Transcript Slide 1

Upper Air Measurements
Pilot Balloon
Radiosonde
Pilot Balloon: Pibal
A pilot balloon can be tracked visually with a single
theodolite that measures the azimuth, relative to True
North, and the elevation angle.
If the balloon ascent rate is known (assumed from balloon
and gas specifications), position of the balloon can be
determined and the wind velocity inferred from successive
balloon positions.
The procedure is to track the balloon optically with the
theodolite and measure the azimuth and elevation angles
at 1 min or 30 s intervals.
Height, z, is determined from the known or assumed ascent
rate.
Pilot Balloon: Single Theodolite Geometry
Side View
Top View
Z
Y
R
S
θ
Φ
Elevation
S
Azimuth
θ
z = R sin Φ
s = R cos Φ
X
x = S sin θ
y = S cos θ
Solve for x and y:
X = z cot Φ sin θ
Y = z cot Φ cos θ
X and y define the horizontal position of the balloon relative to the theodolite,
S is the ground range to balloon, R is slant range to the balloon
(R and S are your author's naming convetions)
Pilot Balloon: Single Theodolite Geometry
For 30 s intervals
u = (xn-xn-1)/(30 secs)
v = (yn-yn-1)/(30 secs)
x, & y are in meters.
u, & v are in meters/secs.
Horizontal wind speed = (u2 +v2)1/2
Wind direction is computed following Appendix E.
Calculate the wind profiles from the theodolite lab.
Each student needs to digitize their lab book data and
input the data into either Excel or a text file.
A Pibal Spreadsheet will be on the class website.
Once the profiles are calculated:
Plot the profiles in two plots:
1. wind speed vs. z
2. wind direction vs. z
Comment on the wind structure.
U and V to Wind Direction
Top View
u = -WS sin θ
v = -WS cos θ
v WS
θ
u
WindDir = atan(u/v) with quadrant corrections
θ is the geographic angle of the wind.
WinDir is geographic but it needs quadrant corrections:
θ in this block
is the quadrant
correction
From: http://www.wrh.noaa.gov/sew/fire/olm/transport.htm
Double Theodolite
Sometimes you do not know the exact inflation
Of the balloon, so you can not calculate its ascent rate.
You may also want to underinflate a balloon so that you
get a more detailed view of the boundary Layer
In these cases you can find the elevation of the balloon
using 2 theodolites.
You must know the
distance between them
and their azimuth angle of the other theodolite.
Radiosonde
vs
Piball
What are the advantages of a radiosonde?
Radiosonde returns:
•Position: Needs 2 GPS's
•Rh Tdp
•Temperature
•Pressure is often derived from Hydrostatic Equation
•Pibal Returns only wind speed and wind direction
•But Requires no electronics