft and velocity binned mass-flux at five cloud depth normalized levels. The variance showed a general decrease with increase in height in the cloud layer while the skewness is weakly positive in the cloud layer and negative near cloud top. The updraft fraction decreases with height with the decrease mainly occurring in the upper half of the cloud layer. The downdraft fraction increases with decrease in height with the increase being almost linear. The velocity of eddies responsible for maximum mass-transport decreases from of 0.4 ms-1 near cloud base to 0.2 ms-1 near cloud top. The half-hour periods were then classified based on the surface buoyancy flux as stable or unstable and it was found that the variance near cloud top is higher during the stable periods as compared to the unstable periods. Classification was also made based on the cloud depth to BL depth ratio (CBR) being greater or less than 0.3. The variance profile was similar for the classification while the skewness was almost zero during periods with CBR less 0.3 and positive during periods with CBR greater than 0.3. A 14 hour period of stratocumulus cloud on March 25, 2005 was analyzed to study the diurnal changes in the turbulence structure and mass transports. The variance near cloud base during the day time when the BL turbulence is primarily due to surface buoyancy production was higher than during the nighttime when the BL turbulence is driven by radiative cooling near the cloud top. Output from a one dimensional radiative transfer model was analyzed to study the vertical structure of the radiative fluxes. A radiative velocity scale analogous to the surface convective velocity scale is proposed to assess the relative importance of radiative cooling near cloud top in generating turbulence compared with the surface buoyancy production. An attempt was also made to calculate the hourly liquid water flux by combining the high temporal resolution (20 sec) liquid water content estimates from the radar reflectivity and a microwave radiometer with the radar observed vertical velocity. The liquid water flux was found to peak at a level below the cloud top and show a divergence with height that was similar to that from model simulations.

Clouds are one of the major uncertainties in climate change predictions caused by their complex structure and dynamics. Numerous cloud processes are acting from cloud-scale down to mm-scale and interplay with each other as well as with atmospheric processes. This complexity on the one hand and the high spatial resolution required to analyse the small scale processes on the other hand cause difficulties in cloud research. One important and until now insufficiently understood process in cloud microphysics is the entrainment process. It defines the turbulent transport of sub-saturated environmental air into the cloud region. Subsequent mixing leads to the evaporation of cloud droplets resulting in negatively buoyant air at cloud edge. One distinguishes between two types of entrainment processes: cloud top and lateral entrainment. While the first type is mostly detected at the top of stratiform clouds, lateral entrainment plays an important role for the dynamics of cumulus clouds. Within in this thesis, highly-resolved measurements with a resolution down to the centimeter scale performed with the helicopter-borne measurement payload ACTOS (Airborne Cloud Turbulence Observation System) are used to study both types of entrainment processes. Shear-induced cloud top entrainment leads to a turbulent inversion layer (TIL) atop of a stratocumulus layer consisting of clear air. The TIL seems to be coupled with the underlying cloud layer due to the turbulence intensity. With increasing thickness of the TIL the turbulence inside is damped monotonically leading to a maximum layer thickness and inhibiting direct mixing between cloud top and free troposphere. At the edges of shallow trade wind cumuli, shear-induced lateral entrainment generates a subsiding shell. Its evolution is analysed based on detailed measurements in continuously developing shallow cumuli. With the cloud evolution, the subsiding shell grows at the expense of the cloud core region and an increasing downdraft velocity is observed within this region. These observations are confirmed with the simulation of an idealised subsiding shell. The results present unique observations at the edges of clouds and are an appreciable progress in cloud research which decisively influence future research.

The marine stratocumulus clouds are highly sensitive to aerosol perturbations. In this study, we have explored the cloud susceptibility to aerosol using satellite observation and multi-model simulations over the Southeast Pacific Ocean (SEP). The climatology of satellite observation indicates that SEP is a relatively clean area with low aerosol optical depth (AOD). The SEP is a region of marine stratocumulus deck with cloud fraction (CF) reaching as high as 90% in many regions, with relatively low (140 cm−3) cloud droplet number concentration (CDNC) over the marine environment, and it increases as it moves towards the coast. The joint histogram analysis shows that the AOD-CDNC relation shows positive sensitivity and a non-linear CDNC-LWP (liquid water path) relationship; however, a negative sensitivity is dominant. The multimodel analysis shows that most models have a strong positive AOD-CDNC sensitivity, suggesting that the cloud albedo effect leads to net cooling. The general circulation models (GCM) reveal a negative radiative forcing (-0.28 to -1.36Wm−2) at the top of the atmosphere (TOA) when using the flux method. It supports the positive AODCDNC sensitivity and the resulting negative radiative forcing in GCMs. However, the CDNC-LWP shows a diverse relation in the models. In the GCMs, the effect of cloud microphysics is not considered while estimating the net radiative forcing. To include the effect of cloud microphysics in the radiative forcing estimates, we have proposed a statistical approach to calculate the net radiative forcing. The results show that the net radiative forcing is sensitive to the LWP change due to the aerosol perturbation. / Die marinen Stratocumulus-Wolken reagieren sehr empfindlich auf Aerosol-Störungen. In dieser Studie haben wir die Anfälligkeit der Wolken für Aerosol anhand von Satellitenbeobachtungen und Multi-Modellsimulationen über dem Südostpazifik (SEP) untersucht. Die Klimatologie der Satellitenbeobachtung zeigt, dass der SEP ein relativ sauberes Gebiet mit geringer Aerosol optischer Dicke (AOD) ist. Der SEP ist eine Region mit mariner Stratocumulus-Decke mit einer Wolkbedeckungsgrad (CF), der in vielen Regionen bis zu 90% erreicht, mit einer relativ niedrigen (140 cm−3) Wolkentröpfchenanzahlkonzentration (CDNC) über der marinen Umgebung, und sie nimmt in Richtung Küste zu. Die gemeinsame Histogramm-Analyse zeigt, dass die AOD-CDNC-Beziehung eine positive Sensitivität und eine nicht-lineare CDNC-LWP-Beziehung (Flüssigwasserpfad) aufweist; allerdings ist eine negative Sensitivität vorherrschend. Die Multi-Modellanalyse zeigt, dass die meisten Modelle eine stark positive AOD-CDNC-Empfindlichkeit aufweisen, was darauf hindeutet, dass der Wolkenalbedo-Effekt eine Nettokühlung bewirkt. Die allgemeinen Zirkulationsmodelle (GCM) zeigen einen negativen Strahlungsantrieb (-0,28 bis -1,36Wm−2) am Oberrand der Atmosphäre (TOA), wenn die Flussmethode verwendet wird. Dies unterstützt die positive AOD-CDNC-Empfindlichkeit und den daraus resultierenden negativen Strahlungsantrieb in GCMs. Der CDNC-LWP zeigt jedoch unterschiedliche Abhängigkeiten in den Modellen. In den GCMs wird die Wirkung der Wolkenmikrophysik bei der Abschätzung des Netto-Strahlungsantriebs nicht berücksichtigt. Um die Auswirkungen der Wolkenmikrophysik auf den Strahlungsantrieb einzubeziehen, haben wir einen statistischen Ansatz zur Berechnung des Nettostrahlungsantriebs gewählt. Die Ergebnisse zeigen, dass der Nettostrahlungsantrieb empfindlich auf die LWP-Änderung durch die Aerosolstörung reagiert.

In this study, four and a half months of ceilometer data from Östergarnsholm are used to analyze cloud related to processes in the boundary layer. Measurements are divided into two categories, which are defined by wind direction: a continental and a marine sector. The results show that there are significant differences in the height of the lowest cloud bases detected for the two sectors, where cloud base heights are lower for the marine wind sector during unstable and neutral conditions. The ceilometer’s ability to detect several cloud base heights simultaneously is utilized to test whether a double layer structure (DLS) can be detected. The results of this particular analysis are inconclusive as to whether a DLS has been observed or not. Detected cloud base heights differ greatly from heights suggested by the lifting condensation level (LCL). A new empirical formula for lowest cloud base height is then derived using the measurements. The Ceilometer’s estimations of sky cover are assessed to be of reasonable quality. This is suggested by computed high correlation with incoming shortwave radiation at noon for three months.  However, histograms of cloud cover measurements suggest that the ceilometer tends to probably either overestimate or underestimate cloud cover.  Large differences in cloud cover were observed for the two wind sectors during unstable conditions. For the months of July and August, a diurnal cycle in cloud cover for the continental wind sector was observed which suggest the presence of Stratocumulus. Measurements performed during upwelling conditions closely resemble those of the marine wind sector performed during stable conditions.

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.

L'aérosol anthropique peut affecter le cycle de vie des nuages de couche limite, en modifiant l'albédo du nuage et en inhibant la formation des précipitations. Cependant, il est difficile de quantifier ces impacts à partir d'observations. Les interactions aérosol-nuages de couche limite sont donc étudiées à l'aide des modèles numériques à résolution fine (LES), qui disposent de paramétrisations détaillées de la turbulence, du transfert radiatif et de la microphysique nuageuse. Dans cette étude, nous examinons plus spécifiquement les impacts de l'aérosol sur le cycle diurne de stratocumulus marins. Des simulations LES d'un cycle de 36 heures sont réalisées pour des concentrations d'aérosol typiques de masses d'air pures et polluées. Partant d'un même état initial, les simulations divergent rapidement. L'augmentation de la concentration des noyaux de condensation nuageux conduit à une augmentation de la concentration des gouttelettes, à une diminution de leur diamètre, et ainsi à l'inhibition de leur sédimentation et de la précipitation de bruine. Le contenu en eau liquide au sommet nuage augmente et l'entraînement sommital est renforcé. De plus, l'absorption du rayonnement solaire à la base du nuage n'est plus compensée par l'évaporation de la bruine et le découplage de la couche nuageuse est renforcé. Dans l'ensemble, la couche limite polluée est mieux couplée la nuit et plus découplée le jour que la couche limite marine pure. Ces simulations permettent d'identifier des signatures mesurables de l'impact des aérosols sur la dynamique de la couche limite, et ainsi de mieux concevoir les expériences de terrain qui visent à quantifier ces impacts.

Auf der Azoreninsel Graciosa, etwa 1600 km westlich von Portugal, betreibt das U.S. Department of Energy eine American Radiation Measurement (ARM) Station, welche mit einer Vielzahl von Messgeräten ausgestattet ist. Durch ihre Lage im subtropischen Hochdruckgürtel eignet sich diese Station gut zur Erforschung von mariner Stratocumulusbewölkung. Für eine Klimatologie der Eigenschaften dieser Bewölkung wurden Messwerte von Radiosonden, einem Ceilometer, einem Distrometer und einem Mikrowellenradiometer aus dem Zeitraum vom 01. März 2014 bis 31. August 2018 verwendet. Daraus wurde der Tages- und Jahresgang der Wolkenhöhe und -dicke, des Flüssigwasserpfades, des Entkopplungs- und Niederschlagsverhaltens und der Inversionsstärke abgeleitet. Die Auswertung basiert auf einer Methode von Zhang et al. (2010) zur Detektion von Wolkenschichten aus Radiosondenmessungen, die für die Abgrenzung mariner Stratocumulusbewölkung von anderen Wolkenarten angepasst und anschließend mit den Messungen eines Ka-Band-Wolkenradars evaluiert wurde. Während die Parameter im Tagesgang nur geringe Änderungen zwischen 00 UTC und 12 UTC aufwiesen, konnten im Jahresgang deutliche Variationen bei Wolkenhöhe, Niederschlagsverhalten, Entkopplung und Inversionstärke gezeigt werden. Die beobachteten Variationen wurden mit den Jahresgängen der solaren Einstrahlung, der Temperatur sowie der synoptischen Variabilität über den Azoren verglichen. / The U.S. Department of Energy operates an American Radiation Measurement (ARM) station on Graciosa (Azores), an island located approximately 1600 km west of Portugal. Due to the wide variety of installed measuring devices and its location in the subtropical belt of high pressure the station is well suited for studying marine stratocumulus. To derive a climatology of the properties of these clouds, data from radiosondes, a ceilometer, a disdrometer and a microwave radiometer were used for the period from 1st March 2014 to 31st August 2018 to get the daily and annual cycle of cloud height and thickness, liquid water path, precipitation, inversion strength and the presence of decoupling. The evaluation is based on a method developed by Zhang et al. (2010) to detect cloud layers from moisture profiles in the sounding data. The method was adapted for the distinction between marine stratocumulus and other cloud types. For the evaluation of the method data from a Ka-band cloud radar were used. While only minor changes occurred in the daily variability of the properties, a clear annual cycle was observed for cloud height, precipitation, inversion strength and decoupling of the stratocumulus. The observed variations were compared with the annual cycles of solar irradiance, temperature and the synoptic patterns over the Azores.

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第13108号 / 論理博第1556号 / 新制||理||1620(附属図書館) / (主査)准教授 重 尚一, 教授 余田 成男, 教授 石川 裕彦 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DGAM