Ulrike Romatschke

Bio: Ulrike Romatschke is an academic researcher from National Center for Atmospheric Research. The author has contributed to research in topics: Precipitation & Radar. The author has an hindex of 10, co-authored 22 publications receiving 2151 citations. Previous affiliations of Ulrike Romatschke include University of Washington & International Atomic Energy Agency.

Viscosity information from relativistic nuclear collisions: how perfect is the fluid observed at RHIC?

Abstract: Relativistic viscous hydrodynamic fits to Brookhaven Relativistic Heavy Ion Collider data on the centrality dependence of multiplicity, transverse, and elliptic flow for square root s = 200 GeV Au+Au collisions are presented. For standard (Glauber-type) initial conditions, while data on the integrated elliptic flow coefficient v(2) are consistent with a ratio of viscosity over entropy density up to eta/s approximately 0.16, data on minimum bias v(2) seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti-de Sitter conformal field theory conjecture. Some caveats on this result are discussed.

Relativistic Fluid Dynamics In and Out of Equilibrium -- Ten Years of Progress in Theory and Numerical Simulations of Nuclear Collisions

Abstract: Ten years ago, relativistic viscous fluid dynamics was formulated from first principles in an effective field theory framework, based entirely on the knowledge of symmetries and long-lived degrees of freedom. In the same year, numerical simulations for the matter created in relativistic heavy-ion collision experiments became first available, providing constraints on the shear viscosity in QCD. The field has come a long way since then. We present the current status of the theory of non-equilibrium fluid dynamics in 2017, including the divergence of the fluid dynamic gradient expansion, resurgence, non-equilibrium attractor solutions, the inclusion of thermal fluctuations as well as their relation to microscopic theories. Furthermore, we review the theory basis for numerical fluid dynamics simulations of relativistic nuclear collisions, and comparison of modern simulations to experimental data for nucleus-nucleus, nucleus-proton and proton-proton collisions.

Regional, Seasonal, and Diurnal Variations of Extreme Convection in the South Asian Region

Abstract: Temporal and spatial variations of convection in South Asia are analyzed using eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data and NCEP reanalysis fields. To identify the most extreme convective features, three types of radar echo structures are defined: deep convective cores (contiguous 3D convective echo ≥40 dBZ extending ≥10 km in height) represent the most vertically penetrative convection, wide convective cores (contiguous convective ≥40 dBZ echo over a horizontal area ≥1000 km2) indicate wide regions of intense multicellular convection, and broad stratiform regions (stratiform echo contiguous over an area ≥50 000 km2) mark the mesoscale convective systems that have developed the most robust stratiform regions. The preferred locations of deep convective cores change markedly from India’s east coast in the premonsoon to the western Himalayan foothills in the monsoon. They form preferentially in the evening and over land as near-surface moist flow is cap...

Characteristics of Precipitating Convective Systems in the South Asian Monsoon

Abstract: Eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data show how convective systems of different types contribute to precipitation of the South Asian monsoon. The main factor determining the amount of precipitation coming from a specific system is its horizontal size. Convective intensity and/or number of embedded convective cells further enhance its precipitation production. The precipitation of the monsoon is concentrated in three mountainous regions: the Himalayas and coastal ranges of western India and Myanmar. Along the western Himalayas, precipitation falls mainly from small, but highly convective systems. Farther east along the foothills, systems are more stratiform. These small and medium systems form during the day, as the monsoon flow is forced upslope. Nighttime cooling leads to downslope flow and triggers medium-sized systems at lower elevations. At the mountainous western coasts of India and Myanmar, small and medium systems are present throughout the ...

Anomalous Atmospheric Events Leading to the Summer 2010 Floods in Pakistan

Abstract: orogrAphiC rAinstorms seen by trmm. Major rainstorms can occur in mountainous regions when air is lifted over the terrain. When the environment is especially unstable, storms may form that contain locally intense buoyant updrafts, heavy downpours of rain, and sometimes hail. High winds may occur where downdrafts, forming in raining regions, reach the ground and spread out. When a sequence of convective upand downdrafts occurs, the precipitating cloud systems take the form of a “mesoscale convective system” (MCS; for a thorough description see Houze’s 2004 article in Reviews in Geophysics), in which some “convective cells” die out while other new ones form, and the older dying storms morph into regions of wider-spread “stratiform rain.” This process is described by Houze in a 1997 Bulletin article. The stratiform region features gentler rain but covers a much greater area and can last for long periods of time if the environment is favorable. Even if the instability is modest but the environment is humid enough to support the persistence of the stratiform regions of MCSs, or if the MCS stratiform region is enhanced by orographic or synoptic-scale upward motion, a great amount of rain can be produced by the MCS. As a result, if MCSs are the principal rain-producing cloud systems over a region such as the upslope side of the Himalayas, the mountains can affect the rainstorms in two important ways: 1) Airflow over rising terrain may trigger new intense convective cells, thus revitalizing and maintaining the vigorous convective region of the MCS for a longer period of time; and 2) the orographic upward air motions may sustain and broaden the stratiform region of the MCS so that the gentler (but nonetheAnomalous Atmospheric Events Leading to the Summer 2010 Floods in Pakistan

Nonlinear fluid dynamics from gravity

Abstract: Black branes in AdS5 appear in a four parameter family labeled by their velocity and temperature. Promoting these parameters to Goldstone modes or collective coordinate fields—arbitrary functions of the coordinates on the boundary of AdS5—we use Einstein's equations together with regularity requirements and boundary conditions to determine their dynamics. The resultant equations turn out to be those of boundary fluid dynamics, with specific values for fluid parameters. Our analysis is perturbative in the boundary derivative expansion but is valid for arbitrary amplitudes. Our work may be regarded as a derivation of the nonlinear equations of boundary fluid dynamics from gravity. As a concrete application we find an explicit expression for the expansion of this fluid stress tensor including terms up to second order in the derivative expansion.

Nonlinear Fluid Dynamics from Gravity

Abstract: Black branes in AdS5 appear in a four parameter family labeled by their velocity and temperature. Promoting these parameters to Goldstone modes or collective coordinate fields -- arbitrary functions of the coordinates on the boundary of AdS5 -- we use Einstein's equations together with regularity requirements and boundary conditions to determine their dynamics. The resultant equations turn out to be those of boundary fluid dynamics, with specific values for fluid parameters. Our analysis is perturbative in the boundary derivative expansion but is valid for arbitrary amplitudes. Our work may be regarded as a derivation of the nonlinear equations of boundary fluid dynamics from gravity. As a concrete application we find an explicit expression for the expansion of this fluid stress tensor including terms up to second order in the derivative expansion.

Future climate risk from compound events

Abstract: Floods, wildfires, heatwaves and droughts often result from a combination of interacting physical processes across multiple spatial and temporal scales. The combination of processes (climate drivers and hazards) leading to a significant impact is referred to as a ‘compound event’. Traditional risk assessment methods typically only consider one driver and/or hazard at a time, potentially leading to underestimation of risk, as the processes that cause extreme events often interact and are spatially and/or temporally dependent. Here we show how a better understanding of compound events may improve projections of potential high-impact events, and can provide a bridge between climate scientists, engineers, social scientists, impact modellers and decision-makers, who need to work closely together to understand these complex events.

Collective Flow and Viscosity in Relativistic Heavy-Ion Collisions

Abstract: We review collective flow, its anisotropies, and its event-to-event fluctuations in relativistic heavy-ion collisions, as well as the extraction of the specific shear viscosity of quark–gluon plasma from collective flow data collected in heavy-ion collision experiments at RHIC and the LHC. We emphasize the similarities between the Big Bang of our universe and the Little Bangs created in heavy-ion collisions.

Viscosity Bound Violation in Higher Derivative Gravity

Abstract: Motivated by the vast string landscape, we consider the shear viscosity to entropy density ratio in conformal field theories dual to Einstein gravity with curvature square corrections. After field redefinitions these theories reduce to Gauss-Bonnet gravity, which has special properties that allow us to compute the shear viscosity nonperturbatively in the Gauss-Bonnet coupling. By tuning of the coupling, the value of the shear viscosity to entropy density ratio can be adjusted to any positive value from infinity down to zero, thus violating the conjectured viscosity bound. At linear order in the coupling, we also check consistency of four different methods to calculate the shear viscosity, and we find that all of them agree. We search for possible pathologies associated with this class of theories violating the viscosity bound.