Clear air echoes (few small insects) -12 dBZ

Echoes in clear air from insects

Common is summer. Watch for echoes to expand area as sun sets and insects take off for their nocturnal travels

Echoes from birds and bats

Birds take off radially from roosts in morning producing concentric rings of echo. Bats do the same at night. Migrating birds will produce small echo clusters that move across screen.

Birds departingAt 1114 UTC

Blocked beams (topography or buildings)

Blockedbeam

D istan c e (km )

R ef le ct iv ity fa cto r (d B Z )

S tr a t i fo rm a rea C on v ec t io n

B BAltitude (km)

The bright band an enhancement of radar reflectivity at the melting level when snowflakes fallingFrom above aggregate and develop wet surfaces.

Note convectiveand stratiform regionsof squall line. Precipitationestimates in stratiformregion must be carefullyexamined because of bright band effects

Why there is a bright band?

Recall |K2| for water is about 0.93 and for ice is about 0.176

When ice particles falling below 0oC, they start to melt, beginning from the surface. Therefore, they keep the size of the ice particle but coated with water. So the radar received power difference would be:

P ice coated with water / P ice = 0.93/0.176 = 5.284 dBZ Zice coated with water = 10log Zice *5.284 = 10logZice + 10log(5.284) = dBZ Zice + 7.2 dBZ

After totally melted, the ice particles becoming rain drops. The density increased from ~0.4 g/cm3 to 1 g/cm3 and the particle size decreased, so the reflectivity decreased.

The bright band appears as a ring on PPI displays where the radar beam crosses the melting level

An extreme example of bright band contamination of precipitation estimation – radar estimates 6 inches of precipitation in a winter storm on January 31, 2002!

Different radar displays

NEXRAD (WSR-88D)Operations and Scanning

Procedures

Plan-Position Indicator (PPI) Scanning procedure

Echoes close to the radar are at a low elevation

Echoes far from the radar are at a high elevation

Data collected on a cone are projected onto a plane

Precipitation mode scan geometry Severe weather scan geometry

Saves time…fewer elevations

WSR-88D Volume Scanning Procedures

Clear air mode: Fewer elevations, slower antenna rotation toachieve greater sensitivity for sensing clear air turbulence, insectsand clouds, light drizzle or light snowfall.

65 55 45 35 25 15 5

Reflectivity Factor (dBZ)

Radar reflectivity: A measure of the power scattered back to the radar from objects in the path of a radar beam. Proportional to the sum of the sixth power of the diameter of all the particles illuminated by a pulse provided the particles are much smaller than the radar wavelength.

Base reflectivity: Echo intensity at the lowest scan level (0.5o) measured in dBZ

Composite reflectivity: Maximum echo intensity at any scan level measured in dBZ

“Precipitation” mode

“Clear air” mode(often used for snow)

Radar parameters set to increase sensitivity

Storm Total Precipitation: Measured in inches fallen after an NWS selected start time.

Storm total precipitation is the primary tool for flash flood forecasting

Vertically integrated liquid (VIL): VIL is the integration of reflectivity within a column of air. Once thought to be related to potential hail size, but not really a good predictor.

Radial velocity: Velocity component along the radar beam direction (knots)

-51 -37 -24 -10 3 17 30 44

Wind Speed (knots)

12

24

0180 225 270 315 360

Height(kft)

Height(kft)

12

24

0

Wind speed (kt)0 20 40 60 80

12

24

0180 225 270 315 360

Height(kft)

Height(kft)

12

24

0

Wind speed (kt)0 20 40 60 80

-51 -37 -24 -10 3 17 30 44

Wind Speed (knots)

Radial Velocity

A measure of thecomponent of thewind along thedirection of theradar beam

Radial Velocity is important for detection of rotation in storms

Storm relative radial velocity: Radial velocity with the component of storm motion along the beam subtracted. Best display to see mesoscyclones, tornado vortex signatures and microburst signatures.

Radar reflectivity

Radial VelocityRadar reflectivity together withRadial velocity provide meansto identify potential tornadoes

NEXRAD PRODUCTS:

LEVEL 2 DATA: Radar reflectivity and radial velocity at original sampling resolution.

LEVEL 3 DATA: NEXRAD Information and Dissemination

System (NIDS) Products

Individual radarsBase reflectivity, Composite reflectivityStorm total precipitation, Vertically integrated liquidRadial velocity, Storm relative radial velocity

National or regional reflectivity composites

Other radar displays

Range-Height Indicator (RHI) Scanning procedure

Radar is scanned in elevation at a fixed azimuth

Volume scans are accomplished by rotating slowly in azimuthwhile scanning rapidly in elevation

A sequence of RHI scans showing the

development of precipitation in a small

cumulus cloud over Florida

Note ground clutter and echo from tall buildings (echo from radar side lobe)

Time-Height Display (Radar beam pointed vertically as storms move over the radar)

Winter snowbands move across vertically pointing radar

“Reconstructed” RHI displays

Vertical cross section through a mesoscale convective system reconstructed from scans from a PPI volume from a NEXRAD

reflectivity

Radial velocity

Horizontal cross sections

Radar data are interpolated from cylindrical coordinates to Cartesian coordinates and displayed in Cartesian space

If more than one Doppler radar scans the storm, winds can be derived using multiple-Doppler analysis techniques that we will discuss later in the course.

Note location of cross section

Vertical cross sections

Cartesian data can be used to construct vertical cross sections similar to the RHI

If more than one Doppler radar scans the storm, vertical winds can also be derived using multiple-Doppler analysis techniques.

Radar composites: Data from PPI scans from many radars are projected onto a single image to give a map of precipitation distribution.

On the display below, surface data are used to recolor the echo to show where snow and mixed precipitation are occurring. Radar data are not used to make this distinction.

National Weather Service composite image

Assignment #2 If air with precipitation water content of 1 g/m3, assuming mono size distribution (all hydrometeors have same size) How many raindrops would be in 1 m3 (total number density N) if all raindrops have diameter (D) of 1 mm? Calculate 10log(N*D6), units in mm6/m3. How many snowflakes would be in 1 m3 if all snowflakes have diameter of 5 mm and density in 0.3 g/cm3? Calculate 10log(N*D6), units in mm6/m3. How many hails would be in 1 m3 if all hails have diameter of 10 mm and density in 0.9g/cm3? Calculate 10log(N*D6), units in mm6/m3.