Research Interests:

I am interested in understanding, modeling and predicting dominant processes and interactions of hydrological and meteorological systems at different space-time scales. I am in particular interested in space-time patterns of precipitation fields and how these are related to meteorological controls on one hand and to hydrological impacts on the other. Precipitation data from remote sensing systems (radar and satellite) are often used in my research, where their uncertainty is also considered. With my group we investigate extreme precipitation and floods at a range of scales. We develop and utilize process-based and data-driven models in deterministic and stochastic frameworks. We examine climate variability and climate change in present, past and future conditions and their effects on different environmental systems that are of interest in hydrological, geomorphologic, agricultural and ecological fields of research.

Curriculum Vitae

Research

Scattering of Electromagnetic Radiation by Irregular Particles in the Atmosphere and Ocean

I work with models that describe the interaction of solar radiation with particles in Earth's atmosphere and oceans. I am particularly interested in how best (both accurately and efficiently) to model the effects of irregular particles, such as non-spherical particles, porous particles, and particles comprised of disorded/amorphous materials. Similarly, I am interested in how best to model mixtures of different components in the same particle. What are the differing effects of insoluble vs. soluble, solid vs. liquid, and absorbing vs. nonabsorbing components?

Underwater Polarization

While human beings only detect the intensity and wavelength of light, some species, both terrestrial and marine, are able to sense light's third attribute, polarization. Such species use polarization (the direction of the electric field) for different tasks such as orientation, navigation, prey detection, and communication. Under water, where the intensity signal is distorted by refraction and weakened by absorption, the ability to utilize polarization can be even more important than on land. For many years, theoretical models of polarized light under water were based on the theory of single scattering of the direct solar beam by small particles (Rayleigh scattering). Models accounting for scattering by larger particles (Mie scattering) have been approximate, and the most sophisticated models to date do not separate the effects of Mie scattering from effects of non-sphericity and multiple scattering. Furthermore, recent measurements under water reveal deviations from the polarization patterns predicted by models, in clear as well as semi-turbid waters. We are using a step-by-step approach to analyze the separate effects of particle size, particle size distribution, particle composition, particle shape, and varying orders of scattering on the underwater polarization pattern, with the goal of identifying the processes at play in different water types.

Light Pollution and the Light Environment Within Caves

My group has collaborated with scientists from Bar Ilan University and Beit Berl College in conducting the first in situ measurements ever of artificial nighttime light under water in a coral reef ecosystem. We found that the artificial light can penetrate to a depth of 30 m under the water, which could influence such biological processes as the tuning of circadian clocks, the synchronization of coral spawning, recruitment and competition, vertical migration of demersal plankton, feeding patterns, and prey/predator visual interactions. Similarly, we have collaborated in conducting the first in situ measurements ever of the state of light polarization within a mid-littoral cave. We found a relatively high degree of linear polarization of the light and a nearly constant orientation of the electric field of the light in winter months, which would improve the ability of the photosynthetic organisms there (cyanobacteria, microalgae, and macroalgae) to harvest light by orienting their light-harvesting receptors to the direction of the electric field.

Atmospheric Electricity

At Hebrew University, we have an ongoing collaboration with Tel Aviv University, The Open University, and the IDC Herzliya to study transient luminous events (TLEs), such as sprites, blue starters, jets, and elves, which occur above thunderstorm clouds. We have conducted our own observations of sprites and have constructed simple theoretical models to help explain some observed phenomena, such as the circular configuration of simultaneous sprites and the shift in the location of sprites from their parent lightning event.

Radiative Forcing of Climate

We look at how the presence of various types of particles affects the Earth's radiative-convective balance. Which particles cause a warming and which cause a cooling, and how does this change with their horizontal distribution? What is the most accurate way of representing subgrid scale features, such as inhomogeneous aerosol and cloud layers, in global models?

Curriculum Vitae

Research Interests: 

Large-scale atmospheric and climate dynamics/variability 


Atmospheric dynamicists try to understand why the atmosphere moves in the way it does. Because of the inherently chaotic nature of the atmosphere, and because the atmosphere is strongly linked to the oceans, land-surface, biosphere, the types of interactions that occur are very complex. However, a wide range of tools, ranging from observational data to simplified models, can help atmospheric dynamicists understand (and even predict) atmospheric motions.
 
 

More specifically, my research aims at extending the duration of reliable weather forecasts, and to then explore how the dynamical processes that are important on these short timescales may manifest on longer, climate-change timescales. The traditional approach to weather forecasting on one- to two-week timescales utilizes weather forecasting models, but on timescales longer than two weeks, the value of deterministic (or ensemble-based probabilistic) forecasts weakens. This is due to the presence of chaotic variability in the atmosphere. Yet certain modes of variability in the climate system have timescales longer than this two-week threshold, and the key to longer-scale prediction is to take advantage of these modes. By understanding the impacts of these modes of variability on surface weather, the potential for improved forecasts on a monthly timescale can be demonstrated and eventually realized. As many of these processes may be modified under climate change (or alternately, climate change may project onto these climate modes), a better understanding of these modes can also help improve the quality of climate change projections.

Two such classes of modes of variability are stratospheric variability (both in the tropical and polar stratosphere) and tropical tropospheric variability (e.g. the Madden-Julian Oscillation and El Nino), and most of my ongoing research focuses on these phenomena. For example, both polar stratospheric sudden warmings and the Madden-Julian Oscillation have been shown to influence European and Mediterannean weather, but it is unclear (1) what mechanism(s) underlie these connections, (2) how far in advance the  impacts can be predicted, and (3) what governs the magnitude of the surface impact.  As these processes must be represented by climate and weather models in order to actualize the potential improvement in predictability, and because the observational record of key meteorological quantities is relatively short, I also run models (both idealized and comprehensive) in order to test model fidelity and to isolate key processes. The ultimate goal is to improve the predictions and projections of surface weather and climate.

Opportunities: Exciting, funded opportunities for further research exist in these areas for master's students, PhD students, and postdocs. For more information, please contact me.

Curriculum Vitae

Mailing address:
 The Fredy & Nadine Herrmann Institute of Earth Sciences
 The Hebrew University Edmond J. Safra Campus,
 Givat Ram Jerusalem, 91904 Israel