Browsing by Author "Oreopoulos, Lazaros"
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Item A Deterministic Self-Organizing Map Approach and its Application on Satellite Data based Cloud Type Classification(IEEE, 2019-01-24) Zhang, Wenbin; Wang, Jianwu; Jin, Daeho; Oreopoulos, Lazaros; Zhang, ZhiboA self-organizing map (SOM) is a type of competitive artificial neural network, which projects the high dimensional input space of the training samples into a low dimensional space with the topology relations preserved. This makes SOMs supportive of organizing and visualizing complex data sets and have been pervasively used among numerous disciplines with different applications. Not withstanding its wide applications, the self-organizing map is perplexed by its inherent randomness, which produces dissimilar SOM patterns even when being trained on identical training samples with the same parameters every time, and thus causes usability concerns for other domain practitioners and precludes more potential users from exploring SOM based applications in a broader spectrum. Motivated by this practical concern, we propose a deterministic approach as a supplement to the standard self-organizing map.In accordance with the theoretical design, the experimental results with satellite cloud data demonstrate the effective and efficient organization as well as simplification capabilities of the proposed approach.Item ENSO Disrupts Boreal Winter CRE Feedback(AMS, 2023-12-27) Jin, Daeho; Kramer, Ryan J.; Oreopoulos, Lazaros; Lee, DongminTwenty years of satellite-based cloud and radiation observations allow us to examine the observed cloud radiative effect (CRE) feedback (i.e., CRE change per unit change in global mean surface temperature). By employing a decomposition method to separate the contribution of “internal changes” and “relative-frequency-of-occurrence (RFO) changes” of distinct cloud regime (CR) groups, notable seasonal contrasts of CRE feedback characteristics emerge. Boreal winter CRE feedback is dominated by the positive shortwave CRE (SWCRE) feedback of oceanic low-thick clouds, due to their decreasing RFO as temperature rises. This signal is most likely due to El Niño-Southern Oscillation (ENSO) activity. When ENSO signals are excluded, boreal winter CRE feedback becomes qualitatively similar to the boreal summer feedback, where several CR groups contribute to the total CRE feedback more evenly. Most CR groups’ CRE feedbacks largely come from changing RFO: e.g., the predominant transition from oceanic cumulus to broken clouds and more occurrences of higher convective clouds with warming temperature. At the same time, low-thick and broken clouds experience optical thinning and decreasing cloud fraction, and these features are more prominent in boreal summer than winter. Overall, the seasonally asymmetric patterns of CRE feedback, primarily due to ENSO, introduce complexity in assessments of CRE feedback.Item Evidence that deliberate marine cloud brightening can be more effective than previously thought(2023-09-22) Chen, Ying; Haywood, Jim; Wang, Yu; Malavelle, Florent; Jordan, George; Peace, Amy; Partridge, Daniel; Cho, Nayeong; Oreopoulos, Lazaros; Platnick, Steven; Grosvenor, Daniel; Field, Paul; Allan, Richard; Lohmann, UlrikeWith global warming currently standing at approximately + 1.2 °C, climate change is a pressing global issue. Marine cloud brightening (MCB) proposes injecting aerosols into marine clouds to enhance their reflectivity and thereby planetary albedo. However, because it is unclear how aerosols influence clouds, especially cloud cover, both climate projections and the effectiveness of MCB remain uncertain. Here, we use volcanic eruptions to quantify the aerosol fingerprint on tropical marine clouds. We observe a large enhancement in reflected sunlight, mainly due to an aerosol-induced increase in cloud cover. This observational evidence of a strong aerosol impact suggests that the Earth’s climate is highly sensitive to external forcing mechanisms, but also that mitigation of global warming via MCB is more plausible than current climate models suggest. Our results suggest that the best efficacy for MCB practice is to seed clouds in humid and stable meteorological conditions.Item Global Reduction in Ship-tracks from Sulfur Regulations for Shipping Fuel(2022-06-11) Yuan, Tianle; Song, Hua; Wood, Robert; Wang, Chenxi; Oreopoulos, Lazaros; Platnick, Steven; Hippel, Sophia von; Meyer, Kerry; Light, Siobhan; Wilcox, EricShip-tracks are produced by ship-emitted aerosols interacting with marine low clouds. Here we apply deep learning models on satellite data to produce the first multi-year global climatology map of ship-tracks. We show that ship-tracks are at the nexus of cloud physics, maritime shipping, and fuel regulation. Our map captures major shipping lanes while missing others, reflecting the influences of background cloud and aerosol properties. Ship-track frequency is more than 10 times higher than expected from a previous survey. Interannual fluctuations in ship-track frequency reflect variations in cross-ocean trade, shipping activity, and fuel regulations. Fuel regulation can alter both detected ship-track density and pattern of shipping routes due to cost economics. The new fuel regulation, together with the COVID-19 pandemic, reduced ship-track frequency in 2020 to its lowest level in recent decades across the globe and may have ushered in a new era of low ship-track frequency. We estimate the aerosol indirect forcing induced by the fuel regulation to be between 0.02 and 0.22 Wm-2.Item Identifying meteorological influences on marine low cloud mesoscale morphology using deep learning classifications(Copernicus Publications, 2020-11-03) Mohrmann, Johannes; Wood, Robert; Yuan, Tianle; Song, Hua; Eastman, Ryan; Oreopoulos, LazarosMarine low cloud mesoscale morphology in the southeastern Pacific Ocean is analyzed using a large dataset of machine-learning generated classifications spanning three years. Meteorological variables and cloud properties are composited by mesoscale cloud type, showing distinct meteorological regimes of marine low cloud organization from the tropics to the midlatitudes. The presentation of mesoscale cellular convection, with respect to geographic distribution, boundary layer structure, and large-scale environmental conditions, agrees with prior knowledge. Two tropical and subtropical cumuliform boundary layer regimes, suppressed cumulus and clustered cumulus, are studied in detail. The patterns in precipitation, circulation, column water vapor, and cloudiness are consistent with the representation of marine shallow mesoscale convective self-aggregation by large eddy simulations of the boundary layer. Although they occur under similar large-scale conditions, the suppressed and clustered low cloud types are found to be well-separated by variables associated with low-level mesoscale circulation, with surface wind divergence being the clearest discriminator between them, whether reanalysis or satellite observations are used. Clustered regimes are associated with surface convergence and suppressed regimes are associated with surface divergence.Item Large-scale convective systems identified by hybrid cloud-precipitation regimes and their modulations by MJO and QBO(2021-12-24) Jin, Daeho; Kim, Daehyun; Oreopoulos, LazarosTropical convective systems that grow larger than 100,000km2 sizes play a significant role in the water cycle and energy budget of the Earth system. Previously, we developed hybrid tropical cloud-precipitation regimes (TCPRs) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) cloud observations and Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation data at a 1° scale, and demonstrated that TCPRs enabled a simple but effective identification of convective systems at the synoptic scale. The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability, which is characterized as a planetary-scale envelop of convective clouds that propagates eastward over the Indo-Pacific warm pool. Recent studies showed a statistically robust correlation between the MJO and the quasi-biennial oscillation (QBO); MJO-related convective activities are enhanced and suppressed during an easterly and westerly phase of QBO, respectively. While the underlying mechanism of the MJO-QBO relationship has remained elusive, one of the most popular hypotheses is that the weakened stability in the upper troposphere and lower stratosphere during easterly QBO years provides a preferrable condition for deep convection to develop deeper and persist longer. To test the stability hypothesis for the QBO control on the MJO, we examine properties of the convective aggregates of TCPRs in the southern Maritime Continent region, in which the contrast in MJO activities between easterly and westerly QBO years is most pronounced. By taking advantage of TCPRs, we composite the total size, fractions of stratiform clouds to core area, and top height of core for different phases of MJO and QBO, and the results are compared to find any systematic difference in the characteristics of convective aggregates. Our results show that, as consistent to previous studies, bigger convective aggregates tend to occur when the stability weakens. Further insight will be obtained by examining cloud radiative effects and atmospheric energy budget per convective aggregates.Item A New Organization Metric for Synoptic Scale Tropical Convective Aggregation(AGU, 2022-07-27) Jin, Daeho; Oreopoulos, Lazaros; Lee, Dongmin; Tan, Jackson; Kim, Kyu-myongOrganization metrics were originally developed to measure how densely convective clouds are arranged at mesoscales. In this work, we apply organization metrics to describe tropical synoptic scale convective activity. Such activity is identified by cloud-precipitation (hybrid) regimes defined at 1-degree and 1-hourly resolution. Existing metrics were found to perform inadequately for such convective regime aggregates because the large domain size and co-existence of sparse aggregate occurrences with noisy isolated convection often violate assumptions inherent in these metrics. In order to capture these characteristics, the existing “convective organization potential” (COP) metric was modified so as to: (1) focus on local organization and (2) provide increased weight to aggregate size. The resulting “area-based COP” (ABCOP) is found to outperform existing metrics in tropical convective events at synoptic scales. Moreover, this new organization metric can match the performance of existing metrics, or arguably be better, over a wide range of domain sizes.Item Observational evidence of strong forcing from aerosol effect on low cloud coverage(AAAS, 2023-11-08) Yuan, Tianle; Song, Hua; Wood, Robert; Oreopoulos, Lazaros; Platnick, Steven; Wang, Chenxi; Yu, Hongbin; Meyer, Kerry; Wilcox, EricAerosols cool Earth’s climate indirectly by increasing low cloud brightness and their coverage (Cf), constituting the aerosol indirect forcing (AIF). The forcing partially offsets the greenhouse warming and positively correlates with the climate sensitivity. However, it remains highly uncertain. Here, we show direct observational evidence for strong forcing from Cf adjustment to increased aerosols and weak forcing from cloud liquid water path adjustment. We estimate that the Cf adjustment drives between 52% and 300% of additional forcing to the Twomey effect over the ocean and a total AIF of −1.1 ± 0.8 W m−². The Cf adjustment follows a power law as a function of background cloud droplet number concentration, Nd . It thus depends on time and location and is stronger when Nd is low. Cf only increases substantially when background clouds start to drizzle, suggesting a role for aerosol-precipitation interactions. Our findings highlight the Cf adjustment as the key process for reducing the uncertainty of AIF and thus future climate projections.Item QBO deepens MJO convection(Nature, 2023-07-10) Jin, Daeho; Kim, Daehyun; Son, Seok-Woo; Oreopoulos, LazarosThe underlying mechanism that couples the Quasi-Biennial Oscillation (QBO) and the Madden-Julian oscillation (MJO) has remained elusive, challenging our understanding of both phenomena. A popular hypothesis about the QBO-MJO connection is that the vertical extent of MJO convection is strongly modulated by the QBO. However, this hypothesis has not been verified observationally. Here we show that the cloud-top pressure and brightness temperature of deep convection and anvil clouds are systematically lower in the easterly QBO (EQBO) winters than in the westerly QBO (WQBO) winters, indicating that the vertical growth of deep convective systems within MJO envelopes is facilitated by the EQBO mean state. Moreover, the deeper clouds during EQBO winters are more effective at reducing longwave radiation escaping to space and thereby enhancing longwave cloud-radiative feedback within MJO envelopes. Our results provide robust observational evidence of the enhanced MJO activity during EQBO winters by mean state changes induced by the QBO.Item Seeking guidance from active cloud observations to improve climate model subcolumn generators(2022-11-16) Oreopoulos, Lazaros; Cho, Nayeong; Lee, Dongmin; Lebsock, Matthew; Zhang, ZhiboOur objective is to test and improve cloud subcolumn generators used for greater realism of scales in the radiation schemes and satellite simulators GCMs. For this purpose, we use as guidance water content fields from active observations by the CloudSat radar (CPR) and the CALIPSO lidar (CALIOP). Cloud products from active sensors while suffering significant sampling and coverage drawbacks have the advantage of resolving both horizontal and vertical variability which is what the generators are designed to produce. Our first order goal is to test the ability of the generators to deliver realistic 2D cloud extinction (cloud optical thickness) fields using, as in GCMs, limited domain-averaged information. Our reference 2D cloud extinction fields fully resolving horizontal (along the track of the satellites) and vertical variability come from combining CloudSat’s 2B-CWC-RVOD (liquid clouds) and CALIPSO-enhanced 2C-ICE (ice clouds) products. The combined fields were improved by introducing a simple scheme to fill liquid cloud extinction values identified as missing by comparing with coincident 2D (phase-specific) cloud masks provided by the CALIPSO-enhanced 2B-CLDCLASS-LIDAR CloudSat product. Our presentation will demonstrate the substantial improvements for low clouds brought by the filling scheme through comparisons with MODIS-Aqua cloud fraction distributions expressed in terms of joint cloud top pressure – cloud optical thickness histograms. Beyond global comparisons, the nature of the improvements become clearer when comparing mean joint histograms segregated by MODIS Cloud Regime (CR): improvement is by design superior for MODIS CRs dominated by low clouds. With the improved 2D extinction fields at hand, we test the skill of two subcolumn generators, one used in the COSP satellite simulator package, and one with more sophisticated cloud overlap implemented in the GEOS global model, to reproduce joint histograms that are statistically similar to the observed counterparts described above (as interpreted by COSP’s MODIS simulator). Our main comparison metrics are the Euclidean distance between observed and generator-produced global or near-global mean joint histograms, and the statistics of Euclidean distances calculated for individual scenes. One full year of data is used to assess whether the more sophisticated cloud generator produces clouds with greater realism in 2D cloud variabilityItem Shortwave direct radiative effects of above-cloud aerosols over global oceans derived from 8 years of CALIOP and MODIS observations(Copernicus Publications on behalf of the European Geosciences Union, 2016-03-04) Zhang, Zhibo; Meyer, Kerry; Yu, Hongbin; Platnick, Steven; Colarco, Peter; Liu, Zhaoyan; Oreopoulos, LazarosIn this paper, we studied the frequency of occurrence and shortwave direct radiative effects (DREs) of above-cloud aerosols (ACAs) over global oceans using 8 years (2007–2014) of collocated CALIOP and MODIS observations. Similar to previous work, we found high ACA occurrence in four regions: southeastern (SE) Atlantic region, where ACAs are mostly light-absorbing aerosols, i.e., smoke and polluted dust according to CALIOP classification, originating from biomass burning over the African Savanna; tropical northeastern (TNE) Atlantic and the Arabian Sea, where ACAs are predominantly windblown dust from the Sahara and Arabian deserts, respectively; and the northwestern (NW) Pacific, where ACAs are mostly transported smoke and polluted dusts from Asian. From radiative transfer simulations based on CALIOP–MODIS observations and a set of the preselected aerosol optical models, we found the DREs of ACAs at the top of atmosphere (TOA) to be positive (i.e., warming) in the SE Atlantic and NW Pacific regions, but negative (i.e., cooling) in the TNE Atlantic Ocean and the Arabian Sea. The cancellation of positive and negative regional DREs results in a global ocean annual mean diurnally averaged cloudy-sky DRE of 0.015Wm⁻² (range of −0.03 to 0.06Wm⁻²) at TOA. The DREs at surface and within the atmosphere are −0.15Wm⁻² (range of −0.09 to −0.21Wm⁻²), and 0.17Wm⁻² (range of 0.11 to 0.24Wm⁻²), respectively. The regional and seasonal mean DREs are much stronger. For example, in the SE Atlantic region, the JJA (July–August) seasonal mean cloudy-sky DRE is about 0.7Wm⁻² (range of 0.2 to 1.2Wm⁻²) at TOA. All our DRE computations are publicly available¹. The uncertainty in our DRE computations is mainly caused by the uncertainties in the aerosol optical properties, in particular aerosol absorption, the uncertainties in the CALIOP operational aerosol optical thickness retrieval, and the ignorance of cloud and potential aerosol diurnal cycle. In situ and remotely sensed measurements of ACA from future field campaigns and satellite missions and improved lidar retrieval algorithm, in particular vertical feature masking, would help reduce the uncertainty.Item Substantial cooling effect from aerosol-induced increase in tropical marine cloud cover(Nature, 2024-04-11) Chen, Ying; Haywood, Jim; Wang, Yu; Malavelle, Florent; Jordan, George; Peace, Amy; Partridge, Daniel G.; Cho, Nayeong; Oreopoulos, Lazaros; Grosvenor, Daniel; Field, Paul; Allan, Richard P.; Lohmann, UlrikeWith global warming currently standing at approximately +1.2 °C since pre-industrial times, climate change is a pressing global issue. Marine cloud brightening is one proposed method to tackle warming through injecting aerosols into marine clouds to enhance their reflectivity and thereby planetary albedo. However, because it is unclear how aerosols influence clouds, especially cloud cover, both climate projections and the effectiveness of marine cloud brightening remain uncertain. Here we use satellite observations of volcanic eruptions in Hawaii to quantify the aerosol fingerprint on tropical marine clouds. We observe a large enhancement in reflected sunlight, mainly due to an aerosol-induced increase in cloud cover. This observed strong negative aerosol forcing suggests that the current level of global warming is driven by a weaker net radiative forcing than previously thought, arising from the competing effects of greenhouse gases and aerosols. This implies a greater sensitivity of Earth’s climate to radiative forcing and therefore a larger warming response to both rising greenhouse gas concentrations and reductions in atmospheric aerosols due to air quality measures. However, our findings also indicate that mitigation of global warming via marine cloud brightening is plausible and is most effective in humid and stable conditions in the tropics where solar radiation is strong.Item Substantial Radiative Warming by an Inadvertent Geoengineering Experiment from 2020 Fuel Regulations(2023-10-30) Yuan, Tianle; Song, Hua; Oreopoulos, Lazaros; Wood, Robert; Bian, Huisheng; Breen, Katherine; Chin, Mian; Yu, Hongbin; Barahona, Donifan; Meyer, Kerry; Platnick, StevenHuman activities affect the Earth’s climate through modifying the composition of the atmosphere, which then creates radiative forcing that drives climate change. The warming effect of anthropogenic greenhouse gases has been partially balanced by the cooling effect of anthropogenic aerosols. In 2020, fuel regulations abruptly reduced the emission of sulfur dioxide from international shipping by more than 80% and created an inadvertent geoengineering experiment with global scale. Here we show the regulation leads to a radiative forcing of 0.12 Wm-² averaged over the global ocean using a combination of modeling and satellite data. The forcing is estimated to effectively double the warming rate of global mean temperature in this decade with strong spatiotemporal heterogeneity. The warming effect contributes 50% to the measured increase in planetary heat uptake since 2020. The radiative forcing also has strong hemispheric contrast of 0.12 Wm-² and contributes to the measured hemispheric contrast in absorbed solar radiation, which has important implications for precipitation patterns. Our result suggests marine cloud brightening may be a viable geoengineering method in temporarily cooling the climate.Item Understanding the microphysical control and spatial-temporal variability of warm rain probability using CloudSat and MODIS observations(AGU, 2022-05-13) Zhang, Zhibo; Oreopoulos, Lazaros; Lebsock, Matthew D.; Mechem, David B.; Covert, JustinBy combining measurements from MODIS and the CloudSat radar, we develop a parameterization scheme to quantify the combined microphysical controls by liquid water path (LWP) and cloud droplet number concentration (CDNC) of the probability of precipitation (PoP) in marine low cloud over tropical oceans. We demonstrate that the spatial-temporal variation of grid-mean in-cloud can be largely explained by the variation of the joint probability density function of LWP and CDNC in the phase space specified by the bivariate PoP (LWP and CDNC) function. Through a series of sensitivity tests guided by this understanding, we find that in the Southeastern Pacific and Atlantic the stratocumulus to cumulus transition of the is mainly due to the variation of CDNC while the annual cycle is mainly due to the variation of LWP. The results of this study provide a viable way to diagnose the root cause of warm rain problems in global climate models.