MEAN ZONAL WINDS
Experimenting with Seasonal Predictions for High Altitude Balloon Flights
Ralph Wallio, WØRPK   W0RPK  at  netINS.net

Experienced planners of Amateur Radio high altitude balloon missions routinely use winds aloft forecasts and observations to predict ground tracks and landing areas. The prediction process and original software was developed in the late 1980's by Bill Brown, WB8ELK, and most recently available as Balloon Track for Windows by Rick von Glahn, NØKKZ (see http://www.eoss.org/wbaltrak/index.html). Predictions for specific missions are typically created only a few days before they fly with an average ≈12-mile accuracy (see Touchdown Prediction entries at the bottom of http://showcase.netins.net/web/wallio/ARHABrecords.htm).

Seasonal planning for high altitude missions may also be possible but it has not been as widely discussed or utilized. Planners could consider methods discussed below to understand how flight track characteristics change month-to-month through the year. They could then consider these changes as they plan for flights several months in advance. Planners already have a good handle on ascent rates, descent rates and burst altitudes. The only missing link is a long-term view of winds aloft.

This discussion experiments with a long term view of winds aloft available in National Center for Atmospheric Research (NCAR) Technical Note 366, Global Atmospheric Circulation Statistics, 1000-1 mb, by William J. Randel (see http://www.ucar.edu/communications/technotes/technotes301-400.shtml for ordering information). It contains 256 pages of interesting discussion of methods and graphics but we only need the last few pages that contain tables of wind speeds across 12 months, surface to 157,000ft, latitudes 85dS to 85dN.

This technical note discusses the concept of Mean Zonal Winds (MZW) and how they vary month-to-month. Daily data from 12 years, 1979-1990, has been used to develop mean wind speed values at 1-1000mb heights (157,000-0ft) in 5 degree increments of latitude for all 12 months of the year. Statistical deviations from mean values are given for daily and interannual periods (2-10 years). Data for Mean Zonal Temperatures is also available.

Given these MZW values, this experimental process involves six steps:

1) Format MZW values via EXCEL into a table compatible with Balloon Track for Windows (BTW).
2) Run BTW and export track prediction output in CSV format (Comma Separated Variables).
3) Import BTW CSV output to EXCEL and standardize data format.
4) Graph downrange distance vs. altitude and consolidate results.
5) Construct charts containing 12 months of MZW downrange tracks
6) Compare MZW based predictions with results from actual missions.


STEP-1: Preparing MZW values for BTW

NCAR/TN-366 provides individual MZW tables for each month of the year that contain values for 35 latitudes (north and south in 5 degree increments) and 17 altitudes, 1000-1mb (0-157,000ft). Wind speeds are given in meters/second (m/s) which can be converted to knots (KTS = m/s x 1.9426), miles per hour (MPH = m/s x 2.237) or kilometers per hour (KPH = m/s x 3.6).

Wind direction is given only as westerly or easterly, e.g., there is no attempt to predict precise wind directions. In the Northern Hemisphere during the winter, winds at all levels are out of the west (270 degrees). During the summer, lower level winds are out of the west (270 degrees) while higher level winds are out of the east (090 degrees). In the tables easterly winds are indicated with negative speed values, e.g., -10m/s.

From these extensive tables I have extracted wind speed values for 40N latitude. U.S. Standard Atmosphere, 1976 (available from NTIS) is used to convert heights given in millibars to meters and feet ASL.

Mean Zonal Winds (meters/second) at 40N Latitude

Charting Altitude (feet ASL) vs. Wind Speed (mph) data from this table  yields interesting results:

MZW values of interest are then entered into an EXCEL spreadsheet that takes care of unit conversion, wind direction and formatting. In this example MZW values from July 40dN are manually transferred as input to the conversion form:

(EXCEL spreadsheet is available from W0RPK at netINS.net)

Output from the input/conversion form is then copied into a second sheet of the same workbook and then saved from that sheet in CSV format to an appropriately named file, e.g., MZW_July_40N.CSV. The result looks like this when opened with a text viewer like NotePad:

STEP-2: Run BTW and export track prediction output

The CSV file prepared in Step-1 is opened by Balloon Track for Windows (BTW) which is then set up for ascent rate, descent rate and burst altitude (via SET UP and FLIGHT DATA tabs). Using 1000ft/min, 1500ft/min and 100,000ft respectively, BTW looks like this when also set up for launch coordinates of 0 degrees latitude and longitude.

BTW then exports output in CSV format to an appropriately named file, e.g., MZW_JULY_40N_BTW.CSV, (using FILE then EXPORT then COMMA DELIMITED).


STEP-3: Import BTW CSV output to EXCEL and standardize data format

The BTW CSV file is opened as an EXCEL spreadsheet for clean-up, conversion and graphing in a standardized format. A great circle navigation formula is used to calculate distance downrange from launch coordinates (GEOFUNC.XLA, an EXCEL function add-in which performs trigonometric calculations for plane and spherical geometry and a number of other related calculations is available at http://nmml.afsc.noaa.gov/Software/ExcelGeoFunctions/excelgeofunc.htm).

STEP-4: Graph downrange distance vs. altitude and consolidate results

EXCEL is used to develop an X-Y scatter/line chart which plots Distance Downrange (X) vs. Altitude (Y). Note that in this case for July, lower level winds push the balloon and payload to the east (some direction northeast through southeast in actual flights) and then upper level winds push them back to the west until burst at 100,000ft.

This chart predicts a single track caused by a 1000ft/min ascent through Mean Zonal Winds but we could add another track for a 500ft/min ascent rate. The result would bracket the vast majority of flights that ascend in the range 500-1000ft/min. To add the second track we back up to STEP-2 and run BTW using 500ft/min and continue through STEPS 3-4. We then consolidate the two tracks into a single EXCEL chart.


Note that track characteristics are similar but distances are longer when ascent rate is slower. Descent track is exactly the same (1500ft/min was used in both cases) but starts further east in the case of a slower 500ft/min ascent.


STEP-5: Construct charts containing 12 months of MZW tracks

Steps 1-4 can be used to create downrange track data for all 12 months and then EXCEL can be used to combine all 12 tracks into two charts (two charts rather than one because track overlap on a single chart is visually confusing). I have used 90,000ft burst altitudes for all 12 downrange tracks.





Note that track distances for winter months are substantially longer than for summer months (peaking in December and January). Easterly winds during summer months cause the characteristic "loop" (peaking in July and August).


STEP-6: Compare MZW based predictions with results from actual flights

Finally (sez the reader), we can compare seasonal MZW downrange track predictions with results from actual flights. Most amateur high altitude missions include GPS receivers and periodically make position reports via digital downlink telemetry radio channels. These position reports include precise time stamp, latitude, longitude and altitude data fields. Telemetry collected at ground stations is often available for post-flight analysis by others.

I am in the process of collecting telemetry data files from as many flights as are available. Data is structured into a standard EXCEL format and organizing by month-of-the-year. An EXCEL downrange track chart is created for each flight and then consolidated with MZW predicted tracks into monthly charts and tables that follow. In the tables, ASCENT is average rate for entire ascent in feet/minute; DESCENT is approximate descent rate in feet/minute at sea level; and RECORDS is the number of GPS data records available in the data file. Brief comments are added for what appear to be unpredicted or extraordinary situations.

Upper level easterlies are not predicted by MZW. Ascent rate of EOSS-46 can not be determined due to lack of time stamps but it appears to be significantly higher than 1000ft/min. Descent rate appears to be close to 1500ft/min (sea level) as used for MZW tracks.

Upper level easterlies reversing track of KNSP-99A are not predicted by MZW. It is interesting that winds above 105,000ft veer back to the west (this does not happen during summer months).

Note that BEAR-1 flew at 54dN latitude and therefore should not be compared to MZW tracks that are for 40dN latitude. The match of NSTAR-01B and 1000ft/min MZW tracks is very close.