Remote Sensing

7/8/02

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Table of Contents

Remote Sensors are needed to provide space and time coverage of the atmosphere to the degree not capable by traditional (in situ) sensors.

Degree of loss depends to a great extent on the wavelength of the signal.

Electromagnetic waves:

Receives returning reflected energy

At a range of 50 km, the beam will spread to a diameter of 873 meters since: where: S = arc length, or width of spread, R = radius, or distance along beam, f = angle of spread in radians

In the case of precipitation, the radar illuminates a large number of individual targets. The average returned power from this volume of raindrops is given by: where:

For spherical targets uniformly distributed over the volume illuminated with an incident wavelength, l, large compared to the radius, a, of the target, the radar back-scattering cross section, sb, of the volume is given byRayleighís law to be:

The returned power, intensity, is expressed in decibels of reflectivity.

Thus, each bin receives a single intensity value for all the precipitation falling within that volume, regardless whether some reflects high intensity and some reflects low intensity.

When a target is in motion with radial velocity (towards or away from the radar) of n, the reflected signal has a frequency of ní given by: where + means target is moving towards the radar, and - is target moving away.

Nexrad (WSR-88D)(~164)

Terminal Doppler Weather Radar

Airport Surveillance Radars

Installed at airports which do not have TDWR

Palestine, TX, profiler data

Laser Radar (Lidar/Ladar)

Coherently: With reference to the emitted signal.

Medium for different types of lasers include: gases (Helium Neon, Zenon Fluoride); solid state diodes, dyes and crystals (ND:YAG = Neodymium: Yttrium Aluminum Garnet)

Photocounts received at fixed time intervals after a lidar pulse. Fixed time intervals represent heights above the lidar unit when aimed vertically.

Used in detection of Clear Air Turbulence, aerosols, concentrations of gases (water vapor, carbon dioxide, methane, chloroflurocarbons, ozone), Clouds, temperature profiles, winds, etc.

Differential Absorption Lidar (DIAL)

LITE Experiment: Lidar Technology Experiment.

Pressure profiles can be determined based on the amount of ìpressure broadeningî of the actual absorption vs. wavelength curve as compared to a standard absorption vs. wavelength curve for the particular gas under study.

Lidar photocounts in range bins can be related to atmospheric density by: where, b is a constant which depends on the individual lidar system, the type of scattering (Rayleigh) and the transmission of the atmosphere, z is the height, N(z) is the number of photocounts at each height.

Clear Air turbulence is detected by sudden changes in the velocity of aerosols as measured by the phase shift between the transmitted and returned laser energy.

Cut-away of a Sodar unit.

Radio Acoustic Sounding System (RASS)

Tv=Virtual Temperature

Author: Alcorn

Email: alcorn@ariel.met.tamu.edu

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