We investigate experimentally the statistical properties of a wind-generated wave field and the spontaneous formation of rogue waves in an annular flume. Unlike many experiments on rogue waves where waves are mechanically generated, here the wave field is forced naturally by wind as it is in the ocean. What is unique about the present experiment is that the annular geometry of the tank makes waves propagating circularly in an unlimited-fetch condition. Within this peculiar framework, we discuss the temporal evolution of the statistical properties of the surface elevation. We show that rogue waves and heavy-tail statistics may develop naturally during the growth of the waves just before the wave height reaches a stationary condition. Our results shed new light on the formation of rogue waves in a natural environment. DOI: 10.1103/PhysRevLett.118.144503 Rogue waves are rare events of exceptional height that may surge without warnings [1][2][3][4]. This peculiar phenomenon is ubiquitous. It has been observed in different contexts such as gravity and capillary waves [5][6][7][8][9][10], optical fibers [11][12][13][14][15][16][17][18][19], superfluid helium [20], and plasmas [21,22]. Because of their universal and potentially detrimental nature, there is a pressing need to understand their physics in order to predict and control them.The generating mechanisms can be disparate [23]. These include the spatiotemporal linear focusing of wave energy [24,25], the focusing due to bathymetry and currents (see, e.g., [26][27][28]), and the self-focusing that results from the Benjamin-Feir instability [29]. The latter is described by exact breather solutions of the nonlinear Schrödinger (NLS) equation [30], which are coherent structures that oscillate in space and/or time. Interestingly enough, breathers can also exist embedded in random waves [31]. Provided that the ratio of the dominant wave steepness to the spectral bandwidth is Oð1Þ and propagation is unidirectional, large amplitude structures can occur often enough to originate strong deviations from Gaussian statistics [6,15,[31][32][33]. Therefore, breathers have been considered in various fields of physics as a plausible prototype of rogue waves.Such solutions have been reproduced experimentally in wave tanks using prescribed boundary conditions at the wave maker [8]. Indeed, the standard form of the NLS equation describes the nonlinear dynamics of a preexisting (initial) wave field, which propagates without gaining or losing energy. This framework, however, is not transferable in a straightforward manner to systems driven by external forcing. The most obvious example of such a context is the ocean, where the oscillatory motion of the water surface is generated by the forcing of local wind (the resulting wave field is generally known as wind sea). Waves then grow with fetch and/or time until a quasistationary condition is reached, i.e., a fully developed sea [34]. Experimental work in wave tanks where waves are generated only by winds have been reported in the past;...

JEM-EUSO is a space mission designed to investigate Ultra-High Energy Cosmic Rays and Neutrinos (E > 5 · 10 19 eV) from the International Space Station (ISS). Looking down from above its wide angle telescope is able to observe their air showers and collect such data from a very wide area. Highly specific trigger algorithms are needed to drastically reduce the data load in the presence of both atmospheric and human activity related background light, yet retain the rare cosmic ray events recorded in the telescope. We report the performance in offline testing of the first level trigger algorithm on data from JEM-EUSO prototypes and laboratory measurements observing different light sources: data taken during a high altitude balloon flight over Canada, laser pulses observed from the ground traversing the real atmosphere, and model landscapes reproducing realistic aspect ratios and light 5 conditions as would be seen from the ISS itself. The first level trigger logic successfully kept the trigger rate within the permissible bounds when challenged with artificially produced as well as naturally encountered night sky background fluctuations and while retaining events with general air-shower characteristics.

We describe the coordinate transformations that can be used to convert the velocity components measured by a set of sonic anemometers with time-dependent tilt fluctuations into a single, time-independent coordinate system. By applying the planar fit method (PFM) to each anemometer dataset, it is possible, for planar flows, to locate the flow plane at each measurement point and compare its orientation with the topography. Installation on a ship is also considered. An application of this method to intercomparison data has led to the detection of an instrument error due to a misalignment between the assembly of the sonic transducers and the anemometer pedestal. If this error occurs, pedestal levelling does not guarantee that measurements are unbiased. A correction method is proposed and the results of two experiments are shown. Flow planarity at different levels and flow distortion caused by the mast are highlighted. The influence of the error on the evaluation of the Reynolds stresses using PFM or the double rotation method and the triple rotation method is discussed and the tilt corrected stresses calculated using the three methods compared.

The speed of sound in moist air is discussed and a more accurate value for the coefficient of the linear dependence of sonic temperature on specific humidity is proposed. An analysis of speed-of-sound data measured by three sonic anemometers in a climatic chamber and in the field shows that the temperature response of each instrument significantly influences not only the determination of sonic temperature, but also its fluctuations. The corresponding relative contribution to the error in the evaluation of the temperature fluctuations and the turbulent heat fluxes can be as high as 40%. The calibration procedure is discussed and a method of correction is proposed.

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