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Refractivity Measurements Using Phased Array Radar

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... Radar in National Weather Radar ... radar being adapted for weather use. Improve the leadtime of weather warnings ... Local Refractivity. To Obtain Absolute ... – PowerPoint PPT presentation

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Title: Refractivity Measurements Using Phased Array Radar


1
Refractivity Measurements Using Phased Array Radar
  • Boon Leng Cheong

2
Phased Array Radar in National Weather Radar
Testbed
  • Also being referred to as SPY-1A Phased Array
    Radar (PAR)
  • Navy's phased array radar being adapted for
    weather use
  • Improve the leadtime of weather warnings

3
Signals are bin sorted for range
4
Array Processing
5
Standard Weather Scanning in PAR
6
Validation with WSR-88D
7
Refractivity Measurements from Ground Clutters
Using PAR
  • Use refractivity measurement as a proxy to
    estimate humidity map within 25 km
  • Adapt the method by Frédéric 2004, JTECH, 21,
    pp. 560-573

8
Ground Clutters in Close Ranges
9
Refractive Index
  • Refractive Index of a medium is
  • Radio waves propagate in a medium with
    non-uniform refractivity bends and changes speed
  • Near the surface, Near the surface, n 1.0003,
    and ?n in O(10-5) O(10 PPM) within 60 km
  • Radio wave bends less than 1 horizontally within
    60 km

10
Refractivity Function
  • More convenient quantity, Refractivity
  • Refractivity equation
  • where p pressure in millibar (mb)
  • T absolute temperature in Kelvin (K)
  • e vapor pressure in mb

11
Refractivity Function
  • N changes predominantly by water vapor
  • At conditions
  • T 28 C
  • p 1000 mb
  • q 7 g kg-1
  • ?1C ?1 N-unit
  • ?1 mb ?0.3 N-unit
  • ?1 g kg-1 ?6 N-unit

12
Phase and Refractivity
  • Time for electromagnetic wave to travel the 2r
    path depends on refractive index
  • The phase of the received signal changes if the
    integrated refractivity along the path is changed

13
Refractivity Measurements
  • Two measurements of phase
  • Use measurement at t0 as the reference map
  • Ideal reference map is measured at
  • Homogeneous humidity profile
  • nref is calculated for the reference condition

14
Local Refractivity
15
To Obtain Absolute Refractivity
  • We have
  • Gradient of phase change along the radius tells us

16
Idealized-simplified Example
  • Refractivity structure is manifested into phase
    difference

17
Noisy Measurements
18
Ground Clutters in Close Ranges
19
Finding Good Ground Clutters
20
To-do List
  • For refractivity measurements
  • Collect longer dataset for a reference map
  • Alternate(better?) scheme for clutter quality
    index
  • Etc

21
  • Something Else

22
Numerical Simulation
  • Point scatterers in a 3D enclosing volume
  • Positions of scatterers are updated based on the
    mean velocity and turbulent field
  • Scatterer exiting the volume is replaced with a
    new scatterer in the opposite location

Modified from Holdsworth and Reid, 1995
23
Parameters Update
Parameters include reflectivity, wind field,
turbulence, etc.
24
Example Parameters
The parameters can be assumed have simple
structures. OR Something generated with more
sophisticated algorithms and fed into the
simulation scheme
25

26
  • The End
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