Geography 311

Atmosphere

Lecture:  Radar Meteorology

 

The color (underline) statements provide answers to Study Guide.

 

Please click the following website to get the article.

 

http://weather.noaa.gov/radar/radinfo/radinfo.html

 

 

1.    Radar (Radio Detection and Ranging)

       (1).  The process of emitting a signal (pulse of energy), listening for any returned signal

               energy (echo on radar screen), then emitting the next signal (around

               1300 times/second).

       (2).  Radar beam Angles: 0.5o to 19.5o angles, 9-angle levels in total.

2.    NEXRAD (Next Generation Weather Radar system)

       Consists of approximately160 WSR-88D (Weather Surveillance Radar-1988 Doppler)

       products.

3.    Doppler effect (Austrian physicist, Johann Christian Doppler)

       (1).  High pitch:      When trains approach the station (compressed sound waves).

       (2).  Low pitch:      When trains leave the station (stretched sound wave)

       (3).  In atmosphere

               The radar’s computers measure the phase change of the reflected pulse of energy

               which then convert that change to a velocity of the object (winds, raindrops,

               clouds, insects, birds) either away or toward the radar.

4.    3 components of WSR-88D

       (1).  Radar Data Acquisition (RDA):

               hardware tower, dome, pedestal, antenna, fiberglass, klystron transmitter, receiver,

               status and control processor, and signal processor.

       (2).  Radar Product Generator (RPG)

               Algorithms that convert base data from the RAD into hundreds of meteorological

               and hydrological products (39 categories) of various resolutions, data level intervals,

               and elevation angles.

       (3).  Principal User Processor (PUP): computer work station

               Displays products generated at the RPG for retrieval of: reflectivity, mean radial           

               velocity, echo top and precipitation amounts, etc.

5.    Operational modes

       (1).  Clear air modes

               A.    The slowest antenna rotation rate

                      (A). Permits the radar to sample a given volume of the atmosphere (volume

                             scan) longer (longer scan duration increases sensitivity).

                      (B). Images are updated every 10 minutes.

                      (C). Ability to detect airborne dust and particulate matter which may not be

                             seen on the precipitation modes because of longer duration of a

                             volume scan .

                      (D) Better than precipitation modes in detecting light snow, drizzle,

                             and light rain which do not reflect the energy sent from the

                             radar very well.

               B.    Volume coverage patterns (VCP) or volume scans.

                      (A). A VCP consists of the radar making several 360o scans (or azimuth scans)

                             of the atmosphere, sampling a set of increasing elevation angles (the radar

                             antenna is angled 0.5o above the ground at the beginning of the scan).

                      (B). 2 clear air mode VCPs (5 elevation angles: 0.5o, 1.5o, 2.5o, 3.5o and 4.5o)

                             a.     Multiple sweeps at the 2.5o and lower angles.

                                    (a).  2 full sweeps at the 0.5o, 1.5o, and 2.5o angles.

                                            a surveillance/reflectivity sweep and a Doppler/velocity sweep.

                                    (d).  a single sweep at the higher angles (3.5o and 4.5 o)

                                            Reflectivity and velocity data are collected together.

                             b.    Single sweep at the 2.5o and higher angles (3.5o and 4.5o)

                                     Single sweep at 0.5o,1.5o, 2.5o, 3.5o, and 4.5o angles.

                      (C). Time of complete scan of atmosphere :10 minutes at the 5 elevation

                             angles.

       (2).  Precipitation modes

               A.    Rain provides plenty of returning signals (less sensitive radar is permitted)

               B.    See higher atmosphere (vertical structure of storms or clouds)

               C.    Same procedures as clear air modes with more elevation angles.

               D.    2 modes

                      (A). Slower VCP

                             Radar completes volume scan of 9 elevation angles from 0.5o to 19.5o

                             in 6 minutes.

                      (B). Faster VCP

                             a.     Radar completes volume scan of 14 elevation angles from 0.5o to

                                    19.5o in 5 minutes.

                             b.    Severe weather use (faster image and most slices into the

                                    atmosphere, more elevation angles for more detail cloud vertical

                                    structures).

               E.    Can detect most precipitation within 80 nm (nautical miles) of the radar

                      location.

               F.    Can detect intense rain or snow within 140 nm of the radar location.

               G.    Hard to detect light rain, light snow, or drizzle from shallow clouds or objects

                      outside the radar range (clear-air modes can do better job in this regard).

6.    Types of radar images (level III products)

       (1).  Base reflectivity (R)

               A.    Display of echo intensity (reflectivity) in dBZ (decibels of Z, where Z

                      represents the energy reflected back to the radar) for a given elevation

                      angle (radar antenna tilt angle).

               B.    2 types of base reflectivity

                      (A). Short range (S Rng): 124 nm (143 miles)

                             Precipitation that occurs farther than 143 miles will not show on echo

                             screen.

                      (B). Long range (L Rng):      248 nm or 286 miles.

               C.    Detect precipitation, evaluate storm structure, determine hail potential.

       (2).  Composite reflectivity (CR)

               A.    A display of maximum reflectivity from any elevation angle at every range

                      from the radar.

               B.    Long range composite reflectivity:        maximum range 286 miles.

                      (A). Weakness: Blocky appearance: low spatial resolution

                             (2.2 by 2.2 nm grid).

                      (B). ¼ the resolution of the base reflectivity and ½ the resolution of the

                             precipitation products.

                      (C). Study storm structure features and intensity trends of storms. 

                      (D). High radar beam at the maximum range:  only detect intense convective

                             storms and tropical systems.

                      (E).  Long range radar beam my overshoot precipitation at lower level and

                             shows no precipitation images (link to an adjacent radar or National

                             Reflectivity Mosaic to see precipitation).

               C.    Short range composite reflectivity: maximum range: 143 miles.

                      Less blocky appearance (higher resolution, 1.1 by 1.1 nm).

       (3).  One-hour precipitation (OHP)

               A.    A map of estimated one-hour precipitation accumulation on a 1.1 x 1.1 nm

                      grid.

               B.    Assess rainfall intensities for flash flood warnings, urban flood statements

                      and special weather statements.

               C.    Maximum range: 143 miles

               D.    Link to an adjacent radar to see precipitation at distance farther than 143 miles.

       (4).  Storm total precipitation (STP)

               A.    A map of estimated storm total precipitation accumulation, continuously

                      updated, since the last one-hour break in precipitation.

               B.    Locate flood potential and estimate total basin runoff.

               C.    Maximum range: 143 miles.

               D.    Link to an adjacent radar to see precipitation at a distance greater than

                      143 miles.     

       (5).  Base velocity (V)

               A.    A measure of the radial component of wind either toward the radar

                      (negative values or green) or away from the radar (positive values or red). 

               B.    Estimate wind speed and direction, locate atmospheric boundaries, severe

                      weather signatures (tornado or hail), suspected areas of turbulence.

       (6).  Tornado Vortex Signature (TVS)

               A.    Intense azimuth shear (green blocks next to red blocks, see Image Database).

               B.    Denoted by a red triangle with numerical output of location and height.

       (7).  VAD wind profile (VWP)

               A.    A graphic display of wind barbs plotted on a height scale in 500 ft or 1,000 ft

                      increments.

               B.    Weather forecasting, severe weather and aviation.

       (8).  Vertically Integrated Liquid (VIL)

               A.    The water content of a 2.2 x 2.2 nm column of air which is color coded and

                      plotted on a 124 nm map.

               B.    Effective hail indicator, significant storms and areas of heavy rainfall.

7.    Relationship between reflectivity (dBZ) and rainfall intensity (inches/hour)

       (1).  Clear air mode

               dBZ values vary from -28 to +28.

       (2).  Precipitation mode

               dBZ values vary from 5 to 75.

       (3).  Same color scales but different dBZ values.

       (4).  Rainfall intensity (rain rate, one-hour precipitation)

               A.    Trace: dBZ reaches 20

               B.    Light rain (0.1 inch/hour):             dBZ = 30.

               C.    Heavy rain (4 inches/hour):   dBZ = 55

               D.    Hail: good reflector, overestimate precipitation.

       (5).  Surface rain rate/Reflectivity on the National Radar Mosaic

               A.    Taking twice per hour (0015 and 0045, for example)

               B.    Composed of the highest observed reflectivity category within map grid

                      boxes 10 km on a side.

               C.    Position, movement, and evolution of precipitation on a synoptic scale.

               D.    Also distributed in Digital Gridded Binary (GRIB) format under WMO                        

                      (World Meteorological Organization) message header HAXA00 KWBC.

               E.    Reflectivity:  within 10,000 feet of the atmosphere, from raindrops,

                      snowflakes, hailstones, or ice pellets.

               F.    6 precipitation levels

                      (A). 0 rain rate (inch/hour) level: 0 or trace, dBZ < 15.

                      (B). 6 rain rate level: 4+ inches/hour, dBZ: 55 + (55 and higher).

       (6).  Radar Coded messages (RCMs)

               A.    Contains a coded text summary of reflectivity features in the local area, along

                      with information on convective storms and wind profile near the radar.

               B.    The individual radar RCMs are composited into one map grid at National

                      Weather headquarters.

               C.    For grid boxes in which reflectivity is detected by more than one radar,

                      the highest reported value is put into the final product.

               D.    Quality control:

                      (A). Echoes from ground clutter, AP, birds, insects, and airborne particulate

                             matter are removed.

                      (B). moderate snow can be detected.

       (7).  Most radars can not detect clouds and fogs.

8.    Terminology

       (1).  dB (decibel) and dBZ (unit of reflectivity).

               A.    Decibel (dB) is a logarithmic expression for ratio of two quantities.

                      dB = 10 log[P1/P2], where

                             P1 is the actual power measured at a point in space and P2 is the power                              

                             that would have been measured there if an isotropic antenna had been                                

                             used. dB is also called gain of antenna. dB is unitless (a ratio)

               B.    Isotropic antenna:  The simplest kind of antenna that radiates energy equally                       

                             in all directions. dB = 0, because  P1 = P2 and [P2/P2] = 1 and log1 = 0.

               C.    dBZ = 10log[z/1 mm6/m3], where z is the radar reflectivity factor.

               D.    dBm:  power relative to 1mW (milliwatts)

                      dBm = 10log[P1/mW]

       (2).  PRF (Pulse Repetition Frequency)

               A.    Definition:       The rate at which the radar transmits is called the pulse                                                

                                             repetition rate or pulse repetition frequency.

               B.    Unit:        Hertz (1 Hz = 1 cycle/second).

               C.    Doppler radar uses 700 to 3000 Hz (Hertz) frequencies

       (3).  Ground clutter:

               A.    Radio energy reflected back to the radar (echo) from outside the central radar

                      beam, from the earth’s surface or buildings.              

               B.    Appears within a radius of 20 nm of the radar.

               C.    Roughly circular region with echoes that’ show little spatial continuity.

       (4).  Anomalous Propagation (AP)

               A.    Radar beam is refracted almost directly into the ground at some distance from

                      the radar.

               B.    Under highly stable atmospheric conditions (typically on calm, clear nights).

               C.    Much less common than ground clutter.

       (5). Cone of Silence

               Radar can not scan the area directly overhead and is limited by elevation angle

               of 19.5o.

       (6).  Mainlobe

               Bright spot (strongest power) of radar beam.

       (7).  Sidelobe

               off the mainlobe.

       (8).  Backlobe

               Radar energy goes directly behind the antenna.

       (9).  Plan position indicator (PPI)

               A map-like format with the radar at the center surrounded by rings that show the

               distances from the radar.     

       (10).Range-height indicator (RHI)

               A.    A diagram showing the echo height and the distances from the radar.

               B.    Exaggerate the vertical extent of echoes to read height more accurately.

       (11). Folded velocity:

               A.    A region of near maximum approaching (toward the radar) wind velocity                            

                      next to an area of near maximum receding (away from the radar) wind

                      velocity.        

               B.    Causes:   multiple-trip echoes, ground clutter, imperfect rotation of antenna.                           

 

$$$: Please click on the following website for more information (optional)      

               http://weather.noaa.gov/radar/radinfo/about.html

               http://www.srh.noaa.gov/jetstream/remote/doppler.htm

               http://www.spc.noaa.gov/faq/tornado/doppler.htm

               http://en.wikipedia.org/wiki/Doppler_radar

               http://www.noaanews.noaa.gov/stories/s2097.htm

               http://weather.noaa.gov/radar/images/DS.p19r0/SI.kbuf/

               http://ww2010.atmos.uiuc.edu/(Gl)/guides/rs/rad/ptrn/ptrn1.rxml

               http://meted.ucar.edu/hurrican/strike/text/thura.htm

 

$$$: Please study Quiz 3 figures in the Image database.