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Lab 6 Radar Imagery Interpretation

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Canada is in the process of converting all of its weather radars to Doppler radars. ... Track the location of its maximum echoes over time on a map of the area. ... – PowerPoint PPT presentation

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Title: Lab 6 Radar Imagery Interpretation


1
Lab 6 Radar Imagery Interpretation
2
Background
  • Weather radar (radio detection and ranging) is
    another very useful remote sensing tool used in
    meteorological forecasting.
  • Microwave radar was developed in early World War
    II to aid in spotting distant ships and
    airplanes. It was noticed early on that during
    adverse weather conditions widespread
    interference often appeared on the radar screen
    and obscured the military objects of interest.
  • A large body of theoretical and experimental work
    in the 1940s showed that this weather clutter
    arose from the scattering of radar waves by
    precipitation.

3
  • These early findings have been refined and
    elaborated to the point that most of the
    measurable properties of radar signals -
    amplitude, phase, polarization, and frequency -
    can be interpreted in terms of the sizes, shapes,
    motions or thermodynamic phase of the
    precipitation particles.
  • Because of their ability to observe and measure
    precipitation quickly, accurately, and from great
    distances, radars have become essential in
    weather observation and forecasting.

4
  • Heavier precipitation reflects more microwave
    energy back to a radar than lighter rain.
    However, more distant rain also gives a weaker
    return signal. A range-corrected and
    equipment-calibrated measure of reflectivity from
    rain is given by
  • log(Z) log(received power) 2 log(range)
    constant
  • where Z is the radar reflectivity factor. Because
    Z has such a wide range of values, the
    reflectivity is usually expressed as decibels dB
    of Z.

5
  • dBZ 10 log(Z)
  • Larger and more numerous drops reflect more radar
    energy
  • Z SD6/ V
  • where D is drop diameter, V is volume of air
    holding the drops, and the sum is over all
    raindrops within that volume.

6
  • But the number and diameter of drops also
    determines the rainfall rate.
  • When the above three equations are combined and
    empirically tuned to the observations, the result
    is a formula for converting radar echo intensity
    in dBZ to rainfall rate R
  • R cR100.0625dBZ
    Eqn 1
  • where cR 0.036 mm h-1 .
  • Six discrete levels of radar echo intensity are
    often used, corresponding to descriptive rainfall
    categories.

7
Figure 8-12 Rainfall intensity chart (Stull 2000)
8
  • Owing to their particular shapes, snowflakes
    produce echoes of different intensity than
    raindrops of the same size.
  • The snowfall rate S is therefore often inferred
    from the radar reflectivity factor from
  • S cS100.05dBZ
  • where cS 0.018 cm h-1 .
  • This relationship assumes that 1 cm of melted
    snow equates 1 mm of water, i.e. that the falling
    snow has a density of 100 kg m-3 .

9
Example
  • Question What is the rainfall rate R for an echo
    of 43 dBZ?
  • Answer Use Eqn 1
  • R cR100.0625dBZ 0.036 mm/h(100.062543)
    17.5 mm/h

10
Example 2
  • Question What is the reflectivity Z for an echo
    of 43 dBZ?
  • Answer Use dBZ 10 log(Z)
  • Then log(Z) dBZ/10 43/10 4.3
  • Thus Z 104.3 19952.6

11
Radar Displays
12
  • There are several major types of radar displays
  • Plan Position Indicator (PPI) This is a map
    displaying radar reflectivity or echoes on polar
    coordinates in plan view.
  • With elevation angle fixed, the antenna scans
    360o in azimuth with the beam sweeping across a
    conical surface in space.
  • The data are then used to give the horizontal
    echo structure.

13
  • Range Height Indicator (RHI) This is similar to
    a PPI, where the antenna scans in elevation with
    azimuth fixed, and emphasizes the vertical
    structure of a precipitating system.

14
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16
  • Constant-Altitude Plan Position Indicator
    (CAPPI) It is a composite radar display
    constructed by assembling radar data from many
    PPIs at successive elevation angles to obtain the
    pattern of the data at a specified constant
    altitude.
  • CAPPIs are the common radar images displayed by
    Environment Canada (at 1.5 km above the surface
    from the radar site).

17
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18
  • A Doppler radar is a weather radar that detects
    and interprets the Doppler effect in terms of
    radial velocity of a target.
  • The signal received by a radar from a moving
    target differs in frequency from the transmitted
    frequency by an amount that is proportional to
    the radial component of the velocity relative to
    the radar.

19
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20
  • A useful introduction to Doppler radar can be
    found on NOAA's website.
  • Canada is in the process of converting all of its
    weather radars to Doppler radars.
  • The full Canadian network of weather radars,
    including the one at Baldy Hughes, B.C. (40 km
    southwest of Prince George) can be found on
    Environment Canada's website.

21
Materials
  • You are provided with two sequences of Doppler
    radar imagery covering parts of North America
  • A sequence of Doppler radar images of a severe
    thunderstorm outbreak that occurred in the
    midwestern U.S. (near St. Louis, Missouri) in May
    of 2003.
  • Another sequence of Doppler radar images of the
    winter storm that affected the Prince George area
    on 26-27 February 2006.

22
Questions
  • 1) Examine carefully the sequence of CAPPI images
    (base reflectivity) for the midwestern U.S.
    assuming ambient air temperatures are well above
    0oC and answer the following questions
  • A) What is the precipitation rate for the maximum
    echoes observed on these radar images (65 dBZ )?
  • B) What type of precipitation does this intensity
    typically represent?
  • C) What is the radar reflectivity factor (Z ) for
    this echo?

23
  • D) What is the precipitation rate for an average
    echo observed on these radar images (30 dBZ)?
  • E) Observe the evolution of the severe
    thunderstorm just to the south of St. Louis.
    Track the location of its maximum echoes over
    time on a map of the area. From this trajectory,
    linearly extrapolate the direction in which the
    storm is expected to move if it does not
    dissipate. Highlight the areas where a severe
    thunderstorm warning should be issued.

24
  • 2) Now examine the other sequence of CAPPI images
    based on the Baldy Hughes Doppler weather radar
    (centre point on the images) recorded between
    1850 UTC and 2220 UTC on 27 February 2006
    assuming ambient air temperatures are below
    freezing and answer the following questions
  • A) Tabulate and plot the approximate
    precipitation rate for Prince George over time
    during the storm. When does the snow finally end
    falling in Prince George?
  • B) Integrate in time the snowfall rate over
    Prince George to obtain the total snow
    accumulation during this period.
  • C) Are the observations from Prince George
    Airport consistent with the radar observations?
    Recall that there is an 8-hour difference between
    Pacific Standard Time (PST) and Universal Time
    Coordinates (UTC).

25
  • D) Focus on a persistent and coherent radar echo
    over several images. Based on the time elapsed
    between images and the distance between
    concentric circles on the maps, determine the
    approximate speed of the system. Please indicate
    the approximate area on which your calculations
    are based.
  • E) What do the radar echoes between Burns Lake
    and Fort St. James (140 km northwest of Baldy
    Hughes) represent? Do you observe similar echoes
    elsewhere?
  • F) What other errors of interpretation may arise
    from the remote sensing of precipitation by
    land-based weather radar? You may consult
    Environment Canada's website for hints.

26
  • G) What is the horizontal resolution of the CAPPI
    images from the Baldy Hughes weather radar?
  • H) A typical value of the radar reflectivity
    factor for this snowstorm is Z 30 dBZ . Convert
    this quantity into a snowfall rate and express it
    in units of mm h-1 snow water equivalent (swe)
    using the assumption that the snow has a density
    of 100 kg m-3.
  • I) Compare the precipitation rate for the average
    echo of Z 30 dBZ deduced for rain (U.S.
    midwestern case) and for snow (Prince George
    case).

27
  • J) Discuss some of the possible reasons that
    would explain differences in the average
    precipitation rates observed in the two case
    studies.
  • K) Bonus Question Imagine a situation in which
    solid precipitation is falling from clouds
    situated several kilometres above the surface.
    The air temperature in the clouds is about -15oC
    whereas the near-surface air temperature is 5oC.
    As the snow falls into a layer of above freezing
    air near the surface, it will melt. What will be
    the impact of this phase change on the radar
    reflectivity if it set to detect snowfall only?

28
Sources
  • Environment Canada
  • United States National Weather Service
  • McGill Weather Radar
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