Finescale structure and microphysics of coastal stratus

ABSTRACT Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases. Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2-1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions

Remote Sensing of Precipitation from Airborne and Spaceborne Radar

Weather radar measurements from airborne or satellite platforms can be an effective remote sensing tool for examining the three-dimensional structures of clouds and precipitation. This chapter describes some fundamental properties of radar measurements and their dependence on the particle size distribution (PSD) and radar frequency. The inverse problem of solving for the vertical profile of PSD from a profile of measured reflectivity is stated as an optimal estimation problem for single- and multi-frequency measurements. Phenomena that can change the measured reflectivity Z(sub m) from its intrinsic value Z(sub e), namely attenuation, non-uniform beam filling, and multiple scattering, are described and mitigation of these effects in the context of the optimal estimation framework is discussed. Finally, some techniques involving the use of passive microwave measurements to further constrain the retrieval of the PSD are presented

Millimeter-wave and terahertz imaging techniques

This thesis presents the development and assessment of imaging techniques in the millimeterwave (mmW) and terahertz frequency bands. In the first part of the thesis, the development of a 94 GHz passive screener based on a total-power radiometer (TPR) with mechanical beamscanning is presented. Several images have been acquired with the TPR screener demonstrator, either in indoor and outdoor environments, serving as a testbed to acquire the know-how required to perform the research presented in the following parts of the thesis. In the second part of the thesis, a theoretical research on the performance of near-field passive screeners is described. This part stands out the tradeoff between spatial and radiometric resolutions taking into account the image distortion produced by placing the scenario in the near-field range of the radiometer array. In addition, the impact of the decorrelation effect in the image has been also studied simulating the reconstruction technique of a synthetic aperture radiometer. Guidelines to choose the proper radiometer depending on the application, the scenario, the acquisition speed and the tolerated image distortion are given in this part. In the third part of the thesis, the development of a correlation technique with optical processing applicable to millimeter-wave interferometric radiometers is described. The technique is capable of correlating wide-bandwidth signals in the optical domain with no loss of radiometric sensitivity. The theoretical development of the method as well as measurements validating the suitability to correlate radiometric signals are presented in this part. In the final part of the thesis, the frequency band of the imaging problem is increased to frequencies beyond 100 GHz, covering the THz band. In this case the research is centered in tomographic techniques that include spectral information of the samples in the reconstructed images. The tomographic algorithm can provide detection and identification of chemical compounds that present a certain spectral footprint in the THz frequency band.Postprint (published version

Entwicklung neuartiger Mess- und Auswertungsstrategien für ein scannendes Wolkenradar und deren Anwendungsbereiche

Die Datengrundlage dieser Arbeit bilden Messungen in Wolken mit einem scannenden Radar. Zur Analyse der wolkenphysikalischen Prozesse werden neuartige Verfahren entwickelt, die (i) die getrennte Betrachtung von Teilprozessen in der Schmelzschicht ermöglichen und die es (ii) erlauben, mittels einer neuen, in dieser Arbeit eingeführten Größe (""residuale Geschwindigkeit""), kleinräumige Fluktuationen in Wolken zu identifizieren und ihre dynamische Entwicklung zu beschreiben

Entwicklung neuartiger Mess- und Auswertungsstrategien für ein scannendes Wolkenradar und deren Anwendungsbereiche

Die Datengrundlage dieser Arbeit bilden Messungen in Wolken mit einem scannenden Radar. Zur Analyse der wolkenphysikalischen Prozesse werden neuartige Verfahren entwickelt, die (i) die getrennte Betrachtung von Teilprozessen in der Schmelzschicht ermöglichen und die es (ii) erlauben, mittels einer neuen, in dieser Arbeit eingeführten Größe ("residuale Geschwindigkeit"), kleinräumige Fluktuationen in Wolken zu identifizieren und ihre dynamische Entwicklung zu beschreiben

Entwicklung neuartiger Mess- und Auswertungsstrategien für ein scannendes Wolkenradar und deren Anwendungsbereiche

Die Datengrundlage dieser Arbeit bilden Messungen in Wolken mit einem scannenden Radar. Zur Analyse der wolkenphysikalischen Prozesse werden neuartige Verfahren entwickelt, die (i) die getrennte Betrachtung von Teilprozessen in der Schmelzschicht ermöglichen und die es (ii) erlauben, mittels einer neuen, in dieser Arbeit eingeführten Größe ("residuale Geschwindigkeit"), kleinräumige Fluktuationen in Wolken zu identifizieren und ihre dynamische Entwicklung zu beschreiben

SPATIAL FILTERING OF CLUTTER USING PHASED ARRAY RADARS FOR OBSERVATIONS OF THE WEATHER

Phased array radars are attractive for weather surveillance primarily because of their capacity for extremely rapid scanning through electronic steering. When combined with the recently developed beam multiplexing technique, these radars can provide significantly improved update rates, which are necessary for monitoring rapidly evolving severe weather. A consequence of beam multiplexing, however, is that a small number of contiguous time series samples are typically used, creating a significant challenge for temporal/spectral filters typically used for clutter mitigation. As a result, the accurate extraction of weather products can become the limiting performance barrier for phased array radars that employ beam multiplexing in clutter-contaminated scattered fields. By exploiting the spatial correlation among the signals from the elements of the phased array antenna, the effect of clutter contamination can be reduced through a processed called spatial filtering . In contrast to conventional temporal filtering, spatial filtering is used to adaptively adjust the antenna beam pattern to produce lower gain in the directions of the undesired clutter signals. In this dissertation, the effect of clutter mitigation using spatial filtering was studied using numerical simulations of a tornadic environment and an array antenna configuration similar to the NSSL NWRT Phased Array Radar for changes in signal-to-noise ratio, clutter-to-signal ratio, number of time series samples, and diagonal loading for three types of clutter sources that include nearly stationary ground clutter, moving targets such as aircraft, and wind turbine clutter, which has recently been documented to be increasingly problematic for radars. Since such data are not currently available from a horizontally pointed phased array weather radar, experimental validation was applied to an existing data set from the Turbulent Eddy Profiler (TEP) developed at University of Massachusetts, which is a vertically pointed phased array radar. Results will show that spatial filtering holds promise for the future of phased array radars for the observation of the weather in a clutter environment