Download or read book Aerosol cloud precipitation Interaction in Ultraclean Layers and Optically Thin Veil Cloud System in the Stratocumulus to Cumulus Transition written by Kuan-Ting O and published by . This book was released on 2019 with total page 114 pages. Available in PDF, EPUB and Kindle. Book excerpt: Recent observational studies have shown that ultraclean layers (UCLs) and optically thin veil clouds associated with precipitating deep cumulus are common features of the marine boundary layer in the stratocumulus to cumulus transition. The very low number concentration of cloud droplet and cloud condensation nuclei in UCLs, strong precipitation in the associated cumulus, together with the low optical thickness of optically thin veil clouds, make such a system particularly appealing for the study of aerosol-cloud-precipitation interactions. More importantly, low cloud radiative properties biases (i.e., too few, too bright low cloud bias) in the current generation of global climate models (GCMs) seems strongly associated with the uncertainty in representing optically thin veil clouds, and these clouds may serve as an important constraint on the too few, too bright problem. However, systematic investigation of (1) global contribution and seasonal variability of optically thin veil clouds and (2) aerosol-cloud-precipitation interactions in UCLs and optically thin veil clouds is still lacking. We aim to investigate these problems with aircraft remote sensing, satellite measurements and a cloud resolving model. The dissertation is organized into the following three sections: • Using high resolution aircraft remote sensing measurement to characterize optically thin veil clouds in the stratocumulus to cumulus transition (SCT): Aircraft remote sensing measurements (i.e., lidar and radar) taken abroad NSF/NCAR GV-HIAPER research flights flown during the Cloud System Evolution in the Trades field campaign (CSET) sampled marine air masses between Sacramento, California (38.68N, 121.58W), and Kona (19.68N, 156.08W) are used in our study. Optically thin veil clouds, defined as the subset of low clouds with cloud bases > 1 km that do not fully attenuate high-spectral-resolution lidar signal (HSRL) (i.e., indicating optical depths 3), comprise considerable cover of low clouds (~ 40 %) over the SCT. It is found that optically thin veil clouds are also geometrically thin with cloud thickness ~ 200 m, and commonly reside in the upper boundary layer with average cloud base 1.5 km. • Investigating deeper, precipitating PBLs associated with optically thin veil clouds in the Sc-Cu Transition using spaceborne satellite measurements: Variability and vertical structure of optically thin veil clouds over SCT regions around the globe are investigated using both passive and active satellite observations. These observations reveal pronounced relationships between optically thin veil clouds, strong precipitation, deep planetary boundary layer (PBL) height and low cloud droplet number concentration (CDNC). The results are in agreement with the hypothesis that the low optical thickness of veil clouds over the SCT is contingent on the low CDNC caused by strong precipitation scavenging occurring in active cumuli, a process whose efficiency is strongly dependent on maximum condensate amount in updrafts and thus is highly constrained by PBL height. • Exploring aerosol-cloud-precipitation processes in UCLs and optically thin veil clouds system using a cloud resolving model: Characteristics of UCLs and optically thin veil clouds are investigated in the cloud resolving model (CRM). The domain mean cloud and aerosol properties in UCLs and optically thin veil clouds from CRM simulations agree with recent observational studies in general. The simulation results show that the detrainment from active precipitating cumulus produces the stratiform veil clouds, which are strongly depleted in particle concentration due to very efficient coalescence-scavenging process in ascending parcels passing through cumulus towers. The simulation shows a median CDNC in thin veil clouds of 5.8 cm−3, implying that majority of thin veil clouds are UCLs as well and confirming the strong connection between veil clouds and UCLs. In addition, there is a strong correlation between surface precipitation and the fraction of low clouds that are UCLs, and such correlation implies the importance of precipitation scavenging for the formation of UCLs. A cloud resolving model coupled with a prognostic aerosol scheme is used in our study, enabling characterization of the spatiotemporal variability of aerosol in the boundary layer. The results show that depletion of aerosol concentration starts first in the upper boundary layer that is associated with in-cloud coalescence scavenging process. The evaporation of veil clouds leaves very low CCN number concentration (Na

Download or read book Ultraclean Layers and Optically Thin Clouds in the Stratocumulus to Cumulus Transition written by Kuan-Ting O and published by . This book was released on 2017 with total page 39 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Aerosol Cloud Interactions from Urban Regional to Global Scales written by Yuan Wang and published by Springer. This book was released on 2015-05-05 with total page 100 pages. Available in PDF, EPUB and Kindle. Book excerpt: The studies in this dissertation aim at advancing our scientific understandings about physical processes involved in the aerosol-cloud-precipitation interaction and quantitatively assessing the impacts of aerosols on the cloud systems with diverse scales over the globe on the basis of the observational data analysis and various modeling studies. As recognized in the Fifth Assessment Report by the Inter-government Panel on Climate Change, the magnitude of radiative forcing by atmospheric aerosols is highly uncertain, representing the largest uncertainty in projections of future climate by anthropogenic activities. By using a newly implemented cloud microphysical scheme in the cloud-resolving model, the thesis assesses aerosol-cloud interaction for distinct weather systems, ranging from individual cumulus to mesoscale convective systems. This thesis also introduces a novel hierarchical modeling approach that solves a long outstanding mismatch between simulations by regional weather models and global climate models in the climate modeling community. More importantly, the thesis provides key scientific solutions to several challenging questions in climate science, including the global impacts of the Asian pollution. As scientists wrestle with the complexities of climate change in response to varied anthropogenic forcing, perhaps no problem is more challenging than the understanding of the impacts of atmospheric aerosols from air pollution on clouds and the global circulation.

Download or read book Aerosol Cloud Climate Interactions written by Peter V. Hobbs and published by Academic Press. This book was released on 1993-07-22 with total page 259 pages. Available in PDF, EPUB and Kindle. Book excerpt: Aerosol and clouds play important roles in determining the earth's climate, in ways that we are only beginning to comprehend. In conjunction with molecular scattering from gases, aerosol and clouds determine in part what fraction of solar radiation reaches the earth's surface, and what fraction of the longwave radiation from the earth escapes to space. This book provides an overview of the latest research on atmospheric aerosol and clouds and their effects on global climate. Subjects reviewed include the direct and indirect effects of aerosol on climate, the radiative properties of clouds and their effects on the Earth's radiation balance, the incorporation of cloud effects in numerical weather prediction models, and stratospheric aerosol and clouds.

Download or read book Aerosol Cloud Precipitation Interactions in the Trade Wind Boundary Layer written by Eunsil Jung and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation includes an overview of aerosol, cloud, and precipitation properties associated with shallow marine cumulus clouds observed during the Barbados Aerosol Cloud Experiment (BACEX, March-April 2010) and a discussion of their interactions. The principal observing platform for the experiment was the Cooperative Institute for Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft that was equipped with aerosol, cloud, and precipitation probes, standard meteorological instruments, and a up-looking cloud radar. The temporal variations and vertical distributions of aerosols observed on the 15 flights show a wide range of aerosol conditions that include the most intense African dust event observed at the Barbados surface site during all of 2010. An average CCN varied from 50 cm-3 to 800 cm-3 at super-saturation of 0.6 %, for example. The 10-day backward trajectories show that three distinctive air masses (originality of air mass as well as the vertical structure) dominate over the Eastern Caribbean (e.g., typical maritime air mass, Saharan Air Layer (SAL), Middle latitude dry air) with characteristic aerosol vertical structures. Many clouds in various phases of growth during BACEX are sampled. The maximum cloud depth observed is about less than 3 km and in most of the clouds is less than 1 km. Two types of precipitation features were observed for the shallow marine cumulus clouds with different impacts on boundary layer. In one, precipitation shafts are observed to emanate from the cloud base with evaporation in the sub-cloud layer (stabilize the sub-cloud layer). In the other, precipitation shafts emanate mainly near the cloud top on the downshear side of the cloud and evaporate in the cloud layer, leading to destabilizing the cloud layer and providing moisture to the layer. Only 42-44 % of clouds sampled were purely non-precipitating throughout the clouds; the remainder of the clouds showed precipitation somewhere in the cloud, predominantly closer to the cloud top. The relationship between aerosol (Na), cloud droplets (Nd), and precipitation rates (R) is addressed to explore aerosol-cloud-precipitation interactions. A robust increase in Nd with increase in aerosol concentrations is documented. Further, a strong linear relation between sub-cloud CCN and cloud-base Nd is observed in updrafts. The sensitivity of Nd to changes in vertical velocity perturbations w ́ (i.e., dlnNd /dlnw ́), is greater in the regimes of high aerosol concentrations, suggesting a slight increase in updrafts (or w ́) in polluted conditions can lead to greater increases in Nd. Suppression of precipitation with aerosol is a common feature during BACEX. To quantify this decrease of precipitation toward higher aerosol concentration, the sensitivity of precipitation to changes in aerosol (i.e., precipitation susceptibility S0 ) is examined. S0 exhibits three regimes and peaks at intermediate range of cloud thickness. Further, the removal of Nd , due to the rain (wet scavenging), makes susceptibility stronger overall. In addition to the aerosol feeding clouds from the sub-cloud layer, small cumuli can alter the aerosol properties of their immediate environment through cloud and precipitation processes. In the warm cumuli studied, the depletion of aerosols near cloud field (so-called cloud halos/shell regimes) are notable, and the reduction of aerosols is more significant in precipitating clouds compared with non-and/or light-precipitating clouds. The modification of boundary layer aerosol by cloud processes is also explored. The comparisons of the thermodynamic structures observed over Africa with those at Barbados indicate that layers below the SAL are moistened by surface fluxes and convective processes as the air masses are advected across the Atlantic over 7-10 days.

Download or read book Aerosol Cloud Interactions written by Udaya Bhaskar Gunturu and published by . This book was released on 2010 with total page 184 pages. Available in PDF, EPUB and Kindle. Book excerpt: (cont.) low or high, they are comparatively less efficient. As the most important part of our study, we examine the response of deep convection to changing initial aerosol concentration. Different aerosol concentrations from those representing pristine to polluted atmospheres are considered. We look at the buoyancy of the cloud and the microphysical evolution. It is found that the dynamics and microphysics are tightly coupled and we infer that to understand aerosol-cloud interactions in deep convective clouds, both - dynamics and microphysics - and their interaction have to be taken into consideration. Our results show that the response of a deep convective cloud to changing aerosol concentration is very different from the much well understood reponse of shallow clouds or small cumulus clouds. In general, increase in aerosol concentratin is seen to invigorate convection and lead to greater condensate. Although the cloud droplet size decreases, collision-coalescence is not completely inefficient. The precipitation in high aerosol regime is seen to occure in short spells of intense rain. A very interesting anomalous response of deep convection to initial aerosol concentration is observed at intermediate aerosol concentrations. The cloud lifetime, and precipitation are seen to increase in this regime. A possible mechanism to explain this anomalous behavior is proposed and the available circumstantial support for the mechanism from extant observations is presented. It is proposed that the efficient collection of rain and cloud droplets by ice and graupel particles in the middle troposphere is primarily responsible for this increased cloud lifetime and precipitation.

Download or read book An Airborne Remote Sensing Perspective on Cloud and Precipitation Properties from Southeast Atlantic Stratocumulus Clouds written by Andrew Michael Dzambo and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Stratocumulus (StCu) clouds cover a majority of the Earth's subtropical oceanic basins, and play an important role in the global energy balance. Cloud and precipitation processes in StCu are complex, and aerosol effects add further complexity to the cloud-precipitation-climate paradigm, where these interactions are among the most widely uncertain processes in present-day climate models. The NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign between 2016-18 observed cloud-aerosol-precipitation interactions over the Southeast Atlantic Ocean. One of the 20+ instruments deployed during ORACLES was the Airborne Precipitation Radar - 3rd Generation (APR-3). The APR-3 collected over 18 million profiles during the three deployments. A precipitation retrieval algorithm (called 2C-RAIN) was adapted from the CloudSat 2C-RAIN-PROFILE precipitation retrieval algorithm to meet ORACLES science objectives. The majority of 2C-RAIN precipitation rates were under 0.01 mm/hr (0.25 mm/day). The sampling environments were considerably different in 2016 compared to 2017 and 2018, necessitating further investigation accounting for environmental controls. Cloud water path (CWP) retrievals were added to the 2C-RAIN algorithm. This retrieval expanded the utility of APR-3 measurements by collocating cloud and precipitation properties (namely CWP and RWP) for the investigation of aerosol indirect effects. This work find typical CWP to RWP ratios on the order of 50:1 to 200:1, implying CWP dominates the total liquid water path (LWP) signal. When partitioning rain rates with CWP and RWP for aerosol contact and non-contact cases, statistically significant differences are found in stable environments for CWP/RWP but not for retrieved rain rates, likely owing to the 100% and larger uncertainties associated with precipitation rate retrievals. Finally, evaporation processes are investigated between drizzling virga and surface precipitation. Evaporation rates/fluxes and corresponding latent cooling rates, between surface precipitation and virga, are on the order of 2:1 implying that surface precipitation contributes the most latent cooling to the local environment. Evaporating virga, regardless, cannot be ignored when studying latent heating and cooling. The development of the 2C-RAIN database for ORACLES, and analyses presented here, pave the way for additional observation-based studies in an area where satellite measurements have limited viability.

Download or read book Aerosol Cloud Interactions in Southeast Pacific Stratocumulus written by Rhea George and published by . This book was released on 2013 with total page 145 pages. Available in PDF, EPUB and Kindle. Book excerpt: The influence of anthropogenic aerosols on cloud radiative properties in the persistent southeast Pacific stratocumulus deck is investigated using MODIS satellite observations, in situ data from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), and WRF-Chem, a regional model with interactive chemistry and aerosols. An albedo proxy is derived based on the fractional coverage of low cloud (a macrophysical field) and the cloud albedo, with the latter broken down into contributions from microphysics (cloud droplet concentration, N[subscript d] and macrophysics (liquid water path). Albedo variability is dominated by low cloud fraction variability, except within 10-15° of the South American coast, where cloud albedo variability contributes significantly. Covariance between cloud fraction and cloud albedo also contributes significantly to the variance in albedo, which highlights how complex and inseparable the factors controlling albedo are. N[subscript d] variability contributes only weakly, which emphasizes that attributing albedo variability to the indirect effects of aerosols against the backdrop of natural meteorological variability is extremely challenging. Specific cases of aerosol changes can have strong impacts on albedo. We identify a pathway for periodic anthropogenic aerosol transport to the unpolluted marine stratocumulus>1000 km offshore, which strongly enhances N[subscript d] and albedo in in zonally-elongated `hook'-shaped arc. Hook development occurs with N[subscript d] increasing to polluted levels over the remote ocean primarily due to entrainment of a large number of small aerosols from the free troposphere that contribute a relatively small amount of aerosol mass to the marine boundary layer. Strong, deep offshore flow needed to transport continental aerosols to the remote ocean is favored by a trough approaching the South American coast and a southeastward shift of the climatological subtropical high pressure system. DMS significantly influences the aerosol number and size distributions, but does not cause hooks. The Twomey effect contributes 50-80% of the total aerosol indirect effect (AIE) both near sources and offshore during hook events. Meteorological variability between simulations can swamp the signal of AIEs, particularly due to the binary model cloud fraction field and distinguishing AIE requires determination of appropriate spatial and temporal averaging scales over which AIE is significant above this noise.

Download or read book Aerosol Cloud Precipitation Interactions in Moist Orographic Flows written by Andreas Mühlbauer and published by Sudwestdeutscher Verlag Fur Hochschulschriften AG. This book was released on 2009 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Aerosols are ubiquitous in the Earth's atmosphere and influence the climate system through their interactions with clouds and radiation. With their ability to serve as cloud condensation nuclei and ice nuclei aerosols influence microphysical processes in clouds thereby potentially affecting precipitation. In this book the possible effects of aerosols on orographic precipitation are investigated with a numerical model.

Download or read book Physics of Precipitation written by Helmut Weickmann and published by American Geophysical Union. This book was released on 1960 with total page 449 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Cumulus Dynamics written by Chao Jih-Ping and published by . This book was released on 1969 with total page 152 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Interaction Between Aerosol Particles and Stratocumulus Clouds written by Szabo-Takacs Beata and published by LAP Lambert Academic Publishing. This book was released on 2011-12 with total page 84 pages. Available in PDF, EPUB and Kindle. Book excerpt: The aim of the paper was to develop a numerical model to study the interaction between aerosol particles and stratocumulus clouds in different air mass types. A detailed microphysical scheme was incorporated into an idealized two-dimensional kinematic model to investigate the role of the aerosol particles in the formation of the cloud, regeneration and the wash out of the aerosol particles. The calculations were made with different cloud condensation nuclei size distributions and concentrations typical for maritime, rural and remote continental air mass types, furthermore with two different updraft profiles. The water droplets were formed on soluble ammonium-sulfate aerosol particles. The drops grew by condensation and collision coalescence in the updraft core, but they evaporated due to the subsaturation in the downdraft region. The model clearly simulated the regeneration of the aerosol particles. The majority of the water soluble particles were washed out by nucleation scavenging. The efficiency of the scavenging of the water insoluble particles depended on the concentration of the water soluble aerosol particles. Results of the simulation show that in the case of the stratocum

Download or read book Proceedings written by and published by . This book was released on 2000 with total page 700 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Suppression of Marine Stratocumulus Clouds Due to Reduced Cloud Condensation Nuclei written by Neil T. Smith and published by . This book was released on 2000-09-01 with total page 68 pages. Available in PDF, EPUB and Kindle. Book excerpt: Cloud researchers have documented a variety of processes at work in the formation and dissipation of clouds in the marine boundary layer (MBL). Cloud rifts occasionally mark a distinct exception to the continuity and broad coverage more commonly observed with these clouds. A possible explanation for the presence of large features of broken cloudiness embedded in stratocumulus is the removal of CCN by nucleation scavenging and drizzle. A cloud rift feature embedded in marine stratocumulus was observed in satellite imagery on July 16, 1999. A CIRPAS Twin Otter aircraft flew repeated crossings of the rift boundary while completing a comprehensive survey of the area. A comparison of microphysics and thermodynamics on opposite sides of the rift boundary indicate that these rifts form where low aerosol concentrations enhance drizzle production. Marine boundary layer aerosol concentrations in the rift were only 1/6 that observed below the background stratocumulus. Cloud droplets in rift clouds were 3-5 microns larger than droplets in stratocumulus and exhibited a broader size distribution. Drizzle observations were strongly correlated with the rift and calculations support a drizzle hypothesis for rift formation and maintenance. Aerosol losses can be accounted for in drizzle droplets and the disruption of the cloud layer evolves in a manner described by Ackerman (1993).

Download or read book Aerosol cloud precipitation Interactions written by Edward Gryspeerdt and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book On the Evolution of Stratocumulus in the Subtropical Oceans written by Ryan Maxwell Eastman and published by . This book was released on 2021 with total page 138 pages. Available in PDF, EPUB and Kindle. Book excerpt: This work analyzes cloud processes on daily timescales in stratocumulus (Sc) cloud decks over eastern subtropical oceans. These clouds have a significant effect on climate. Subtropical Sc decks reflect abundant solar radiation, but emit infrared radiation at a temperature nearly as warm as the surface, causing a net cooling to the climate system. Climate models tend to poorly represent subtropical Sc, often producing too few clouds, which are too bright. The size and significance of these cloud decks combined with their poor representation in models motivates further research into how clouds are responding to their environment, and what internal processes drive observed behavior. To address this knowledge gap, a substantial Lagrangian framework is developed here, where thousands of cloudy parcels are followed for several days over the eastern subtropical oceans. These parcels are repeatedly sampled at 12-hour intervals, creating a large-scale dataset of environmental and cloud variables that can incorporate a time dimension with a variety of lead times. This framework is applied here in several ways: The first chapter incorporates the Lagrangian framework in two climate models as well as in observations for the same year. The model comparison shows which daily-scale cloud processes are well simulated and which are poorly simulated. The final two chapters address transitions in cellular structure in Sc cloud decks, first assessing which variables are associated with 24-hour transitions from classic, closed cell Sc to more broken open cells or disorganized cells, showing that transitions in cellularity are associated with separate mechanisms. In chapter 3, the results from chapter 2 are put in context using an expanded set of trajectories to study multi-step processes that incorporate interplay between environmental variables, cloud processes, and observed changes in cellular structure. The following paragraphs introduce the three chapters. A Lagrangian framework is developed and applied to two GCMs (CAM5 and HadGEM, a.k.a. UKMET) and an observational dataset in order to compare the daily-scale evolution of cloud cover and cloud variables between models and observations in the eastern subtropical oceans. Cloud cover in both models is less extensive than cloud cover seen by MODIS. Observed rain rates, as estimated by CloudSat-tuned AMSR/E 89 GHz brightness temperatures, is comparable to the HadGEM rain rates, while CAM5 rain rates appear too heavy. Inversion height estimated by MODIS and CALIPSO-observed cloud tops falls between the too-shallow CAM5 inversion and the too-deep HadGEM inversion. Lagrangian decorrelation timescales are similar in the modeled and observed environments, with e-folding times on the order of 12-36 hours for most cloud variables, shorter for cloud water path and cloud cover. Predictor variables, both meteorological and internal to the boundary layer, are tested as drivers of changes to cloud variables. Increased subsidence is modeled and observed to decrease cloud cover, inversion height, precipitation, and cloud water content. Modelled clouds tended to be oversensitive to changes in SST, while cloud microphysical and precipitation processes were poorly simulated by both models, indicating a need for improvement for the simulation of these processes. In chapter 2, mesoscale cellular convection (MCC) is classified by daytime MODIS L2 cloud liquid water path in 256km square boxes spaced 128 km apart in stratocumulus decks in the southeastern subtropical oceans. A Lagrangian framework is applied to MCC observations taken 24 hours apart in order to assess meteorological conditions and cloud properties associated with transitions from closed cell MCC to open cells, or from closed cells to disorganized cells more akin to trade cumulus. Results suggest that higher rain rates, observed by the CloudSat-tuned AMSR/E 89 GHz brightness temperatures, are associated with the closed-to-open MCC transition along with reduced cloud drop concentration as seen by MODIS and strong wind speeds, sourced from the ERA5 reanalysis. Strongly contrasting with the closed-to-open MCC transition, the closed-disorganized MCC transition is associated with entrainment warming and drying in a rapidly deepening boundary layer, observed by CALIPSO-tuned MODIS cloud top temperatures. In the closed-disorganized transition, the boundary layer appears to deepen in response to declining subsidence and reduced humidity in the lower troposphere, as well as a warmer sea surface. In chapter 3 the ability of wind speed to induce the closed-to-open MCC transition in subtropical marine stratocumulus is further investigated. This Lagrangian framework is expanded to use trajectories that span 96 hours. A new analysis is created to assess the power of a variable to predict a 24-hour closed-to-open or closed-to-disorganized MCC transition relative to a closed-to-closed case. Predicting power is compared for a large set of variables at various lead times up to 72 hours prior to the MCC transition. Results show that strong wind speeds precede heavy drizzle as a predictor of the closed-to-open transition. Further Lagrangian analysis shows that strong winds are associated with heavier rain in closed cell Sc and increasing rain rates over the 12 hours past the strong wind observation. The positive relationship between wind and rain is explained by rearranging the relationship between latent heating, humidity, and wind speed from LHF ~ wind/RH to wind ~ RH x LHF. This means that in a capped boundary layer, wind speed represents a combination of the moisture flux and moisture content, so that wind is pumping moisture by evaporating seawater into a closed system, which drives increased rainfall. The rainfall can initiate the closed-to-open transition through cold-pool convergence processes that have been shown to sustain open cells in prior modeling work. The closed-to-open MCC transition is compared to the closed-to-disorganized transition: Two different systems emerge, where the closed-to-open transition occurs when the boundary layer is overloaded with moisture, while the closed-to-disorganized transition occurs when the boundary layer dries due to excess entrainment. These results indicate that closed cell stratocumulus clouds rely on a balance between moisture input from wind and entrainment drying from the incorporation of free-tropospheric air. Excesses of moisture or drying can break apart the closed cells.

Download or read book Investigation of the Cloud Microphysics and Albedo Susceptibility of the Southeast Pacific Stratocumulus Cloud Deck written by David Painemal and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.