Removal of range uncertainty of CW Wind Lidar by frequency modulation.

<p>Frequency Modulation (FM) is a well-known technology but was never used in Continuous Wave (CW) Wind Lidars. The reason is because range and velocity can only be resolved for single hard targets like vehicles but not for dispersed targets like atmospheric aerosol. Here we present a FMCW system that uses the established focusing method for ranging and in addition a frequency modulated transmit signal. The origin of the scattered radiation is localized by focusing in a limited measuring volume. Because of this – by applying FM – the unavoidable range-velocity ambiguity of CW Wind Lidars can be resolved similarly as for hard targets. This is particularly important in conditions with fog or low hanging clouds (coastal or mountainous areas) or distant moving obstacles behind the measuring volume, or generally spoken in cases with strong gradients of the backscatter cross section. While out-of-focus contributions is a well-known concern of CW Lidar, we will show examples from FMCW field measurements first time revealing quantitatively the range uncertainty based on focus distance. Not surprisingly this uncertainty increases with height range, where the focus becomes less well-defined. Furthermore, the FMCW Wind Lidar allows also to correct uncertainties of mechanical focus distance setting. This is also mainly important at larger ranges where the focus distance becomes very sensitive to mechanical tolerances. Moreover, auxiliary measurements of wind direction, that are needed by CW systems for removing the sign-ambiguity of velocity, are obsolete, and there is no lower threshold of measurable windspeed. As a consequence wind measurements are feasible in street canyons, forest clearings and any other environment with strong vertical gradients.</p>

Low-Frequency Sea Surface Radar Doppler Echo

Observed sea surface Ka-band normalized radar backscatter cross section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave range. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent NRCS averaging within the footprint account for the NRCS and DV LF variance with the exception of VV NRCS for which almost half of the LF variance is attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for |MTF|< 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations.

Equation for the Microwave Backscatter Cross Section of Aggregate Snowflakes Using the Self-Similar Rayleigh–Gans Approximation

In this paper an equation is derived for the mean backscatter cross section of an ensemble of snowflakes at centimeter and millimeter wavelengths. It uses the Rayleigh–Gans approximation, which has previously been found to be applicable at these wavelengths due to the low density of snow aggregates. Although the internal structure of an individual snowflake is random and unpredictable, the authors find from simulations of the aggregation process that their structure is “self-similar” and can be described by a power law. This enables an analytic expression to be derived for the backscatter cross section of an ensemble of particles as a function of their maximum dimension in the direction of propagation of the radiation, the volume of ice they contain, a variable describing their mean shape, and two variables describing the shape of the power spectrum. The exponent of the power law is found to be −. In the case of 1-cm snowflakes observed by a 3.2-mm-wavelength radar, the backscatter is 40–100 times larger than that of a homogeneous ice–air spheroid with the same mass, size, and aspect ratio.

Radar efficiency and the calculation of decade-long PMSE backscatter cross-section for the Resolute Bay VHF radar

The Resolute Bay VHF radar, located in Nunavut, Canada (75.0° N, 95.0° W) and operating at 51.5 MHz, has been used to investigate Polar Mesosphere Summer Echoes (PMSE) since 1997. PMSE are a unique form of strong coherent radar echoes, and their understanding has been a challenge to the scientific community since their discovery more than three decades ago. While other high latitude radars have recorded strong levels of PMSE activities, the Resolute Bay radar has observed relatively lower levels of PMSE strengths. In order to derive absolute measurements of PMSE strength at this site, a technique is developed to determine the radar efficiency using cosmic (sky) noise variations along with the help of a calibrated noise source. VHF radars are only rarely calibrated, but determination of efficiency is even less common. Here we emphasize the importance of efficiency for determination of cross-section measurements. The significant advantage of this method is that it can be directly applied to any MST radar system anywhere in the world as long as the sky noise variations are known. The radar efficiencies for two on-site radars at Resolute Bay are determined. PMSE backscatter cross-section is estimated, and decade-long PMSE strength variations at this location are investigated. It was noticed that the median of the backscatter cross-section distribution remains relatively unchanged, but over the years a great level of variability occurs in the high power tail of the distribution.