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AT 351 Lab #7
Precipitation and Weather Radar
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 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 (14 Points)
Refer to Image 1
1. 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
2. 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
3. Starting from the surface dew point temperature, follow the purple constant mixing
ratio lines up. Starting from the surface temperature, follow the green dry adiabatic lines
up until your 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. This level is located at approximately:
a. 1000mb
b. 800 mb
c. 700 mb
d. 300 mb
4. At the LCL, the lifted air parcel is
surrounding environment
a. stable, warmer
b. unstable, warmer
c. stable, colder
d. unstable, colder
because it is
than the
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5. The parcel path follows the moist adiabat (the curving blue line) up once it becomes
saturated at the LCL. The LFC is located where the parcel path intersects the
environmental temperature profile. The LFC is located at approximately:
a. 1000 mb
b. 800 mb
c. 700 mb
d. 300 mb
6. 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 28th?
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
7. What type of sounding does this resemble?
a. Continental Sounding
b. Tropical Sounding
Part 2: Weather Radar (21 Points)
Refer to Images 2,3
A weather radar (or any radar for that matter) consists of a pulsating beam of microwave
energy that is reflected by raindrops and snowflakes that are in the beam’s path. That
reflected microwave energy reaches a receiver on the radar, which then calculates the
difference between the sent microwave signal and the received signal.
8. What does the word RADAR stand for?
a. Ranging And Detection of Antenna Radios
b. RAdio Detection And Ranging
c. Radio And Doppler Audio Detection
d. Recording And Documenting Antenna Radios
9. Radar reflectivity is measured in units of
a. Kelvin (K)
b. Decibels (dBZ)
c. Watts (W)
d. Joules (J)
:
10. An area of strong reflectivity is often referred to as a(n)
a. Microburst
b. Wave
c. Tornado
d. Echo
:
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These areas of strong reflectivity are displayed as contours on a map background. The
National Weather Service and other agencies operate more than 110 weather radars at
various sites nationwide. They separate reflectivity into weak and strong regimes, which
provides information as to the kind of precipitation that is falling.
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. What 2 colors represent the strongest reflectivity in image 2?
a. Red and Yellow
b. Purple and Black
c. Blue and Green
d. Red and Purple
12. At what time period was the strongest reflectivity covering the largest area in Fort
Collins, and what was the reflectivity measured for this time period (in the units of
reflectivity you chose above)?
a. 2000 MDT, 30-35 units
b. 2145 MDT, 50-55 units
c. 2015 MDT, 45-50 units
d. 2145 MDT, 65-70 units
13. 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.
14. Mark a big X on Image 3 where the largest amount of rain fell.
Is this X in proximity to (near) the strongest radar reflectivities?
a. Yes
b. No
15. From above we see that strong reflectivity corresponds to:
a. Light rainfall
b. No rainfall
c. Heavy rainfall
d. Fog
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16. Thus, larger particles tend to reflect
particles.
a. More
b. Less
of the microwave beam than smaller
17. Other types of precipitation that might have strong radar reflectivity are (circle all that
apply)?
a. Hail
b. Drizzle
c. Flurries
d. Heavy Snow
Part 3: Application (10 Points)
18. 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.
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Image 1
The Denver Atmospheric Sounding for 6 AM on July 28, 1997
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Image 2
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Image 3
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