Search

Enhanced meteorological observations were made during the 2015 Pan and Parapan American Games in Toronto in order to measure the vertical and horizontal structure of lake-breeze events. Two scanning Doppler lidars (one fixed and one mobile), a C-band radar, and a network including 53 surface meteorological stations (mesonet) provided pressure, temperature, humidity, and wind speed and direction measurements over Lake Ontario and urban areas. These observations captured the full evolution (prior, during, and after) of 27 lake-breeze events (73% of observation days) in order to characterize the convective and dynamic processes driving lake breezes at the local scale and mesoscale. The dominant signal of a passing lake-breeze front (LBF) was an increase in dew-point temperature of $$2.3 \pm 0.3 \,^{\circ }\hbox {C}$$ , coinciding with a $$180^{\circ }$$ shift in wind direction and a decrease in air temperature of $$2.1 \pm 0.2 \,^{\circ }\hbox {C}$$ . Doppler lidar observations over the lake detected lake breezes 1 hour (on average) before detection by radar and mesonet. On days with the synoptic flow in the offshore direction, the lidars observed wedge-shaped LBFs with shallow depths, which inhibited the radar’s ability to detect the lake breeze. The LBF’s ground speed and inland penetration distance were found to be well-correlated ( $$r = 0.78$$ ), with larger inland penetration distances occurring on days with non-opposing (non-offshore) synoptic flow. The observed enhanced vertical motion $$({>} 1\hbox { m s}^{-1})$$ at the LBF, observed by the lidar on 54% of lake-breeze days, was greater (at times $${>} 2.5\hbox { m s}^{-1}$$ ) than that observed in previous studies and longer-lasting over the lake than over land. The weaker and less pronounced lake-breeze structure over land is illustrated in two case studies highlighting the lifetime of the lake-breeze circulation and the impact of propagation distance on lake-breeze intensity.

Wind fronts associated with cold-air outbreaks from the Chinese continent in the winter are often observed over the northern South China Sea and are well studied. However, wind fronts caused by another type of synoptic setting, the sudden increase or freshening of the north-east monsoon, which is caused by the merging of two anticyclonic regions over the Chinese continent, are also frequently encountered over the northern South China Sea. For the first time, such an event is investigated using multi-sensor satellite data, weather radar images, and a high-resolution atmospheric numerical model. It is shown that the wind front generated by the freshening of the north-east monsoon is quite similar to wind fronts generated by cold-air outbreaks. Furthermore, we investigate fine-scale features of the wind front that are visible on synthetic aperture radar (SAR) images through variations of the small-scale sea-surface roughness. The SAR image was acquired by the Advanced SAR of the European Envisat satellite over the South China Sea off the coast of Hong Kong and has a resolution of 150 m. It shows notches (dents) in the frontal line and also radar signatures of embedded rain cells. This (rare) SAR image, together with a quasi-simultaneously acquired weather radar image, provide excellent data with which to test the performance of the pre-operational version of the Atmospheric Integrated Rapid-cycle (AIR) forecast model system of the Hong Kong Observatory with respect to modelling rain cells at frontal boundaries. The calculations using a horizontal resolution with 3-km resolution show that the model reproduces quite well the position of the notches where rain cells are generated. The model shows further that at the position of the notches the vorticity of the airflow is increased leading to the uplift of warmer, moister air from the sea-surface to higher levels. With respect to the 10-km resolution model, the comparison of model data with the near-surface wind field derived from the SAR image shows that the AIR model overestimates the wind speed in the lee of the coastal mountains east of Hong Kong, probably due to the incorrect inclusion of the coastal topography.

International audience; Weather radar refractivity depends on low-level moisture, temperature, and pressure and is available at high space-time resolutions over large areas. It is of definite meteorological interest for assimilation, verification, and process-study purposes. In this study, the path-averaged refractivity change is simulated from the Arome cloud-resolving atmospheric system analyses and compared with corresponding radar observations over a 35-day period with various meteorological conditions. For that, a novel post-processing procedure is applied to radar data to improve its quality. Also, an observation operator is developed that ingests Arome analyses and simulates a 3-h path-averaged refractivity change. A sensitivity study shows that simulated path-averaged refractivity change is immune to the modelling of the beam height as long as it remains below approximately 60 m above the ground. Comparisons show overall consistency between observed and simulated path-averaged refractivity change, with discrepancies at times that suggest an improvement in analyses once radar refractivity change observations are assimilated. Finally, errors introduced when retrieving local refractivity from path-averaged refractivity are estimated and it is found for our dataset that such retrievals halve the range of usable observations.

A study of an interesting meteorological episode over the Owens Valley, California, USA during the Terrain-Induced Rotor EXperiment was conducted using a recently adapted statistical interpolation method to retrieve wind-velocity vectors from Doppler lidar data. This vector retrieval method has been adapted from radar data assimilation techniques. Results show that the method allows better preservation of local variations in the flow field than other techniques. In addition, a high resolution Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) run is used to understand the large-scale flow within the valley and compared with lidar retrievals. Observations from 1030 UTC to 1230 UTC (0230 local time to 0430 local time) on March 27, 2006 are presented. Lidar observations show complex and uncharacteristic flows such as sudden bursts of westerly cross-valley wind mixing with the dominant up-valley wind. Model results from COAMPS and other in-situ instrumentation are used to corroborate and complement these observations. The optimal interpolation technique for Doppler lidar data vector retrieval appears well suited for scenarios with complex spatial variations in the flow field.

Cloud radars at X, Ka and W-bands have been used in the past for ocean stud- ies of clouds, but the lack of suitable stabilization has limited their usefulness in obtaining accurate measurements of the velocity structure of cloud particles and the heights of cloud features. A 94 GHz (W-band) radar suitable for use on shipboard studies of clouds has been developed that is small and lightweight and can maintain the radar's beam pointing in the vertical to reduce the affects of the pitch and roll of the ship. A vertical velocity sensor on the platform allows the effects of the ship's heave to be removed from the measured cloud particle motions. Results from the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Exper- iment (VOCALS-Rex) field program on the NOAA vessel Ronald H. Brown demonstrate the improvements to the cloud measurements after the ship's motion effects are removed. The compact design of the radar also makes it suitable for use in aircraft studies. The radar is being repackaged to fit in an aft bay of a NOAA P3 aircraft to observe sea-spray profiles during ocean storms.

International audience; Refractivity depends on meteorological parameters such as temperature and water vapour pressure and can be measured using a weather radar. A realistic atmospheric simulation from the Meso-NH numerical model is used in order to describe and establish the relation between refractivity and the dynamic and thermodynamic phenomena responsible for the development and propagation of convection. These investigations lead to discussion of the complementarity between the refractivity and the convective available potential energy. The relation observed between the refractivity signal and the meteorological parameters calls the refractivity measurement into question, since it is based on phase differentiation with time and space and can be degraded by phase aliasing problems. These aliasing problems increase with the radar frequency (perceptible in the S-band, serious in the C-band, and more serious in the X-band) and also with the integration range and sampling time. Thus, a statistical approach permits us to simulate the possibility of measuring the refractivity with operational radar during convective events. A typical case in the south-east region of France is selected to simulate measurements by radar (S-band, C-band, X-band) in convective systems, in order to evaluate the measurement feasibility, particularly in terms of phase ambiguity, related to temporal and spatial sampling, of a future implementation of the refractivity measurement over the French operational radar network. This numerical statistical approach is completed with a similar study using in-situ measurements performed at the Trappes station. The seasonal and diurnal dependencies of aliasing are investigated, leading to clarification of the impact of the turbulent fluxes on the refractivity measurement.

At Frankfurt airport (EDDF) vertical soundings of the lower atmosphere from two independent sources are available. One of them is a wind and temperature profiler (wind temperature radar and radio acoustic sounding system, WTR/RASS) located at the western end of the main pair of runways. The second source is aircraft meteorological data relay (AMDAR), i.e. measurements operationally collected by approaching and departing aircraft. Together, both offer a rare opportunity to compare the performance of these widely used systems. We use 1 year of continuous data to calculate systematic temperature and wind vector differences between both measurement systems. The differences show a clear season-dependent structure in conjunction with typical inversion heights. Possible causes for this behaviour are discussed. Second, we compare the ability of both systems to identify inversion and wind-shear layers above the airport. AMDAR-detected layers are typically higher than wind profiler detections. The layer base is usually detected with more agreement than the top. The mutual probability of detection of inversions is found to be mostly between 40 and 60%.

This paper is the second of a series devoted to the observation and analysis of coherent structures in the cloudy Atmospheric Boundary Layer (ABL) such as horizontal rolls or thermal cells. In the first paper, the TRAC (Turbulence Radar Aircraft Cells) experiment which is the observational support of this investigation based on coupled radar-aircraft measurements, was presented along with an overview of the main results of this campaign held in June 1993 in France. Here the analysis is focused on the spatial characteristics (length-scale, orientation ... ) of the coherent structures, their temporal evolution and vertical distribution deduced from the radar reflectivity fields acquired in clear air at several levels for five different ABLs. For that, an original and efficient image processing method able to extract the major mode of the organisation was developed. These characteristics are examined in relation to the dynamic and thermodynamic state of the ABL using mean and turbulent information as observed by the in-situ aircraft. These experimental results are the basis of a future Large Eddy Simulation modeling of an organised ABL which is in progress and will be the concern of the third paper in the series.

The scientific objective of the TRAC experiment (Turbulence Radar Aircraft Cells) was to investigate the respective roles played by small-scale turbulence and coherent structures in the vertical transfer within the atmospheric boundary layer (ABL). Field research held in June 1993 in France was based on coupled aircraft and Doppler radar measurements. The results discussed here are mainly focused on the evaluation of the performance of the radar in the 3D description of the clear air ABL, which was the technical goal of TRAC. During the experiment, the radar was able to provide continuous and coherent echo fields over a range of several tens of kilometres, extending up to about 3 km. Good agreement was obtained in the ABL between the radar-derived turbulent quantities and airborne measurements. As depicted by the reflectivity fields, coherent organizations were found to be a common feature of the eleven ABL cases analyzed. These organizations evolved during the day between a banded structure and a cellular pattern. A very weak correlation was found between the reflectivity field and the atmospheric parameters measured by the aircraft. However, in terms of characteristic scale, the reflectivity field appeared to be strongly related to the water vapour field. The inhomogeneity induced by the coherent circulations questions the representativity of one-dimensional sampling of these 3D fields and suggests the need to adapt the traditional statistical approach of the ABL.

Boundary-layer meteorologicalmeasurements were made before, during, and after theMay 10, 1994 partial (94%) solar eclipse over thedesert at the Atmospheric Profiler Research Facilityat White Sands Missile Range, New Mexico, U.S.A. A largenumber of sensors were located in the middle of theTularosa Basin to measure mean and turbulentquantities and the refractive index structureparameter (Cn2). This event permitted a rareopportunity to measure, examine, and document theeffects upon the atmospheric boundary layer of asudden cut-off and subsequent turn-on of the sun'sradiant energy. At the peak of the eclipse, whichoccurred for more than three hours, all of the heatexchange parameters were affected, the turbulentprocesses were diminished, and the refractive indexstructure parameter decreased dramatically. A time-heightdisplay from the FM-CW radar shows a Kelvin–Helmholtzwave that developed during theeclipse. The results of several analyses arepresented to document and characterize the eclipse-modifiedboundary layer.

Sensitive Doppler radars regularly detect fine lines ofenhanced reflectivity in mesoscale boundary-layer convergence zones. Recent studies have concludedthat these ‘fine lines’ are attributable primarily to backscatter fromconcentrations of small, weakly flying insects (‘aerial plankton’)entrained in the convergence zones. Such concentrations are likely tobe attractive to aerial predators that feed on small insects, raising thequestion of whether the presence of the predators themselves maycontribute significantly to the radar-observed fine lines.

Based on continuous observations of the planetary boundary layer (PBL) with anL-band (1357.5 MHz) boundary-layer radar (BLR) at a hilly location in Japan, we have discovered that on clear days, a thin enhanced echo layer corresponding to the top of the PBL (or mixed layer) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward velocities were observed below the echo layer (or inside the PBL), reaching 1500 m in the afternoon.

The measurement of turbulent wind quantities with a single Doppler radar requires a horizontal homogeneity assumption. When the terrain is not horizontally homogeneous, then the measurement of the various moments is contaminated by the gradients of these moments. However, by scanning the radar at three different elevation angles, the contamination from these gradients can be removed from the second-moment quantities of interest.

For the last two decades, radar has been used to probe the planetary boundary layer (PBL) in a number of different ways. The sensitive, high-power radars at Wallops Island, Virginia, were used to examine the refractive index structure of the PBL over hundreds of square kilometers with a resolution of several hundred meters. In these studies, fields of convective cells were documented over land and over the ocean. Vertically pointing FM-CW radars were used to observe the vertical structure of the PBL with resolution as good as 2 m. In addition, FM-CW radars, as well as conventional radars, have detected birds and insects in the PBL. Within the last seven years, dual Doppler radar was used to document the details of the velocity structure of the PBL in three dimensions. Recently, it has been shown that radar can be a useful tool for turbulence and diffusion measurements in the boundary layer.

Some early results from an oceanographic experiment staged off Marineland, Florida, in December 1975 are presented, viz., intercomparisons between the X-band and L-band imagery obtained by the Environmental Research Institute of Michigan's (ERIM) dual-wavelength, dualpolarization multiplexed radar. This radar allows direct comparison since the images are produced simultaneously. The wave data obtained from the radar imagery are compared with surface measurements of waves obtained with a pitch- and-roll buoy. The conclusions are only applicable to medium and low wind and wave conditions encountered during the Marineland test. The results indicate that X-band images provide superior quality wave imagery and more useful Fourier Transforms compared to L-band under equivalent signal-to-noise ratios and resolution. Optimum wave imagery is seen when waves propagate in the range direction. Comparisons betweenin situ measurements and X-band imagery of the same area indicate that the dominant wave direction can be obtained from imagery to within a few degrees. A one-dimensional spectrum obtained from X-band imagery compares favorably with an equivalent wave frequency spectrum obtained from the pitch- and-roll buoy after suitable transformation asing linear wave theory.

Analysis of a vast series of angel echo observations made by a millimetric vertically-pointing radar reveals four major types of echo organization on height-time recordings. The relationship between the formation and development of these structures and the thermodynamic state of the atmosphere is clarified. The meteorological indicator role played by angel echoes is then discussed. In particular it is shown that in certain conditions, angel echoes can delineate the boundaries of small cumulus.

The experimental Seasat-A satellite, scheduled for launch in 1978, is designed to provide measurements of sea-surface temperature, surface wind speed and direction, sea state and directional wave spectra. The instrument package of the satellite includes a highly precise radar altimeter, a wind-field scatterometer, a synthetic-aperture imaging radar, a scanning multifrequency microwave radiometer and a radiometer operating in the visual and infrared. Tides, currents and storm surges on the marine geoid may be monitored through analysis of range information from the radar altimeter and a satellite ephemeris; sea-ice reconnaissance is also planned.

Features associated with the upper limit of convection, observed by turbulence probes supported by a tethered kite balloon and by high-power Doppler radar, are described. The observations illustrate the interaction of thermal plumes with the capping inversion (and stable air aloft) and confirm the existence of non-turbulent, intermittently turbulent and fully turbulent layers. Evidence is presented for entrainment processes occurring on scales ranging from a few metres to several hundred metres. Individual distortions of the inversion interface, tracked by the radar, have a lifetime of about 5 minutes. Other, larger scale (i.e., > 1 km) perturbations of the top of the boundary layer were observed over longer periods, and are thought to be due to topographical effects.

The usual operation of a synthetic-aperture radar (SAR) assumes that the sensor platform moves at a constant velocity along a straight line and that objects to be imaged are stationary. Moving ocean waves perturb the Doppler frequencies in the SAR phase histories, and when processed in a conventional manner, they produce images of waves that are dispersed and thus defocused in the azimuth (along-track) direction. This defocusing can be compensated in the processor by readjusting the azimuth focus by an amount proportional to the velocity of the wave. The relationship among the parameters of the radar system, the SAR processor, and the relative target velocity is, theoretically determined for radars that operate at both X- and L-band frequencies. Experimental observations support these calculations. The effect of varying spectrum sampling, range focus, and number of coherent averages or looks is also observed.

The satellite-borne radar altimeters on GEOS 3 and SEASAT produce high-precision measurement of distance from the satellite to the ocean surface. However, the precision of the GEOS 3 altimeter (~50 cm) and especially the forthcoming SEASAT (~10 cm) instrument far exceeds our ability to determine the position of either satellite using conventional electronic or laser-tracking methods. Thus special techniques are required to prevent the uncertainty of the satellite position from degrading the value of the altimeter data. The altimeter data themselves provide a solution to this problem. Using the condition that intersections of passes of altimeter data must measure the same time-invariant part of the sea-surface height, the root-mean-square error of 292 intersections of 47 passes of GEOS 3 altimeter data from the Atlantic Ocean was reduced from 17 m to 44 cm. Simulations of the SEASAT problem also show that altimeter data can aid in determining the satellite orbit, and have their greatest value when radar or laser tracking is sparse.

Radar measurements of wave height are compared with independent measurements made during the JONSWAP-2 experiment by Waverider and pitch-roll buoys, a shipborne wave recorder and a laser profilometer. The radar data were recorded by a Naval Research Laboratory (NRL) nanosecond-pulse X-band radar altimeter flown in a NASA C-54 aircraft at 3-km altitude under various wind and sea conditions. Averages of 800 pulses of the pulse-limited altimeter data were used to calculate maximum-likelihood estimates of significant wave height (SWH) and skewness of the sea-surface height distribution. The mean values of the radar-estimated SWH were in good agreement with the other measurements. The standard deviation of the values of the radar measurements was typically 10% of the average wave height. A two-dimensional computer simulation of the sea surface indicates that the major portion of the observed standard deviation is attributable to the relatively small sea-surface area illuminated by the radar (125 m × 900 m) rather than to instrumental error. Increasing the number of pulses averaged reduced the variance in the estimates without changing the means. The mean value of the skewness parameter was generally near zero but the standard deviation was typically 0.25. The estimate of SWH did not change when the skewness parameter was constrained to zero.

The physics of the interaction of electromagnetic waves with the ocean surface has been an active area of research for a number of years. This paper contains the results of satellite and aircraft experiments to investigate the ability of active microwave radars to infer surface wind speeds remotely. Data obtained from the recent National Aeronautics and Space Administration (NASA) Skylab experiment are compared with surface wind speeds measured by low-flying aircraft and ships-of-opportunity and found to give useful estimates of the ocean wind field. Also investigated was the influence of varying wave height on radar measurements of wind speed by measuring the backscattering cross-section for constant wind speed but variable wave conditions. It is found that this effect is of little importance.

High-frequency (HF) radar observations of sea waves apparently travelling against the wind are discussed. In some cases, it appears that the wave angular spectrum does contain components travelling against the wind. Some data taken on the Barbados Island during BOMEX have been analyzed, and they show that such components increase in amplitude with distance from a windward shore. It is shown that the order of magnitude of the observed growth rates can be explained by resonant non-linear interaction within the wave spectrum.

Three-dimensional air motion, observed with radar in a convective planetary boundary layer under light wind conditions, is organized in cellular structures scaled from 2 to 5 km; simultaneous acoustic echosonde measurements indicate that near the ground there are smaller cells than those observable by the radar. At heights of 400 m, cell scales become as large as those observable by the radar. Pressure fluctuations at the ground measured with spatial pressure transducers, at the same time as the radar and echosonde observations, traveled at the same speed and direction as the radar volume-averaged wind. The length scale determined at the ground with pressure fluctuation data was between 1.2 and 2.4 km, smaller than the largest radar-measured motion scales.

Several features of the maintenance of breaking gravity waves and turbulence in a marine inversion are examined. A formulation is proposed for a critical Richardson number based on a mutual response of the mean and turbulent states to a wave-like disturbance. The energy balance, based on averaged aircraft soundings, is examined to ascertain the order of magnitude of the component terms in the vicinity of a contemporaneous radar echo. Some physical mechanisms are discussed which may explain some aspects of the sustained existence of the echo layers.

A radar equation which can be applied to any bistatic acoustic sounder is derived. The equation reduces to the expression normally used for the special case of the monostatic sounder. Numerical results using this equation are given for specific acoustic sounders, including the relative contributions to the scattering from the temperature and velocity parameters (C T 2 and C V 2), the effect of wind speed on the scattered intensity and the measurement of horizontal wind velocity.

A special-purpose radar has been used to observe the vertical distribution of large, strong-flying insects migrating in the nocturnal boundary layer over the central-western plains region of New South Wales. During the period of take-off flight at dusk, the density of insects decreased monotonically with height, and a distribution of this type persisted for much of the night in the zone of steady temperature lapse extending from the top of the inversion layer to the flight ceiling at about 1 km. Later in the night, the insects often became concentrated in the warm air at the top of the inversion layer. The lower boundary of this concentration sometimes became sharply defined, but above the zone of maximum insect density there was usually a smooth decrease of insect numbers with altitude. When the boundary layer was disturbed by an atmospheric density current, the direction and vertical distribution of the migration was permanently changed; solitary wave disturbances, however, had only a transient effect.

Radar sensors are being used to provide two-dimensional imagery of the ocean surface. The radar image has a brightness variation which is proportional to the local backscatter cross-section of the surface at the wavelength of observation. The backscatter cross-section is, in turn, a function of the local surface topography and, in the case of a coherent radar system, of the surface dynamics. The synthetic-aperture radar imaging technique produces very high resolution images essentially independent of the distance to the surface being imaged. This technique is discussed in some detail. The emphasis is on recent observations of a large variety of ocean surface patterns. Deep ocean waves have been observed under a variety of weather conditions, including hurricanes. Breaking waves are observed because of the increase in roughness and the presence of foam. Discrete wave-like patterns, which resemble internal wave trains, have been observed in numerous locations; and eddy-shaped, linear, curvilinear and periodic slicks have also been seen. The different models for wave image formations are briefly reviewed. Specifically, the roughness modulation, tilt modulation, and orbital velocity models are discussed. Finally, it is shown that surface randomness does not destroy the coherence of the signal needed to generate the synthetic-aperture image because of the short-term coherence of the small Bragg waves.

A number of troposphere-monitoring experiments were pressed into service during a large tropospheric disturbance which lasted for several days in December 1974. The experiments operated intensively included a monostatic Sodar system, a satellite radio beacon receiver at very low elevation angles, Meteorological Radiosondes, a 250-kW weather radar, a microwave LOS link and a microbarograph. This multi-pronged campaign yielded very useful results and the correspondences between various observations were striking. The Sodar and microwave link observations were visibly affected only when δN reached a value of about −70 units; however, the satellite beacon observations showed anomalous behaviour even for lower δN values. Rather spectacular coincidences occurred on one night: the large fading (up to 10 db) in the LOS link; highly disturbed conditions displayed in the entire Sodar range; the largest tropospheric effect on the satellite beacon system; and the most active superrefraction of the radar echoes. It was also noticed that these effects are obvious only during nights, while they remain masked or dampened in the sun-lit hours.

Remote measurements of wind velocity are performed, up to 100 km away from the shore, by using some features of the Doppler spectrum of HF radio waves backscattered by the sea. The variation with radio-frequency of the ratio of the amplitudes of the two Bragg lines in this spectrum shows a discontinuity occurring at a value simply related to the wind velocity. The validity of these measurements is discussed by comparison with meteorological data obtained by conventional methods.

From the mid 1960s to the present, the normalized radar cross-section (NRCS) of the ocean has been measured using airborne radars operating over a frequency range of 0.4 to 14 GHz. Analyses of these data have shown that the NRCS was proportional to the ocean surface wind speed raised to some power, but the values of the exponent remained in dispute. This paper extends previous work and uses these NRCS measurements to demonstrate that to the first order, the NRCS is a function of only the friction velocity at the ocean's surface. Further analyses characterize the dependence of the NRCS on radar variables such as frequency, incidence angle, polarization, etc. Finally, recommendations are made for using Ku-band radars at large incidence angles for remote sensing of the wind friction velocity vector.

Short gravity-capillary waves, the predominant radar scatterers under many oceanic and radar-viewing conditions, are modulated in amplitude, velocity and orientation by the larger-scale motions of the ocean surface. These modulations render the larger scales observable to microwave radar. The high data rate and advanced technology of modern radar systems make it possible to measure these modulations and, in some cases, to display them as images of the ocean surface. While the modulation of orientation and velocity are straightforward to understand, the amplitude modulation is a dynamic response of the equilibrium short gravity-capillary waves to larger scale driving forces including straining by orbital velocities of large waves. Microwave studies of the growth and equilibrium of short wind-generated waves are reviewed. It is shown that the response of these waves to straining can produce modulations in radar cross-section greater than those due to tilting but which are wind-speed dependent. The net modulation depends strongly on the direction as well as the magnitude of the wind speed. Quantitative determination of ocean wave-height spectra from measured modulations will therefore be a complicated procedure. Measurements of ocean waves with CW Doppler and Synthetic Aperture radars are discussed in the light of these findings.