Part 2 Flashcards
• A range-corrected and equipment-calibrated measure of reflectivity from rain is given by:
log(Z) = log(received power) + 2 log(range) + constant
• log(Z) = log(received power) + 2 log(range) + constant
where Z is
the radar reflectivity factor
dBZ
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
dBZ=
• dBZ = 10 log(Z)
Larger and more numerous drops
- reflect more radar energy:
- Z = ΣD6/ V
- where D is drop diameter, V is volume of air holding the drops, and the sum is over all raindrops within that volume.
………………………………. determines the rainfall rate
the number and diameter of drops
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
• where cR = 0.036 mm h-1 .
Six discrete levels of radar echo intensity are often used, corresponding to
descriptive rainfall categories
Rainfall intensity chart
owing to ……………………….. snowflakes produce ………………………….
Owing to their particular shapes, snowflakes produce echoes of different intensity than raindrops of the same size
• 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 .
- 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
What is the rainfall rate R for an echo of 43 dBZ?
What is the reflectivity Z for an echo of 43 dBZ?
MDR Images
manually digitized radar
Manually Digitized Radar (MDR) images are
low resolution radar depictions available from the National Weather Service
Manually Digitized Radar (MDR) images are low resolution radar depictions available from the National Weather Service. These images show
the echo intensity in six categories.
There also a number of symbols indicating precipitation type,
cell movement,
line movement, and echo coverage.
Severe thunderstorm and tornado watches,
line echo wave patterns, and
hook echoes are also indicated.
Manually Digitized Radar (MDR) it shows
The height of cloud tops is also indicated in hundreds of feet above sea level.
Reflectivity Features
bright band
Precipitation typically forms
high in the atmosphere where the temperature is below freezing
Precipitation typically forms high in the atmosphere where the temperature is below freezing. As ice crystals form
aloft and fall toward the surface, they collect each other to form large snowflakes.As the snowflakes fall, they pass through a level where the temperature rises above freezing. When the snowflakes start to melt, they initially develop a water coating.
Water is about ……………………… more reflective than ice at ……………………… wavelengths,
9X
microwave
Water is about 9X more reflective than ice at microwave wavelengths, so these large wet snowflakes produce
a high reflectivity
As the flakes continue to fall and melt, they
collapse into rain drops
As the flakes continue to fall and melt, they collapse into rain drops. The rain drops are
smaller and fall faster
As the flakes continue to fall and melt, they collapse into rain drops. The rain drops are smaller and fall faster, so ………………………………………. , reducing the radar reflectivity
both the size of the particles and their concentration are reduced
The rain drops are smaller and fall faster, so both the size of the particles and their concentration are reduced, reducing the radar reflectivity. All of these processes lead to
the formation of a narrow ring of high reflectivity near the melting level. This ring, called the “bright band”
Locating Tornadoes
hook echoes and velocity couplets
Tornadoes are often located at
the center of a hook-shaped echo on the southwest side of thunderstorms.
Tornadoes are often located at the center of a hook-shaped echo on the southwest side of thunderstorms. The hook is best observed in
the reflectivity field
Tornadoes are often located at the center of a hook-shaped echo on the southwest side of thunderstorms. The hook is best observed in the reflectivity field.
This image shows
a reflectivity field containing several hook echoes
Another way to determine if a storm is tornadic is to
examine the radial velocity field.
Another way to determine if a storm is tornadic is to examine the radial velocity field. ………………………………………., is detectable as a velocity couplet.
A mesocyclone, the small rotating circulation with its center beneath the updraft of a supercell thunderstorm
The couplet is oriented so that
a concentrated area of radial winds moving away from the radar appears on one side of the beam axis, while a concentrated area of radial winds moving toward the radar appears on the opposite side of the beam axis.
tornado vortex signature (TVS)
When the central pixels near the beam axis show exceptionally strong winds, this signature is called a tornado vortex signature (TVS)
This image shows the TVS in the velocity field from the same Tennessee and Kentucky storms. Negative values (blue-green) denote movement toward the radar and positive values (yellow-red) represent movement away from the radar.
Here is the velocity couplet associated with the Newcastle hook echo (red and green pixels adjacent to each other).
Hurricanes on Radar
circular areas of moderate to high reflectivity
Hurricanes show up clearly on radar as
circular areas of moderate to high reflectivity, often surrounding a low reflectivity center.
This image shows the reflectivity field from the eye wall of Hurricane Andrew. The symmetry shown in this image indicates that Andrew was a very well developed hurricane. The ring of orange are the high reflectivities associated with the convection found in the eye wall.
This image shows the reflectivity field from a scan of Hurricane Erin on August 2, 1995. The lack of symmetry indicates that Erin was a rather weak hurricane, especially compared to Andrew.
The velocity field of Hurricane Erin reveals the strong counterclockwise rotation responsible for the inward flow on the storm’s north side and the outward flow on the south side. Negative values (blue-green) denote movement toward the radar and positive values (yellow-red) represent movement away from the radar.
This image shows the reflectivity field from a scan of hurricane Roxanne on October 19, 1994. High reflectivities north and east of the eye are associated with strong convection present in the eye wall
Using radar data, a forecaster can determine the
nature of any existing weather systems and follow their movement and evolution.
Using radar data, a forecaster can determine the nature of any existing weather systems and follow their movement and evolution. This is a valuable tool for
for making short term weather predictions.
By analyzing radar loops such as this, forecasters can determine
the intensity and movement of thunderstorms, helping them to issue advanced warnings and advisories to cities which lie in the paths of the storms.
This QuickTime movie (803k) shows
a large area of heavy rain developing around St. Louis. After seeing this area develop on radar, meteorologists issued flood watches for Missouri and southern Illinois
radar data is used to issue
special weather statements
This image shows a map of radar-estimated precipitation totals for a 12 hour period
This image shows a map of radar-estimated precipitation totals for a 12 hour period. Since the radar reflectivity is
closely related to the precipitation rate, the total amount of precipitation falling on a region over a fixed period of time can be determined by analyzing reflectivity field over that period.
the heaviest amounts are indicated in
…. should be issued
The heaviest amounts are indicated in yellow and red. Flash flood warnings would be issued for the stream and river basins which drain these areas.
This image shows the different scales on which snow can occur. The large snow band extending across the figure is associated with a large storm system moving across the country.
This image shows the different scales on which snow can occur. The large snow band extending across the figure is associated with a large storm system moving across the country.
Superimposed on the large system, is a
smaller scale snow band located off the west shoreline of Lake Michigan. Each of these bands individually produced heavy snow and where they intersected near Chicago, the snow was particularly intense.
Hail
indicated by regions of
very high reflectivity
This image shows some severe thunderstorm cells in western Illinois and eastern Missouri on April 19, 1996. The small regions of high reflectivity (shades of purple and gray) located near the edges of these storms are likely to be regions where large hail is falling. Typically, radar reflectivities associated with hail have values exceeding 60 dBZ.
