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Transcript
Severe and Unusual Weather
ESAS 1115
Spotter Training and
Radar Meteorology
Part 2 – Introduction to
Radar Meteorology
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Meteorological Sensors


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Two types of two types of sensors:
• Remote vs. In-situ
• Active vs. Passive
Our passive eyes can only see
features of the storm
In order to see the inner workings of
a thunderstorm, and to understand it
better, we need an active remote
sensor – weather radar
Radar will allow us to interrogate
information about the storm by
detecting precipitation and wind
information within
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Radio Detection and Ranging
NEXRAD – Next Generation Radar
WSR-88D – Weather Surveillance Radar, 1988 Doppler
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Important Angles
 The azimuth angle is the




Prof. Paul Sirvatka
compass angle from north
(360°)
Targets are indicated by
A/R (Azimuth/Range)
Birds eye view is displayed
on a PPI (Planned Position
Indicator)
A cross sectional display is
an RHI (Range Height
Indicator)
Volume scan
ESAS 1115 Severe and Unusual Weather
Gate Size
 Reflectivity
Information is
displayed in pixellike units of area
called “gates”
 Gate size is
determined by pulse
length and angular
beam width
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Reflectivity




dBm = 10log(power returned/1milliwatt)
Z is reflectivity
dBZ is a logarithmic scale similar to dBm
1dBZ is the power returned by a sphere of
1mm6/m3
 Doubling of power results in a linear increase
of 3dBZ
 Z is proportional to D6
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Clear Air Mode
Range from -28 to 28 dBZ
16 4dBz increments
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Precipitation Mode
Range from 5 to 75 dBZ
16 5dBz increments
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
The Significance of Clear Air
Mode During Snowfall
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
Base Reflectivity – Various Elevation Angles
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
Base (or Storm Relative) Velocity –
Various Elevation Angles
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
1 Hour Precipitation
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
Storm Total Precipitation
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
VIL – Vertically Integrated Liquid
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
Echo Tops
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
VAD – Velocity Azimuth Display
VWP – VAD Wind Profile
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
NIDS - Nexrad Information
Dissemination Service
Composite Reflectivity
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Interpreting Doppler Radar
Radial Velocity is the velocity toward or away from a radar
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Using the Zero Isodop
When the radial is
perpendicular to the the wind,
the radar displays zero
velocity - This “zero zone” is
called the “Zero Isodop”.
0%
100%
100%
When the wind velocity
is parallel to the radial,
the full component of
the wind is measured
Prof. Paul Sirvatka
0%
What percentage
of actual wind
will the radar detect?
00 = 100% - Parallel
150 = 97%
300 = 87%
450 = 71%
600 = 50%
750 = 26%
900 = 0% - Perpendicular
ESAS 1115 Severe and Unusual Weather
Interpreting Doppler Radar
 Winds will flow
perpendicular to the
zero isodop from green
to red
 Veering wind profile is
denoted by an “S” on
the overall winds
display
 Veering winds with
height indicate warm
air advection and
hence rising air
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Interpreting Doppler Radar
 Backing wind profile is
denoted by a
backwards “S” on the
overall winds display
 Backing winds with
height indicate cold air
advection and hence
sinking air (subsidence)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
What Does this Represent?
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Veering Winds on VWP
Velocity Azimuth Display (VAD)
Wind Profile
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Hurricane Katrina
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Divergence
Divergence is indicated by
radial shear (along the radius)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Rotation
Cyclonic rotation is indicated
by azimuthal shear (from one
azimuth to another)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Low-level Rotation and
Storm Top Divergence
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Rotation or Convergence?
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Automated Detection of
Meteorological Phenomena
 Algorithms help detect significant features
• MDA – Mesocyclone Detection Algorithm
• Meso - “donut”
• Persistent, strong and detected over a large depth of
the storm
• TDA – Tornado Detection Algorithm
• TVS – Tornado Vortex Signature
• Strong gate to gate shear
• HDA – Hail Detection Algorithm
 SCIT - Storm Cell Identification and Tracking
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Composite Reflectivity with
Storm Attribute Table
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Composite Reflectivity
with Symbols
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Gate to Gate Shear (TVS)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
TVS
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
TVS
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Azimuth Resolution Considerations
Rotational couplet identification can be affected by azimuth resolution. As the diagram shows, the closer a
rotation is to the radar the more likely it will be identified correctly. If the rotation is smaller than the 1 0
beam width (possible at long ranges) then the rotation will be diluted or averaged by all the velocities in that
sample volume. This may cause the couplet to go unidentified until it gets closer to the radar.
Azimuth 3
Weak inbound,
weak outbound


Range 0
Prof. Paul Sirvatka
Rotation too small
to be resolved

Strong inbound,
strong outbound

(example)
Stronger inbound
than outbound
120 nm
Azimuth 2
Azimuth 1
ESAS 1115 Severe and Unusual Weather
Strong TVS
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
SRV vs. Base Velocity
with Subtle Rotation
Base Velocity
When diagnosing straight line winds use base
velocity. The strength of an advancing line of
storms producing straight line winds is the sum of
the winds produced by the storms, plus the
movement of the storms.
Prof. Paul Sirvatka
Storm Relative
When diagnosing rotation, use storm relative
velocity. SRV subtracts out the motion of a storm
to display pure rotational characteristics of that
storm.
ESAS 1115 Severe and Unusual Weather
FAR vs. POD
 FAR (False Alarm Ratio) – An event is
warned for but does not occur results in a
false alarm
 POD (Probability of Detection) – An
event that occurs and has been warned
for results in a 100% POD
 A high POD is achieved at the expense
of an increased FAR
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather







Doppler Dilemma
Radial velocity and range is limited by
PRF
High PRF’s result in short unambiguous
ranges and vice versa
Low PRF’s result in velocity aliasing and
vice versa
Given a PRF, the radar can determine
the radial velocity within some range, +
or – some velocity, called the velocity
interval
Any velocity beyond that range will
“fold over” into the incorrect value
Bad data must be “dealiased”
The Doppler Dilemma: There is no single
PRF that maximizes both Rmax and
Vmax
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Velocity Aliasing and Dealiasing
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Three-Body Scatter Spike
(TBSS)
 The “flare” appears
further than the main
core along the same
radial as the highest
core
 The added distance the
beam makes from stone
to ground to stone (3
bodies) results in a
display further than
reality
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
TBSS Examples
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Same Storm, Different Radar
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
TBSS in Velocity Data
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Anomalous Propagation (AP)
In the wake of this line of thunderstorms,
a low-level inversion created by the cold pool
results in superrefraction and thus AP
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
AP and Ground Clutter
AP is enhanced by strong returns
from buildings (ground clutter)
Cooling in the evening hours results in a
low-level nocturnal inversion and AP
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Sunset Spike
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Sunset Spikes
Sunset spikes
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Sunset Spike
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
What the Heck Is This?
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather