Professor Bjorn Stevens will give a three lecture course on Atmospheric Water, Aerosol Forcing, and Lower Tropospheric Mixing.
February 6th, 2014 at 4pm in MIT 32-124
Understanding of water, a deceptively simple molecule with an unusual appetite for infrared radiation, provides the foundation for much of what we know about climate, and climate change. Understanding water helps explain basic properties of the climate system, like the globally averaged surface temperature and the strength and patterns of the hydrological cycle, and rationalizes how these might change. Likewise, a lack of understanding of water, particularly controls on the distribution of its condensed phases and their coupling to circulations, is responsible for most of the uncertainty about the future trajectory of the climate system. All of which makes better understanding atmospheric water climate science's foremost intellectual challenge.
February 20th, 2014 at 4pm in MIT 54-915
Since its inception, modern climate science has accepted the fact that the aerosol is one of the climate systems most important and uncertain forcings. In the 1970s it was argued that the radiative effects of the aerosol might portend an accelerated arrival of a new Weltwinter (ice age), in the 1980s aerosol effects were linked to massive climate consequences of nuclear war, in the 1990s it was proposed that aerosols were masking a much larger sensitivity of global temperatures to rising concentrations of CO2, and in the first decade of this century aerosol effects on the hydrological cycle and weather extremes have received substantial attention. In this talk I will argue that the role of aerosol forcing has been greatly exaggerated, but to the extent the aerosol is responsible for a substantial forcing of the climate system this forcing was realized in the middle of the last century, and for both reasons the aerosol has ceased to be important for present or future changes in the global climate.
February 7th, 14th, and 21st, 2014 at 4pm in MIT 54-915
Feb. 7 - Water plays a very important role in driving mixing in the lower atmosphere. In this lecture we briefly review basic concepts in moist thermodynamics and outline the role of water in turbulent mixing processes in the lower atmosphere, focusing on its role in three canonical marine boundary layer regimes: the cloud-free convective boundary layer, the stratocumulus-topped boundary layer, and cumulus-capped boundary layers.
Feb. 14 - The role of temperature on the distribution of water, and hence the efficiency of mixing, is reviewed with an attempt to better understand and constrain feedbacks on radiative forcing associated with marine boundary layer clouds in a variety of climates. Recent work that highlights the role of lower tropospheric mixing on the modeled response to climate forcing will be discussed.
Feb. 21 - (As necessary to elaborate on the first II lectures, or to meet with students to discuss their related work.)
Professor Bjorn Stevens is a director at the Max-Planck-Institute for Meteorology where he leads the Atmosphere in the Earth System Department and is a professor (§17) at the University of Hamburg. Prior to moving to Hamburg Dr. Stevens was a full professor of Dynamic Meteorology at the University of California of Los Angeles. His research blends modeling, theory and field work to help articulate the role of clouds and atmospheric convection on the climate system. Dr. Stevens has made pioneering contributions to our understanding of mixing and microphysical processes on the structure and organization of marine boundary layer clouds, whose statistics regulate the flow of energy through the Earth system. Small changes in such clouds can greatly amplify, or dampen, perturbations to the Earth system.
Dr. Stevens received a PhD in Atmospheric Science in 1996 from the Colorado State University in Ft Collins CO, and holds a Bachelor and Masters of Science in electrical engineering from Iowa State University. He has contributed more than 90 scholarly articles to the peer reviewed literature. Dr. Stevens serves on a number of international advisory boards, has served as editor of leading journals in his field and has been honored by a number of awards, including fellowships from the Advanced Study Program of the National Center for Atmospheric Research, and the Alexander von Humboldt Society. In 2002 he was chosen as the recipient of the prestigious Clarence Leroy Meisinger Award of the American Meteorological Society for "pioneering advances in understanding and modeling of cloud-topped boundary layer.
Bodas-Salcedo, A., Williams, K. D., Ringer, M. A., Beau, I., Cole, J. N. S., Dufresne, J.-L., Koshiro, T., Stevens, B., & Wan, Z. (2014). Origins of the solar radiation biases over the Southern Ocean in CFMIP2 models. Journal of Climate, 27, 41-56. doi:10.1175/JCLI-D-13-00169.1.
Bellon, G., & Stevens, B. (2013). Time scales of the trade wind boundary layer adjustment. Journal of the Atmospheric Sciences, 70, 1071-1083. doi:10.1175/jas-d-12-0219.1.
Crueger, T., Stevens, B., & Brokopf, R. (2013). The Madden-Julian like oscillation in ECHAM6 and the introduction of a objective MJO score. Journal of Climate, Available online early. doi:10.1175/JCLI-D-12-00413.1.