CHAPTER 7:: UPPER ATMOSPHERE MEASUREMENTS
Introduction
A three-dimensional picture of temperature, pressure, relative humidity, and wind speed and direction in
the atmosphere is essential for weather forecasting and meteorological research.
History
During the latter part of the 19th century and the first quarter of the 20th century, the upper air information
was obtained mainly by meteorographs sent aloft on tethered kites, which automatically recorded, on a
single sheet, the measurements of two or more meteorological parameters such as air pressure, temperature,
and humidity.Kite Station, late 1800s and early 1900s This system had several weaknesses:
* The average altitude reached was only about 10,000 feet although some flights reached nearly 20,000
feet.
* The data could not be evaluated until after the observation was completed and the kite and
meteorograph were brought back to the ground.
* Kites could be flown only in good weather and only if winds were neither too light nor too strong.
* There was danger of the kite breaking away and the meteorograph endangering lives and property.
From 1925 to 1937, upper-air data were obtained by attaching meteorographs to airplanes, observations
called APOBS. While planes continued to be used for this purpose until 1943, their use was greatly
curtailed after radiosonde observations were inaugurated in 1937. Airplane observations also had several
weaknesses:
* average altitude was approximately 17,000 feet and the maximum about 20,000 feet.
* data could not be evaluated until the observation was completed and the plane had landed.
* observations could not be taken in stormy weather.
Upper air measurements
Radar Theodolites track the movement of the rawindsonde.
  elevation angle
  azimuthal angle
By tracking the balloon we can obtain R, S , ,  , and from this we can obtain the position ( x, y ) and
height h.
S  R cos  ; x  S cos  ; y  S sin 
Then we can calculate the winds by using equation 5.4
x
y
 u;
v
t
t
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(5.4)
This technology gives the x, y as a function of time.
Radiosonde
The radiosonde is a small, expendable instrument package that is suspended below a large balloon filled
with hydrogen or helium. The radiosonde consists of sensors used to measure several meteorological
parameters coupled to a radio transmitter and assembled in a lightweight box. The meteorological sensors
sample the ambient temperature, relative humidity, and pressure of the air through which it rises.
As the radiosonde is carried aloft, sensors on the radiosonde measure profiles of pressure, temperature, and
humidity. These sensors are linked to a battery powered, 300 milliwatt radio transmitter that sends the
sensor measurements to a sensitive ground receiver. The transmitter operates on a frequency ranging from
1668.4 to 1700.0 MHz, while the ground receiver can detect and process signals ranging from 1655 to 1705
MHz. By tracking the position of the radiosonde, information on wind speed and direction aloft is also
obtained.
The instrument package is attached to a biodegradeable plastic parachute, and together, they are suspended
from a balloon. The balloon is a spherically-shaped film of natural or synthetic rubber (neoprene) and is
inflated with a lighter-than-air gas such as hydrogen or helium. Even though various sizes are available,
600-gram and 1200-gram balloons are most widely used. The balloon, parachute and radiosonde, known
collectively as a flight train, ascend at an approximate rate of 1000 feet per minute depending on weather
conditions, amount of gas, and balloon size.
During the flight train's ascent, the radiosonde continuously transmits temperature, relative humidity, and
pressure readings to the ground-based Radiosonde Tracking System which is housed in a fiberglass dome
above the inflation shelter. Wind speed and direction are determined for each minute of the flight, generally
90 minutes. They are determined from changes in the position and direction of the flight train as detected
by the Radiosonde Tracking System. When winds are incorporated into the observation, it is termed a
rawinsonde observation, and all National Weather Service upper air stations take rawinsonde observations.
The altitude reached by rawinsonde observations varies for several reasons:
* bursting height of the balloon;
* faulty receiving equipment;
* atmospheric interference.
Those observations that reach the bursting altitude of the regular 600-gram balloon attain an average height
slightly in excess of 90,000 feet. The average bursting altitude for stations using the larger 1,200-gram
balloon exceeds 100,000 feet. The average altitude reached for all observations is approximately 94,000,
while a number of observations have risen to 125,000 feet or more. When the balloon reaches its elastic
limit and bursts, the parachute slows the descent of the radiosonde, minimizing the danger to lives and
property.
Significant changes in the meteorological parameters are entered into a minicomputer, which converts them
into coded messages. These messages are relayed through various communications systems to all parts of
the world. The information in these messages is extremely important in weather forecasting, climatology,
and research. The National Center for Environmental Prediction (NCEP) prepares daily upper air charts and
forecasts based on the rawinsonde observations. After the data has been used, it is archived at the National
Climatic Data Center (NCDC), where it is available to anyone upon request. A Complete Flight Train in
Flight
Although all the data from the entire flight is used, data from the surface to 400 millibars (mb) or about
24,000 feet is considered minimally acceptable for NWS operations. Thus, a flight is considered a failure
and a second radiosonde is released if the balloon bursts before reaching the 400 mb level or if more than 6
minutes of data between surface and 400 mb are missing.
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Worldwide, there are more than 900 upper-air observation stations using 15 major types of radiosondes.
Most stations are located in the Northern Hemisphere and all observations are taken at the same times each
day at 00:00 and 12:00 UTC (Greenwich Mean Time), 365 days per year. Observations are made by the
NWS at 93 stations - 72 in the conterminous United States, 13 in Alaska, 10 in the Pacific, and 1 in Puerto
Rico.
Through international agreements, radiosonde data are exchanged between countries and are applied to a
broad spectrum of efforts. Applications include: data initialization for global and regional numerical
prediction models; input for air pollution/dispersion models; severe storm, general, aviation, and marine
forecasts; ground truth for satellite data; weather research; and climate change studies. NWS Rawinsonde
Observing Network in the United States
Approximately one-third of the radiosondes released by the National Weather Service are found and
returned to the Instrument Reconditioning Branch in Kansas City, Missouri, where they are repaired and
reissued for further use, some as many as seven times. Instructions printed on the radiosonde explain the
use of the instrument, state the approximate height reached, and request the finder to mail the radiosonde,
postage free, at any Post Office or hand it to a U.S. rural carrier. The finders receive the satisfaction in
knowing that they are helping reduce the overall cost of operating the Government.
At present, the rawinsonde observation is the most accurate and cost-effective method for sampling the
atmosphere. When combined with meteorological satellites and aircraft reports, a complete profile of the
atmosphere to very high altitudes is possible.
NWS Operational Use of Radiosonde Observations
Accurately predicting changes in the atmosphere ranging from severe thunderstorms to global climate
change requires adequate observations of the upper atmosphere. The NWS radiosonde network is the
primary source of upper-air data and will remain so into the foreseeable future.
The current operational numerical weather prediction (NWP) models for weather and hydrologic prediction
are run on computers at the National Center for Environmental Prediction in Camp Springs, Maryland.
Radiosonde data are fed into the models and, with other observations, assimilated to provide initial
conditions for the model predictions. The highest resolution operational weather prediction model
computes weather parameters on a mathematical grid 48 by 48 km horizontally, 38 levels vertically, and in
200 second time steps out to 48 hours. Experimentally, a 29 by 29 km, 50 vertical level model is being run
regularly in addition to the 48 km resolution model.
Radiosonde sites in the conterminous United States are about 375 km (235 miles) apart. Since the NWP
model grid resolution is much smaller than the spacing of radiosonde sites, the loss of data from only one
or two radiosonde sites can have significant impacts on model forecasts. Studies have shown that when
missing radiosonde observations occur, small scale weather features above the surface can be lost or
inaccurately positioned in the data analysis, causing significant errors in the model predictions. This
especially holds true during severe weather episodes.
Importance of Radiosonde Data to Local Weather Prediction
NWS meteorologists also analyze individual radiosonde soundings to help them prepare short-term, local
weather forecasts. Individual soundings help forecasters determine many local weather parameters
including, atmospheric instability, freezing levels, wind shear, precipitabie water, and icing potential. The
following are examples of local weather phenomena that are predicted with the aid of sounding data:
* Severe thunderstorms
* Tornadoes
* Microbursts
* Flash floods
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* Ice storms
* Aircraft icing conditions and turbulence
* Cloud heights
* Maximum temperature
Unfortunately, the present spatial and temporal resolution of the radiosonde network hinders the NWS from
optimizing the accuracy of local weather forecasts because they occur on much smaller time and space
scales. Just one missing radiosonde sounding can degrade the forecast accuracy of severe weather events
and thus it is essential that all radiosonde sites report observations on a routine basis.
GPS Dropsondes
GPS Dropsondes are instruments deployed from aircraft to measure temperature, humidity, pressure, and
especially winds between flight level and the surface. Use of the Global Positioning System (GPS) for wind
finding is a technical revolution. The sonde moves with the horizontal wind as it falls at 10 m/s (2000
ft/min) so that the wind can be determined from the Doppler shifts of the GPS signals. It carries capacitive
temperature, moisture, and pressure sensors. Readings are telemetered to the airplane at half second
intervals. Vertical resolution for all measurements is 5 m. With its finer vertical resolution, superior
thermodynamic sensors, and GPS navigation, it is a great improvement over earlier versions. It promises to
revolutionize understanding of hurricane motion and intensity change.
The National Center for
Atmospheric Research
(NCAR) GPS
Dropsonde system is also
known as AVAPS (Airborne Vertical Atmospheric Profiling System). Since its debut in 1997, it has flown
on numerous missions in support of operational weather forecasting and atmospheric research, with
impressive results. The dropsonde incorporates a pressure, temperature, humidity sensor module (RSS903)
and a new GPS receiver module (GPS111), both designed by Vaisala, Inc.. The sensor specifications are
shown in the following table.
Dropsonde Sensor Specifications
Range
Accuracy
Resolution
Pressure
1080-100 hPa
± 1.0 hPa
0.1 hPa
Temperature
-90 to +60 C
± 0.2 C
0.1 C
Humidity
0-100%
± 5%
1.0%
Horiz Wind
0-200 m/s
± 0.5 m/s
0.1 m/s
Driftsondes
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What is a driftsonde?
It is a new type of observing system that shows great promise in tracking weather above hard-to-reach parts
of the globe, as well as make soundings that will fill critical gaps in data coverage over oceanic and remote
arctic and continental regions. Forecast sensitive regions that would be targeted for driftsondes are (1)
relatively void of in-situ measurements from radiosondes and commercial aircraft, such as the remote
Pacific and Atlantic oceans, (2) covered with extensive cloud shields so that satellite measurements are
limited. The across the ocean driftsonde flights will provide synoptic-scale high-vertical resolution
atmospheric profiles made by GPS dropsondes that would be difficult or impossible to obtain by
deployment of aircraft alone.
The driftsonde system consists of a low-cost zero-pressure polyethylene balloon (roughly 30 feet tall ) that
has a duration of approximately five to seven days or a moderate duration super pressure balloon that can
fly for weeks or even months, either type balloon has an attached NCAR gondola. Housed in the gondola
are the system electronics which includes an embedded computer, a GPS navigation system, flight level
pressure, temperature, and humidity (PTH) sensors, a ballast control system, a battery power system, an
Iridium satellite two-way communication system, and up to approximately 60 dropsonde tubes.
How does a driftsonde work?
The special type of balloon used in the driftsonde ascends to the lower stratosphere, at heights of around
60,000 to 65,000 feet, and drifts with the winds that prevail at these altitudes. Scientists at an operations
center communicate with the driftsonde through Iridium communications satellites. With a remote
command, they can release dropsondes that a driftsonde carries. The dropsonde falls through the
atmosphere, collecting weather data twice per second and sending the data back to the driftsonde gondola.
Later, the information is relayed to the operations center and then on to forecast centers.
The use of driftsondes was made possible by four major studies: THORPEX (The Observing System
Research and Predictability Experiment), a 10-year program under the World Meteorological Organization
designed to accelerate skill in predicting high-impact weather events; the AMMA (the African Monsoon
Multidisciplinary Analysis), a multiyear program designed to advance understanding of the West African
monsoon; the Antarctic climate-change study in association with the International Polar Year (2007-08);
and a North Pacific campaign in 2008 focused on predicting intense winter cyclones, floods, severe
thunderstorm outbreaks, and fire weather over North America.
Related information
Balloon Type: Either a 4 to 7 day Zero Pressure balloon (e.g. 363 cu. meter for an altitude of 100 mb up to
a 10,000 cu. meter for an altitude of 10 mb) or a Super Pressure balloon (e.g. 12 meter diameter for an
altitude of 10 mb) that can fly for weeks or even months.
Scientific Payload: A typical gondola weight with 50 GPS MIST dropsondes is ~22 kgms. Other scientific
instruments can be integrated into the on board system for flight level measurements.
Launch Site Requirements: Some type of shelter for protection of the balloon during inflation (e.g. Hangar)
along with a reasonably flat smooth area for launch (e.g. in front of hangar). The wind speed should be
below 10 knots during balloon launch.
Zero Pressure Balloons
A zero pressure balloon is made from very thin (7.5 micron) plastic sheet, like a laundry bag. It is filled
with helium through a tube connected to the upper portion, and it has a vent to the atmosphere at the very
bottom.
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The balloon is filled only partially on the ground. As the balloon rises, the gas bubble expands to fill out the
balloon. When the gas bubble expands to completely fill the balloon, excess gas exhausts through the vent
at the bottom; at this point, the ascent stops, and the balloon will hover at a constant altitude.
At night, after the sun goes down, the gas bubble cools and shrinks. This causes the balloon to begin to
descend. If the balloon mission must go for several days, some ballast material, such as sand, can be
dropped to maintain altitude. But during the next day, the gas will heat and expand again, but since the
system is now lighter, it will ascend higher. To maintain its altitude, the system has to dump some gas.
And so on until the supply of ballast is used up. Eventually, it will hit the ground, but this may take several
days.
A zero pressure balloon MUST be provided with a means of terminating the flight to keep it from floating
in civil airspace at night. This is usually vent at the top of the balloon which can be opened via a radio
command. Another method is to put some weight at the top and to cut away the payload; then the weight at
the top will cause the balloon to turn upside down so that the gas can exhaust through the vent.
Because of the difficulty and possible failure of the termination means, it is prefered to use rubber (latex)
balloons, which will self-destruct (almost always) at altitude.
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