Determining the Variations of Wind with Height. Assignment 13. Due April 22

Discussion: Pibals are an inexpensive way to determine winds aloft below cloud base. They are still used in many developing countries, in field programs, and in air quality work. Single theodolites require significant assumptions about the rate of ascent of the balloon. Part of this exercise is to determine the sensitivity of the inferred horizontal winds to these assumptions. Two theodolites tracking a single pibal can determine the horizontal wind without those assumptions being required. The pibals being used are 30 gram balloons.

Review the program ~jhorel/pibal/single.f to understand the assumptions and method required to convert single thedolite measurements to horizontal winds.

Determine the lifting force of 2 pibals with differing amounts of helium. Record these numbers in grams.

Track the 2 pibals and record azimuth and elevation to the nearest tenth of a degree as a function of time (in 30 second intervals). Interpolate any missing observations.

Plot the azimuth and range on the accompanying chart. How did the winds change with height?

Write for each balloon separately the azimuth and elevation of each observation in a file in fortran f5.1 format.

Run the program ~jhorel/pibal/single with the name of one of the balloon files listed on the command line. Answer all of the questions required (base, lifting force, total weight, time interval). Output from the program will be in the file pibal_output. Use the vertical velocity determined from the program. Determine the horizontal wind speed and direction as a function of elevation Write the horizontal wind speed and direction as a function of elevation in a simple tabular format.

Plot the azimuth and range on the accompanying chart. How did the winds change with height? Contrast the winds determined from the pibal to those evident in the SLC sounding at 12z. What features in the wind field agree or disagree at the 2 different times and locations?

Repeat running the program with a vertical velocity 1/2 that and double that determined earlier. What was the impact of changing the ascent rate on the horizontal winds?

What other assumptions or operational aspects of the pibal launch and tracking could introduce errors?