Our ability to understand how solar energy is distributed and deposited on our planet has greatly improved over the past two decades, enabled by new global observing systems such as the NASA A-­‐Train satellite constellation. But some of the outstanding questions regarding the radiative effects of atmospheric constituents and the surface cannot be answered with satellite observations alone because the derivation of surface and atmospheric energy budget terms from remote sensing requires a number of assumptions and parameterizations. Aircraft observations can fill some of these gaps because they allow the direct measurement of flux densities above, below and inside atmospheric layers of interest – alongsidein-­‐situ measurements of atmospheric constituents and their optical properties. In my talk,I will show that the combination of airborne spectrally resolved radiation measurements with three-­‐dimensional radiative transfer modeling allowed the resolution of a long-­‐standing issue in so-­‐called radiation closure experiments where cloud absorption from measurements appeared to be consistently higher than expected from the calculations. This led to the discovery of “colored” or spectrally dependent net horizontal photon transport, which is relevant not only to energy budget parameters such as radiative forcing and absorption, but also to remote sensing. The radiative effect of aerosols in homogeneous or inhomogeneous cloud fields can be understood as a spectral perturbation to the radiative signature of the underlying cloud field. Interpreting the combined signal from aerosols and clouds by means of their distinct fingerprints has become one of the goals of an emerging focus area: “cloud-­‐aerosol spectrometry.” I will present some of the results of this new research direction and discuss how multiple observational techniques including spectral imaging and active sounding could be combined to gain a more complete understanding of the radiative effects of clouds, aerosols and gases in future research.

Location: Physics Bldg., Room 401

Dust cycle is an emerging core theme in the Earth system science. Dust emitted from deserts and disturbed soils can have significant impacts on climate, human health, ecosystems, and biogeochemical cycle. On the other hand, dust emissions can be affected by changes in rainfall, wind speed, and vegetation cover. The impacts of dust are far-reaching because of the global movement of dust. Each year about 60 million tons of dust is imported into North America from both coasts, which is dominated by the trans-Pacific transport of dust with both Asian and African origins. This surprisingly large magnitude of dust import is comparable with domestic emissions in North America. Meanwhile, 43 ~ 58 million tons of trans-Atlantic dust from North Africa is deposited into the Amazon basin every year, providing nutrients needed for maintaining the health and productivity of Amazon rainforest, an important ecosystem in regulating global climate. The trans-Pacific aerosol transport, additional source of particulate pollution for the U.S., increases the surface concentration of fine particulate matter (PM2.5) by a magnitude that is much larger in the west than in the east, because of the geographical proximity and high topography of the west. However, when the influences on meteorology by imported aerosols are also considered, the more populous east shows the PM2.5 increase that is comparable to the west. In this presentation I will discuss how these impacts have been assessed utilizing advanced aerosol remote sensing measurements from the MODerate resolution Imaging and Spectroradiometer (MODIS) and the Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) in conjunction with a regional modeling system. Future research will also be discussed.

Location: Physics Bldg., Room 401

It is increasingly realized that clouds are at the heart of physical climate science. They are the most important player in the energy balance of the Earth by interacting with both shortwave and longwave radiation. Tiny changes in cloud properties can have major consequences for our climate.

Here I concentrate on how aerosols affect clouds, the so-called aerosol indirect effects (AIEs), which remain one of the most uncertain factors in our scientific understanding and projection of climate change. Two cloud regimes will be discussed.

In one, aerosols invigorate maritime tropical convection at a large scale. The invigoration effect manifests in characters of precipitation radar reflectivity vertical profiles, cloud top ice particle size and cloud glaciation temperature. Furthermore, lightning, as a hallmark of strong convection, increases at a rate of 20-40 times per unit increase of aerosol optical depth. Aerosol-induced lightning changes also have interesting implications for ozone chemistry and wildfire activity.

In the other, aerosols change cloud properties of trade cumuli at a large scale. They decreased cloud droplet size, decreased precipitation efficiency and increased cloud amount. In addition we find significantly higher cloud tops for polluted clouds. Changes in cloud properties caused by aerosols perturbed the energy balance by more than 20Wm-2, almost an order of magnitude higher than aerosol direct forcing alone. It highlights the strong leverage of AIE in this cloud regime. Furthermore, the precipitation reduction associated with enhanced aerosol leads to large changes in the energetics of air-sea exchange within trade wind boundary layer.

Results from both regimes open up new opportunities for future research in reducing uncertainty surrounding AIEs and climate adaptation/mitigation.

Location: Physics Bldg., Room 401

Phytoplankton are unicellular organisms that are responsible for half of all photosynthetic activity on Earth. According to their functional types, phytoplankton are divided into several different conceptual groups that include calcifiers, nitrogen fixers, DMS producers and silicifiers. Coccolithophores belong to the calcifier group which plays important roles in global carbon cycle processes. When proper nutrition and light conditions are met, phytoplankton can produce massive blooms, which have large environmental impacts. I am primarily interested in how light interacts with particles, particularly phytoplankton and other oceanic particles, and how light can be used to remote sense and monitor ocean water optical properties. In this talk I will cover three general aspects of my research interests: light scattering, radiative transfer, and optical remote sensing. Light scattering studies the interaction of light with single particles within the classical electromagnetic theory and linear optics. Radiative transfer theory covers light field multiply scattered within turbid media consisting of individual particles. It is understood that optical properties of single particles are known knowledge in radiative transfer theory. Generally radiative transfer theory assumes incoherent scattering, which means that there is no systematic phase correlation among light waves scattered by a group of particles. Nonetheless, coherent scattering is significant in coherent backscattering and other phenomena. Optical remote sensing is to use light scattered by particles, singly or multiply, to retrieve information about particles’ properties. In other words, light scattering and radiative transfer theories are building blocks and diagnostic tools for optical remote sensing. I will present two examples of my research efforts in each of the three theoretical categories. In light scattering, the examples are invisible particles for monostatic lidar/radar detections and simulation of light scattering by nonspherical coccolithophores. In radiative transfer theory section, I will cover how polarization is treated and two methods are presented to solve light field in a turbid medium: Monte Carlo and the successive order of scattering. In the optical remote sensing section, two applications of polarized radiative transfer solutions are given for ocean color and aerosol remote sensing.
 

Location: Physics Bldg., Room 401

Rising concentrations of greenhouse gases are the main drivers of current and projected future climate change. Reactive gases, including methane (CH4) and ozone (O3), contribute substantially to climate forcing and the chemistry controlling their abundances also responds to climate change. Using multiple global chemical transport models we diagnose factors controlling the year-to-year variations in tropospheric hydroxyl (OH), the main sink for atmospheric CH4. The modeled variations over the last decade are then evaluated against our best observational constraints. Using factors that control OH in the recent past, we project CH4 lifetime and abundance forward in time for a range of future climate scenarios. This simple approach agrees with projections from fully coupled Earth System Models. All this information is combined into an uncertainty analysis of the future CH4 abundance and global warming potential. Key process uncertainties are identified and can guide future research priorities.

Location: Physics Bldg., Room 401

Through the stories of my interaction with four young scientists, I will explore different aspects of Aerosol Remote Sensing. These will include developing an operational algorithm for wide community use, developing a specialized product to retrieve that hard-to-acquire aerosol characterization of spectral absorption, and exploring aerosol-cloud interaction in two different ways. Aerosol remote sensing spans both data producers and data users, and both types of scientists discover new knowledge in pursuit of their projects. My four young scientists each made a significant contribution to science as they pursued their Ph.D.s and post-docs, and also enriched my experience in the process.

Location: Physics Bldg., Room 401

Date: Monday, January 6, 2014
Time: 8:00 am
Location: PHYS 401

TITLE:
A New Differential Absorption Lidar Using Raman Cells to Measure Subhourly Variation of Tropospheric Ozone Profiles in the Baltimore - Washington D.C. region

ABSTRACT:
This proposal will detail the theory and background necessary for the ground based tropospheric ozone Differential Absorption Lidar (DIAL) system at the NASA Goddard Space Flight Center (Greenbelt, MD 38.99° N, 76.84° W, 57 meters ASL), with initial results from 500 m to 10 km in Summer 2013. Current atmospheric satellites cannot peer through the optically thick stratospheric ozone layer to remotely sense boundary layer tropospheric ozone. In order to monitor this lower ozone more effectively, NASA has funded the ground based Tropospheric Ozone Lidar Network (TOLNET) which currently consists of five stations across the US. The Goddard instrument is based on the Differential Absorption Lidar (DIAL) technique, which transmits three wavelengths, 266, 289 and 299 nm. Ozone is absorbed more strongly at 266 nm and 289 nm than at 299 nm. The DIAL technique exploits this difference between the returned backscatter signals to obtain the ozone number density as a function of altitude. The transmitted wavelengths are generated by focusing the output of a quadrupled Nd:YAG laser beam (266 nm) into a pair of Raman Cells, filled with high pressure Hydrogen and Deuterium. Stimulated Raman Scattering within the focus shifts the pump wavelength and the first Stokes shift in each cell produces the required wavelengths. With the knowledge of the ozone absorption coefficient at these two wavelengths, the range resolved number density can then be derived. A interesting scientific validation data set will be examined, which yields accurate initial results. There are currently surface ozone measurements hourly and ozonesonde launches occasionally, but this system will be the first to make continuous routine ozone profile measurements in the Washington, DC - Baltimore area.

Date: Thursday, December 12, 2013
Time: 1:00 pm
Location: PHYS 401

TITLE:
Shedding New Light on Accreting Pulsars

ABSTRACT:
Neutron stars are remnants of stellar evolution, the collapsed cores of massive stars. They are extremely dense and sustained against further gravitational collapse by neutron degeneracy pressure. Many have the strongest magnetic fields found in the universe. Accreting X-ray pulsars are rotating neutron stars which show regular flashes of X-ray emission powered by the accretion of material from a stellar companion onto the magnetic poles of the neutron star. The processes that take place at the poles involve strong gravitational fields, high temperatures and the most extreme magnetic fields. These are conditions that cannot take place naturally on Earth and cannot be reproduced in laboratories.

In recent years, considerable progress has been made regarding the development of physical models describing the accretion process onto the magnetic poles such that these new models can now be tested for the first time. These new models can now provide the first direct connection between physical parameters of the accretion process (magnetic field strength, plasma temperature, plasma density, mass accretion rate) and X-ray spectral shape. The standard empirical and new physical spectral models will be systematically applied to a sample of pulsars. Most of the chosen pulsars show a “cyclotron line”, a spectral feature that allows for a direct measurement of their magnetic fields. This detailed spectral analysis will be mainly based on observations from the Japanese X-ray satellite, Suzaku, due to its instruments’ high sensitivity, spectral resolution, and broad-band coverage. (The project will also include additional satellite data of a Symbiotic X-ray Binary, a member of a small, under-studied subclass of X-ray pulsars accreting from a dense stellar companion wind. Its study will involve modeling the spectrum as well as characterizing the variability and comparing the results with more common X-ray binaries.)

The goal of this project is provide the first steps towards a much needed improvement of our theories and observational analysis of magnetically dominated accretion, which can only be achieved by fitting the best physical models to the best available data.

The super-massive black holes residing at the centers of many galaxies emit jets of relativistic plasma moving near the speed of light, a process which can deposit enormous amounts of energy into the host galaxy and inter-galactic medium. Yet the detailed physics behind these jets, including how they are launched and collimated, how long they remain relativistic, and their total energy content remain poorly understood. Using over 13 years of archival Hubble Space Telescope (HST) observations of the relativistic jet in the archetypal radio galaxy M87, we have produced astrometric speed measurements of the optically bright synchrotron emitting plasma components in the jet with unprecedented accuracy. Building on previous work showing the superluminal nature of the jet in the optical, we have found that the jet motion is incredibly complex, with both transverse motions and flux variations which can be seen by eye in the time series of deep exposures. These observations of M87 provide us with a unique dataset with which to refine theoretical models of the large-scale jet structure, potentially addressing open questions such as the jet collimation mechanism, bulk acceleration and deceleration in the jet, and the presence of a helical structure. I will also present very recent results using data from the HST archive on the optical counterjet and nuclear regions of M87 and discuss the larger implications of these detailed studies of one of the most nearby AGN jets.

Location: Physics Bldg., Room 401