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ATS 351, Spring 2010
Lab #9
Weather Radar - 55 points
1. (5 points) If a radar has a maximum unambiguous range of 300km, what is its PRF? (The speed
of light, c, is equal to 3x108 m/s)
The equation to solve for the unambiguous range is Rmax = c/2*PRF where c is the speed of
light and PRF is the pulse repetition frequency (units of cycles per second). Therefore,
rearrange the equation to solve for PRF: PRF = c/2* Rmax. Rmax is in km and c is in m/s so
convert the Rmax to m first before substituting the value into the equation.
PRF = (3x108 m/s) / (2*300000m) = 500 cycles/s
2. (5 points) Explain the presence of a bright band.
The bright band is a band of high reflectivity observed within the cloud, surrounded above and
below by lower reflectivities. It represents the region in the cloud where snow is melting. As snow
from the upper levels of the cloud falls through the melting layer (the layer in the cloud with 0C),
the snow acquires a coat of water, giving it the appearance as a large raindrop. Because reflectivity
is related to the diameter of the hydrometeor to the sixth power, the water-coated snowflakes have
high reflectivity values and produce the bright band on radar.
3. (5 points) What are the differences between the Clear Air Mode and the Precipitation Mode
(including which features can be identified easier depending on the mode)?
The Precipitation Mode is a faster scan, used when precipitation is present or expected to be
present and is useful for detecting smaller particles (such as hydrometeors). The Clear Air Mode is
a much slower scan used when precipitation is not present. Reflectivities that appear on this mode
are due to scattering by differences in refractivity and scattering from insects and birds. The
reflectivity scale used for the Clear Air Mode is much lower than the Precipitation Mode with typical
maximum reflectivities usually less then 30dBZ as opposed to reflectivities exceeding 70dBZ for the
precipitation mode.
4. (5 points) Using the PPI radar reflectivity image below, answer the following questions:
a. Which letter indicates the location of the radar? D
b. Which letter indicates the region of ground clutter?
C
c. Which letter indicates the region of highest reflectivity values? A
d. Which letter (A or B) indicates a region of lower rainfall rates? B
e. What type of precipitation system is this (supercell, snow, bow echo)?
Bow echo
B
A
C
D
5. (4 points) On the reflectivity and velocity images below, indicate with an arrow (on both) the
most probable location of the tornado?
Fort Collin's Flood Case Study
On July 28th, 1997 a major precipitation event occurred along the foothills in Fort Collins. Over 14
inches of rain fell in a concentrated area in Fort Collins during a 5-hour period. This heavy rainfall
caused the Spring Creek to spill over its banks, which resulted in massive flooding. Much of the
CSU campus was under several feet of water.
This part of the lab will help you understand how the precipitation was initiated, and how to
interpret the radar images associated with the rainfall itself.
Part 1: Precipitation Initiation
6. (2 points) On the afternoon of July 28, the prevailing winds along the Front Range were from the
East. As the warm air mass from off the plains came in contact with the topography of the foothills,
it was forced to rise. Referring to the attached atmospheric sounding, an air parcel being lifted from
the surface would initially be:
a. unsaturated
b. saturated
7. (2 points) As the air was forced to rise up the topography, it initially cooled at:
a. ~6.5 oC/km, the moist adiabatic lapse rate
b. ~10 oC/km, the dry adiabatic lapse rate
c. the environmental lapse rate
d. the air did not cool as it rose
8. (3 points) Refer to Image 1 Starting from the surface dew point temperature, follow the purple
constant mixing ratio lines up and draw a line. Starting from the surface temperature, follow the
green dry adiabatic lines up and draw a line, until this line intersects the dew point line you just
drew. This is the Lifting Condensation Level (LCL), where the air parcel has expanded and cooled
to the point of saturation. At approximately what level is the LCL located?
~800 mb
9. (3 points) The parcel path follows the moist adiabat (the curving blue line) up once it becomes
saturated at the LCL. The Level of Free Convection (LFC) is located where the parcel path
intersects the environmental temperature profile. At approximately what level is the LFC located?
~710 mb
10. (2 points) The mountains of the Front Range rise up to about 600 mb in the atmosphere.
Knowing this, what was the mechanism that initiated the convection in Fort Collins on July 28 th?
a. Buoyant air rising to the LFC
b. Air being forced to rise along the topography of the Front Range to the LFC
c. Saturated air sinking to the surface
Part 2: Weather Radar
Refer to Images 2,3
Image 2 is a time series of Radar Reflectivity during the heaviest rainfall of the storm on July 28,
1997 in 15 minute increments. Please read the caption under the image to understand the
underlying grid system.
11. (3 points) At what time period was the strongest reflectivity covering the largest area in Fort
Collins, and what was the measured reflectivity for this time period (in the correct units)?
2145 MDT, 50-55 dbZ
12. (2 points) Following the contours of strongest reflectivity, would you say this storm was:
a. stationary
b. moving to the northwest
c. moving to the southwest
d. moving due north
Image 3 is a contour map of total rainfall amounts for the lifetime of the storm. Recall our radar
images are only a subset of the total storm, however, they represent the heaviest rainfall of the
storm.
13. (4 points) Compare the reflectivity from image 2 to the precipitation totals from image 3. We
can see that strong reflectivity corresponds to the area of heaviest rainfall accumulation. Explain
why this is the case.
Reflectivity has a 6th power relationship between diameter of a precipitation particle, and a linear
relationship with the number of particles. This means that reflectivity increases linearly as the
number of particles increases, and increases to the 6th power for every one unit diameter increases.
Areas of a storm with more numerous, larger drops tend to reflect more of the microwave radiation
back to the radar, resulting in a higher reflectivity.
Part 3: Application (10 Points)
14. In your own words, describe the mechanisms that caused the Spring Creek to flood its banks
during this rainfall event. For full credit, tell me about atmospheric stability, atmospheric moisture,
storm movement, and rainfall rates.
Air was forced to rise up along the mountainside to the LFC where the air then continued to rise
freely (free convection above the LFC). There was an ample supply of moisture in the atmosphere
to be condensed: moisture contents greater than 5 g/kg existed from the surface to beyond 550 mb
with significant amounts of moisture (greater than 10 g/kg) near the surface. The storm remained
relatively stationary, which resulted in all of the precipitation falling in the same area. The radar
reflectivity plot shows that precipitation greater than 45 dBz fell continuously and precipitation
greater than 50 dBz fell for a significant period during the storm. These large reflectivity values
suggest large rainfall rates, meaning that the storm produced heavy rainfall throughout the period of
20:00 to 22:00 MDT.