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Xu D.,Center for Environmental Sensing and Modeling | Malanotte-Rizzoli P.,Massachusetts Institute of Technology
Dynamics of Atmospheres and Oceans

The upper layer, wind-driven circulation of the South China Sea (SCS), its through-flow (SCSTF) and the Indonesian through flow (ITF) are simulated using a high resolution model, FVCOM (finite volume coastal ocean model) in a regional domain comprising the Maritime Continent. The regional model is embedded in the MIT global ocean general circulation model (ogcm) which provides surface forcing and boundary conditions of all the oceanographic variables at the lateral open boundaries in the Pacific and Indian oceans. A five decade long simulation is available from the MITgcm and we choose to investigate and compare the climatologies of two decades, 1960-1969 and 1990-1999.The seasonal variability of the wind-driven circulation produced by the monsoon system is realistically simulated. In the SCS the dominant driving force is the monsoon wind and the surface circulation reverses accordingly, with a net cyclonic tendency in winter and anticyclonic in summer. The SCS circulation in the 90s is weaker than in the 60s because of the weaker monsoon system in the 90s. In the upper 50. m the interaction between the SCSTF and ITF is very important. The southward ITF can be blocked by the SCSTF at the Makassar Strait during winter. In summer, part of the ITF feeds the SCSTF flowing into the SCS through the Karimata Strait. Differently from the SCS, the ITF is primarily controlled by the sea level difference between the western Pacific and eastern Indian Ocean. The ITF flow, consistently southwestward below the surface layer, is stronger in the 90s.The volume transports for winter, summer and yearly are estimated from the simulation through all the interocean straits. On the annual average, there is a ~5.6. Sv of western Pacific water entering the SCS through the Luzon Strait and ~1.4. Sv exiting through the Karimata Strait into the Java Sea. Also, ~2. Sv of SCS water enters the Sulu Sea through the Mindoro Strait, while ~2.9. Sv flow southwards through the Sibutu Strait merging into the ITF. The ITF inflow occurs through the Makassar Strait (up to ~62%) and the Lifamatola Strait (~38%). The annual average volume transport of the ITF inflow from the simulation is ~15. Sv in the 60s and ~16.6. Sv in the 90s, very close to the long term observations. The ITF outflow through the Lombok, Ombai and Timor straits is ~16.8. Sv in the 60s and 18.9. Sv in the 90s, with the outflow greater by 1.7. Sv and 2.3. Sv respectively. The transport estimates of the simulation at all the straits are in rather good agreement with the observational estimates.We analyze the thermal structure of the domain in the 60s and 90s and assess the simulated temperature patterns against the SODA reanalysis product, with special focus on the shallow region of the SCS. The SODA dataset clearly shows that the yearly averaged temperatures of the 90s are overall warmer than those of the 60s in the surface, intermediate and some of the deep layers and the decadal differences (90s. -. 60s) indicate that the overall warming of the SCS interior is a local effect. In the simulation the warm trend from the 60s to the 90s in well reproduced in the surface layer. In particular, the simulated temperature profiles at two shallow sites at midway in the SCSTF agree rather well with the SODA profiles. However, the warming trend in the intermediate (deep) layers is not reproduced in the simulation. We find that this deficiency is mostly due to a deficiency in the initial temperature fields provide by the MITgcm. © 2013 Elsevier B.V. Source

Yoo B.,Center for Environmental Sensing and Modeling
International Journal of Geographical Information Science

The depiction and navigation of large-scale urban landscapes are limited by the great cost of traditional computer-aided design (CAD) models for large urban environment in terms of both the labor of data entry and the runtime computational expense. This article presents a hybrid modeling approach that enables rapid urban model production from legacy spatial data. Our scheme fills the gap between the low geometry models, such as photo-textured digital terrains, and high geometry models, such as true three-dimensional CAD models. To achieve optimal performance in modeling and rendering, we employ bilayered displacement mapping consisting of global displacement mapping (GDM) for terrains and local displacement mapping (LDM) for buildings. The LDM is performed only within image processing so that the complexity of the models depends only on the area of an urban model. We present a use case of rapid urban model production to compare our approach with the traditional polygonal urban models of a widely used geo-browser. © 2013 Copyright Taylor and Francis Group, LLC. Source

Gryspeerdt E.,University of Oxford | Stier P.,University of Oxford | Grandey B.S.,Center for Environmental Sensing and Modeling
Geophysical Research Letters

The observed strong link between aerosol optical depth (τ) and cloud top pressure (ptop) has frequently been interpreted as the invigoration of convective clouds by aerosol, with increased τ being strongly correlated with decreases in ptop (increases in cloud top height). A strong correlation between τ and cloud fraction (fc) has also been observed. Using satellite-retrieved data, here we show that p top is also strongly correlated to fc, and when combined with the strong sensitivity between fc and τ, a large proportion of the relationship between ptop and τcan be reconstructed. Given the uncertainties about the influence of aerosol-cloud interactions on the τ-fc relationship, this suggests that a large fraction of the τ-ptop correlation may not be due to aerosol effects. Influences such as aerosol humidification and meteorology play an important role and should therefore be considered in studies of aerosol-cloud interactions. © 2014. American Geophysical Union. All Rights Reserved. Source

Lee S.-Y.,Center for Environmental Sensing and Modeling | Wang C.,Massachusetts Institute of Technology
Journal of Climate

Previous studies on the response of the South Asian summer monsoon to the direct radiative forcing caused by anthropogenic absorbing aerosols have emphasized the role of premonsoonal aerosol forcing. This study examines the roles of aerosol forcing in both pre- and postonset periods using the Community Earth System Model, version 1.0.4, with the Community Atmosphere Model, version 4. Simulations were perturbed by model-derived radiative forcing applied (i) only during the premonsoonal period (May-June), (ii) only during the monsoonal period (July-August), and (iii) throughout both periods. Soil water storage is found to retain the effects of premonsoonal forcing into succeeding months, resulting in monsoonal central India drying. Monsoonal forcing is found to dry all of India through local responses. Large-scale responses, such as the meridional rotation of monsoon jet during June and its weakening during July-August, are significant only when aerosol forcing is present throughout both premonsoonal and monsoonal periods. Monsoon responses to premonsoonal forcing by the model-derived "realistic" distribution versus a uniform wide-area distribution were compared. Both simulations exhibit central India drying in June. June precipitation over northwestern India (increase) and southwestern India (decrease) is significantly changed under realistic but not under wide-area forcing. Finally, the same aerosol forcing is found to dry or moisten the July-August period following the warm or cool phase of the simulations' ENSO-like internal variability. The selection of years used for analysis may affect the precipitation response obtained, but the overall effect seems to be an increase in rainfall variance over northwest and southwest India. © 2015 American Meteorological Society. Source

News Article
Site: http://news.mit.edu/topic/mitenvironment-rss.xml

On the Indian subcontinent, the widespread burning of firewood, coal, agricultural waste, and biomass for energy disperses black carbon particulates into the atmosphere. These manmade aerosols not only pollute the air, but also form an “atmospheric brown cloud” that’s disrupting the local climate. A case in point is the South Asian summer monsoon, a complex system of winds that typically bring rain from the tropical Indian Ocean to most of the region, especially in the southwest and northeast. The aerosols, which block solar radiation and cool the ground before they get swept away by rain, have altered the monsoon considerably. In previous investigations of the local climate impact of aerosols present in the atmosphere during the onset of the summer monsoon, MIT researchers found that rainfall increased in the month prior to the monsoon (June) and decreased during the monsoon season (July and August). Now, in a study published in the Journal of Climate, they have determined how aerosols emitted both before and during the monsoon are changing precipitation patterns. “Aerosols can influence the supply of moisture, and a small perturbation can play a big role,” says study co-author Chien Wang, a senior research scientist at MIT’s Center for Global Change Science and in the Department of Earth, Atmospheric and Planetary Sciences. “Pre-monsoonal aerosols push rainfall to the northwest, drying out central India. Aerosols emitted during the monsoon season send the moisture supply in a different direction, and dry out most of the subcontinent.” The most dramatic consequences are enhanced flooding in the pre-monsoon season in the normally dry northwest regions of India and in Pakistan, and reduced agricultural output in central India, which now gets far less rainfall than usual during the monsoon season. The study produced its findings by running a global climate model to simulate the impacts of aerosols emitted before and during the monsoon season in all but the northeast region of India, which is not well-represented in the model. The effects of pre-monsoonal aerosols — increased rainfall in the northwest and decreased rainfall elsewhere, particularly in central India — persist during the monsoon, further reducing soil moisture in central India. Aerosols emitted during the monsoon lower rainfall totals not only in central India, but across the country. When aerosol emissions are simulated throughout both periods, impacts during the monsoonal period are similar to those resulting from monsoonal aerosols only. The research was funded by the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology's Center for Environmental Sensing and Modeling, as well as by grants from the National Science Foundation, the Department of Energy, and the Environmental Protection Agency.

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