Research subject: Changes in deep water formation in the Eastern Mediterranean.

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

Interests: 

physical oceanography (ocean mixing and stirring, internal waves, density currents...), modern and paleo climate dynamics, interaction between biota and climate.

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:

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

My research interests encompass the fundamentals of atmospheric and oceanic dynamics. The complex whirling of eddies of all sizes, the majestic major ocean currents or jet streams in the atmosphere are all examples of large scale dynamical features of the ever evolving fluids that move on the surface of the rotating spherical earth. The amount of fluid (be it gas in the atmosphere or saline water in the ocean) that are transported by these flows is too large for us to grasp at first glance. The Gulf Stream transports about 100 million tons of water every second from low latitudes to the mid-latitudes along the eastern coast of North America; about 3,000 times the amount of water flowing down the Niagara Falls. The Subtropical Jet Stream in the atmosphere transports about half-a-million tons of air (recall that the density of air is about 1/1000 that of water) around the earth from west to east at altitudes of about 10 km and speeds of the order of 100-150 km/h. The existence of the Subtropical Jet Stream enables the Israeli Airline El-Al to fly non-stop from Los-Angeles to Tel-Aviv, which it does in the opposite direction quite seldom. In my Geophysical Fluid Dynamics (GFD) research I attempt to define the exact physical origin of certain fluid dynamical phenomena and to provide exact theoretical descriptions of their observed features (such as the way they change with time and space). I also attempt to highlight the geophysical ramifications of their presence. 

In recent years I developed a theory of non-harmonic (also described as Trapped) waves of Geophysical Fluid Dynamics based on the formulation of a time-independent, Schrodinger eigenvalue equation for zonally propagating wave of the Rotating Shallow Water Equations (AKA Laplace Tidal Equations). The energy levels of the eigenvalue equation provide explicit expressions for the phase speeds of the waves and the associated eigenfunctions describe the meridional amplitude structure of the various waves. This formulation is relevant to many physical settings including the mid-latitude f-/\(\beta-\) plane, the equatorial \(\beta-\)plane and the spherical Earth. In particular, this formulation provided, for the first time, explicit expressions for the dispersion relations of Planetary (Rossby) waves and Inertia-Gravity (Poincare) waves on the spherical Earth (i.e. a rotating sphere but without the centrifugal acceleration. In additon to providing approximate, but highly accurate expressions for Planetary and Inertia-Gravity waves on the spherical earth the formulation of the  Schrodinger eigenvalue equation has provided a clear demonstration that Yanai wave (AKA the mixed Rossby-Gravity mode) exists on a sphere but for different reasons than on the equatorial \(\beta-\)plane: In the latter this wave exists only because the second westward propagating wave is associated with singular zonal velocity while on a sphere the approximate solution by the Schrodinger equation yields an unacceptable (complex) phase speed near the gravity wave phase speed.

In contrast to the planar theories where Kelvin waves exist as additional modes to Inertia-Gravity waves (e.g. modes in which one velocity component vanishes identically so the other velocity and the height fields solve the three scalar Rotating Shallow water Equations) on a sphere these waves do not exist at all but the eastward propagating Inertia-Gravity n=0 mode is nearly non-dispersive.                

These theoretical advances enabled the construction of test cases for the dynamical cores of global scale General Circulation Models and the explicit expressions have also been applied in the interpretation of satellite borne observations of Sea Surface Height Anomalies in the Indian Ocean south of Australia where the nearly zonal coast provides the "wall" at which the waves are Trapped. 

My book entitled “Shallow Water Waves on the Rotating Earth” which was published by Springer in 2015 provides the details of this unified approach to the known waves of Geophysical Fluid Dynamics

My recent CV and list of publications

My updated list of publications