Laboratory #4
ATMS 111 – Understanding the Atmosphere (Lab)
Precipitation
Due: 19 Oct 2015
Purpose: To gain a better understanding of the difficulties in observing precipitation and to have
an appreciation of the localized nature of precipitation.
Reference: Chapter 5, “Essentials of Meteorology An Invitation to the Atmosphere” [Ahrens]
Objective: You will access actual rain gauge data observations taken at locations near the Great
Smoky Mountain National Park and create a rainrate algorithm from radar reflectivity data so that
we might estimate rainfall using radar data which covers a broad region of the mountainous region
of Western North Carolina. You’ll use Excel to draw a graph and create a fit to six pairs of data
(reflectivity, rain gauge). The graph will have radar reflectivity [dBZ] on the x-axis (abscissa) and
rain gauge-estimated rainrate [millimeters per hour] on the y-axis (ordinate) and the data points
and best-fit second order polynomial will all be plotted via Excel. The polynomial mathematical
relationship will be determined using least-squares available within Excel fit to the original data
points. The rain gauge data will be provided in a data file and the reflectivity values will be
determined using “zoomed” radar imagery, both available on the class web page.
Equipment: computer and laboratory notebook.
Hints: Subtracting the times found for each time period in the rain gauge data gives the change in
time between rain gauge bucket tips in time units of hours. Subtracting the total accumulation
found for each time period gives the total accumulation during the period in units of millimeters.
Extension Questions:
(a) Use your rainrate algorithm to determine the estimated rainrate (in mm per hour) for
radar pixels that observe reflectivity values of [i] 10, [ii] 30, and [iii] 50 dBZ. For some radar pixel
locations the reflectivity is a negative number. What should be the estimated rainrate for those
locations having negative radar reflectivity?
(b) Assume a flat earth and calculate the altitude of the lowest radar scan (0.5 o above the
horizon) for a photon beam traveling from the radar site in Greenville, SC to the rain gauge
locations indicated on the “big picture” radar imagery found on the class web page. What might be
a problem using the Greenville, SC radar to estimate rainfall at The Swag and Purchase Knob
locations?
(c) Some of the mountain peaks in Haywood County exceed 6000 feet in elevation. What
might be another problem with using the lowest radar scan data from the Greenville, SC radar to
estimate rainfall for locations in Haywood County?