Fundamentals of Radar Display. Atmospheric Instrumentation

Transcription

1 Fundamentals of Radar Display

2 Outline Fundamentals of Radar Display Scanning Strategies Basic Geometric Varieties WSR-88D Volume Coverage Patterns Classic Radar Displays and Signatures Precipitation Non-weather targets An Echo Identification Approach Parameters Based on Radar Reflectivity Rain Rate (Z-R relationships) Echo Tops (ET) Vertically Integrated Liquid (VIL) Storm Cell Identification and Tracking (SCIT)

3 1. Plan Position Indicator (PPI): Basic Scanning Strategies Radar completes one full rotation at a single elevation angle Observations are mapped onto a display based on (1) azimuth and (2) range from radar (the change in height with range of the radar returns is not shown) Data collected on a cone are then projected onto a flat plane Echoes close to the radar are at a lower elevations Range rings help orient the user Echoes far from the radar are at a higher elevations

4 Basic Scanning Strategies 1. Plan Position Indicator (PPI): An Example Reflectivity Factor (dbz)

5 Basic Scanning Strategies 2. Constant Altitude Plan Position Indicator (CAPPI): Radar completes several full rotations at multiple elevation angles Observations are mapped onto a display based on (1) azimuth and (2) range from radar equivalent to a given altitude assuming standard refraction of all radar pulses Tends to remove ground clutter close to the radar Generation of a simple 3-km AGL CAPPI

6 Basic Scanning Strategies 2. Constant Altitude Plan Position Indicator (CAPPI): An Example

7 3. Range Height Indicator (RHI): Basic Scanning Strategies Radar completes a full elevation scan at a single azimuth Observations are mapped onto a 2-D display based on the (1) elevation angle and (2) range from the radar Data collected along a slice are then projected similarly at different times Note ground clutter and echo from radar side lobe

8 4. Time Height Indicator (THI): Basic Scanning Strategies Radar is aimed vertically and remains stationary Observations are mapped onto a 2-D display based on (1) time and (2) range (or height) Storms pass over stationary Radar aimed vertically Note storm anvil and bright band Height (m) Reflectivity (dbz) Time (hour)

9 Volume Coverage Patterns WSR-88D Scanning Strategies Can not perform RHI or HTI scans Operate using one of three pre-programmed volume coverage patterns by completing a series of PPI scans at increasing elevation angles (1) VCP Clear Air Uses long pulse durations Sweeps 5 elevation angles in 10 min Uses separate surveillance and Doppler scans at the lowest three angles (i.e. the two lowest elevation angles are scanned once for reflectivity and a second time for velocity) Data is used to detect early formation of convection, air mass discontinuities, light snowfall, and to obtain vertical wind profiles Cone of Silence

10 Volume Coverage Patterns WSR-88D Scanning Strategies Can not perform RHI or HTI scans Operate using one of three pre-programmed volume coverage patterns by completing a series of PPI scans at increasing elevation angles (2) VCP Precipitation Uses a short pulse durations Sweeps 9 elevation angles in 6 min Uses separate surveillance and Doppler scans at the two lowest angles Data are used to observe distant storms when no other storms are closer Cone of Silence

11 Volume Coverage Patterns: WSR-88D Scanning Strategies Can not perform RHI or HTI scans Operate using one of three pre-programmed volume coverage patterns by completing a series of PPI scans at increasing elevation angles (3) VCP Severe Weather Uses a short pulse durations Sweeps 14 elevation angles in 5 min Uses separate surveillance and Doppler scans at the two lowest angles Data are used in algorithms to determine storm tracks, shear, mesocyclones, vertical wind profiles, and storm total precipitation amounts Cone of Silence

12 VCP-31: An Example WSR-88D Scanning Strategies Spatial pattern of light snowfall at the lowest elevation angle (0.5 )

13 WSR-88D Scanning Strategies VCP-21 or VCP-11: An Example Spatial pattern of liquid precipitation at the lowest elevation angle (0.5 )

14 Precipitation: Base Reflectivity Return echo intensity at the lowest PPI elevation scan (0.5 ) displayed in dbz Simplest display type Requires minimal data processing User must remember that features far from the radar are at greater altitudes Classic Radar Displays

15 Precipitation: Composite Reflectivity Maximum return echo intensity from any PPI scan over a given location displayed in dbz User can see elevated rain / hail cores Shows full storm size at one level Classic Radar Displays

16 Precipitation: Radar Mosaic Classic Radar Displays Composite radar reflectivity displays from multiple radars are combined to show storm structures and patterns extending across the full radar network User can see synoptic-scale rainfall patterns

17 Classic Radar Signatures Precipitation: Identification of Convective Mode / Type Distinct two- and three-dimensional spatial patterns in the radar reflectivity field are often used to identify the mode (or type) of a convective storm:

18 Classic Radar Signatures Precipitation: Identification of Convective Mode / Type Distinct two- and three-dimensional spatial patterns in the radar reflectivity field are often used to identify the mode (or type) of a convective storm:

19 Classic Radar Signatures Precipitation: Identification of Convective Mode / Type Distinct two- and three-dimensional spatial patterns in the radar reflectivity field are often used to identify the mode (or type) of a convective storm:

20 Classic Radar Signatures Precipitation: Identification of Convective Mode / Type Distinct two- and three-dimensional spatial patterns in the radar reflectivity field are often used to identify the mode (or type) of a convective storm:

21 Precipitation: Bright Band Classic Radar Signatures Results from a combination of three factors as hydrometeors descend through the 0 C level 1. Change in dielectric constant from a small (ice K 2 = 0.18) to a large (liquid K 2 = 0.93) value as hydrometeors melt while passing through the 0 C level increases dbz 2. Change in hydrometeor size as large-diameter aggregate snowflakes collapse inward to smaller-diameter liquid water drops as they melt increases dbz temporarily 3. Change in hydrometeor fall speed from slow (snow) to fast (rain) produce a decrease in hydrometeor concentration per unit volume decreases dbz 0 C

22 Precipitation: Bright Band Classic Radar Signatures Results from a combination of three factors as hydrometeors descend through the 0 C level 1. Change in dielectric constant from a small (ice K 2 = 0.18) to a large (liquid K 2 = 0.93) value as hydrometeors melt while passing through the 0 C level increases dbz 2. Change in hydrometeor size as large-diameter aggregate snowflakes collapse inward to smaller-diameter liquid water drops as they melt increases dbz temporarily 3. Change in hydrometeor fall speed from slow (snow) to fast (rain) produce a decrease in hydrometeor concentration per unit volume decreases dbz

23 Precipitation: Hail Spikes Classic Radar Signatures Anomalous reflectivity spikes located at a greater range from the radar than the hail storm Results from multiple scatterings of a single radar pulse among highly-reflective wet hail before the return echo makes its way back to the radar delayed returns appear to originate from further ranges than in reality

24 Non-Weather Targets: Beam Blockage Classic Radar Signatures Primarily caused by radar pulses being unable to penetrate through hills and mountains Beam Blockage

25 Non-Weather Targets: Beam Blockage Classic Radar Signatures Also caused by tall buildings, trees, water towers, and cell towers located near the radar Partially Blocked Beams

26 Classic Radar Signatures Non-Weather Targets: Insects / Bats / Birds Insects and bats often rest during the day and travel at night take-off at sunset Birds often rest during the night and travel by day take-off at sunrise Bats departing caves at sunset

27 Classic Radar Signatures Non-Weather Targets: Insects / Bats / Birds Insects and bats often rest during the day and travel at night take-off at sunset Birds often rest during the night and travel by day take-off at sunrise

28 Non-Weather Targets: Other Classic Radar Signatures Ground clutter near the radar caused by super-refraction and ducting of radar pulses Aircraft and Ground clutter and diverted aircraft Shuttle Columbia Break-up

29 Determining Target Type: An Echo Identification Approach 1. Echo Strength: Weather dbz Ground Clutter > 60 dbz Birds / Insects < 10 dbz 2. Size / Shape: Weather Globular patterns with clear horizontal (> 10 km) and vertical (> 3 km) extension Interference Thin lines at lowest elevation angle (toward Sun) Hail Spikes Elongation along a radial at ranges beyond storm Birds / Bats Circular rings or disks at single elevation angle 3. Movement: Weather Moves in common direction as a coherent entity Ground Clutter Stationary Birds/ Insects Random motions within a stationary pattern 4. External Clues: Weather Clouds clearly apparent in satellite imagery All Other No clouds on satellite (Use satellite, rawinsonde, and surface observations)

30 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate The definitions of effective radar reflectivity (Z) and rain rate (R) are very similar: Z = 0 N 6 ( D) D dd R = π 6 0 N 3 ( D) D w ( D) dd F where: D = drop diameter (m) N(D) = number of drops per unit volume (m -3 ) [ raindrop size distribution ] w F (D) = fall speed as a function of diameter (m s -1 ) [ fall speeds ] If the raindrop size distribution and fall speeds can be estimated then we can convert the radar reflectivity to a more useful rain rate

31 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate Numerous drop size distributions have been obtained from field observations in all types of convection (warm / cold / intense / light / tropical / midlatitude / polar) How? Optical disdrometers count rain drops and measure their diameters Examples of raindrop images collected by disdrometer in a Florida rain shower LED Slot Detector

32 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate Numerous drop size distributions have been obtained from field observations in all types of convection (warm / cold / intense / light / tropical / midlatitude / polar) Each summarizes drop concentration as a function drop diameter N(D) If an exponential function of the following form is fit to the data N bd ( D) = ae where: a = regression coefficient b = regression coefficient D = drop diameter (m) then the functional fit can be used to relate radar reflectivity (Z E ) to the rain rate (R)

33 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate Raindrop fall speed as a function of drop diameter is well known due to a series of detailed laboratory experiments conducted by Gunn and Kinser (1949)

34 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate Using a little algebra, one can easily show that the radar reflectivity (Z) can be related to the rainfall rate (R) using exponential functional fits for both the raindrop size distribution and the raindrop fall speed to arrive at the following basic Z-R relationships b Z = ar where: a = regression coefficient b = regression coefficient NWS forecasters can select among five Z-R relationships when using WSR-88D radars: Default WSR-88D Z = 300 R 1.4 Rosenfeld Tropical Z = 250 R 1.2 Marshall/Palmer Z = 200R 1.6 East Cool Season Z = 200 R 2.0 West Cool Season Z = 75 R 2.0

35 Parameters Based on Radar Reflectivity Computed Parameters: Rain Rate Storm Total Precipitation Time integral of rain rate computed from base reflectivity using a selected Z-R relationship Primary tool to predict flash flooding

36 Parameters Based on Radar Reflectivity Computed Parameters: Echo Tops (ET) Elevation of the upper-most 10 dbz echo Primary estimate of storm depth / height Computed from volumetric reflectivity fields Analogous to IR cloud top temperatures ET = z10dbz Echo Tops (ET) Base Reflectivity (0.5 elevation angle) Cone of Silence

37 Parameters Based on Radar Reflectivity Computed Parameters: Vertically Integrated Liquid (VIL) Vertical integral of rain rate per unit area computed from the volumetric reflectivity fields VIL = 0 ρ w R dz Primary tool to predict / monitor hail size Larger VIL with lower ETs = Larger Hail where: ρ w = density of liquid water (kg m -3 ) R = rain rate per unit area (mm hr -1 m -2 ) (computed from a Z-R relationship) [see Radar Algorithms #2 on course website] Composite Reflectivity VIL

38 Parameters Based on Radar Reflectivity Computed Parameters: Storm Cell Identification and Tracking (SCIT) The goal is to identify the three-dimensional (3D) centroid of a storm, and then track the centroid from one volume scan to the next volume scan for storm motion estimates 1. Identify a storm at a single elevation angle by searching for gate runs, whereby adjacent radar gates exhibit reflectivity values that exceed a threshold (30-40 dbz)

39 Parameters Based on Radar Reflectivity Computed Parameters: Storm Cell Identification and Tracking (SCIT) The goal is to identify the three-dimensional (3D) centroid of a storm, and then track the centroid from one volume scan to the next volume scan for storm motion estimates 2. Correlate the potential storms across multiple adjacent elevation scans to search for vertically-coherent convective cells Cells identified in at least 4 adjacent elevation scans are then tracked from one volume scan to the next volume scan

40 Parameters Based on Radar Reflectivity Computed Parameters: Storm Cell Identification and Tracking (SCIT) The goal is to identify the three-dimensional (3D) centroid of a storm, and then track the centroid from one volume scan to the next volume scan for storm motion estimates 3. Sequential storm centroids are used to compute cell motions, which help the forecasters issue severe weather warnings

41 WSR-88D Data Available to the Public: All WSR-88D data since 1992 have been archived and are publically available through the National Climate Data Center s (NCDC s) webpage: Level-II: Raw VCP data Radar reflectivity and radial velocity at original sampling resolution Level-III: Derived Products Base Reflectivity Composite Reflectivity Base Radial Velocity Base Storm-relative Radial Velocity Vertically Integrated Liquid (VIL) Echo Tops (ET) Storm Total Precipitation Radar Meteorology

42 WSR-88D Data Available to the Public: Must have software to view the downloaded WSR-88D data Simple FREE options: A licensed PAY option: GR2Analyst Demonstration Radar Meteorology

43 Summary Fundamentals of Radar Display Scanning Strategies Basic Geometric Varieties WSR-88D Volume Coverage Patterns Classic Radar Displays and Signatures Precipitation Non-weather targets An Echo Identification Approach Parameters Based on Radar Reflectivity Rain Rate (Z-R relationships) Echo Tops (ET) Vertically Integrated Liquid (VIL) Storm Cell Identification and Tracking (SCIT)

44 References Atlas, D., 1990: Radar in Meteorology, American Meteorological Society, 806 pp. Crum, T. D., R. L. Alberty, and D. W. Burgess, 1993: Recording, archiving, and using WSR-88D data. Bulletin of the American Meteorological Society, 74, Doviak, R. J., and D. S. Zrnic, 1993: Doppler Radar and Weather Observations, Academic Press, 320 pp. Fabry, F., 2015: Radar Meteorology Principles and Practice, Cambridge University Press, 256 pp. Reinhart, R. E., 2004: Radar for Meteorologists, Wiley- Blackwell Publishing, 250 pp.