Radiosounding experiment
Climatological and hydrological field work
Rietholzbach research catchment
Handout: Andrea Grant, Tracy Ewen, Micah Wilhelm, Martha Vogel
Lecturer: Martha Vogel
1
Introduction
Upper air measurements of temperature, relative humidity, pressure and wind speed and
direction have been taken since the early 1900s. Early measurements were obtained
with recording instruments flown on kites or aircraft. With the development of radio
communications in the 1930s, the first radiosonde was developed by Soviet meteorologist
Pavel Molchanov. This lab will collect data from radiosondes attached to balloons.
A meteorological balloon, carrying instruments and transmitting equipment is released, untethered, rises until it bursts, and falls back using a parachute. The radiosonde
transmits GPS data to a radio receiver so that the payload can be recovered.
Radiosondes are launched twice daily at 12 UTC and 00 UTC at about 900 stations worldwide. In Switzerland the radiosondes are launched in Payerne. They provide
information regarding the vertical structure of the atmosphere which is ingested into
numerical weather prediction models. The term ‘sonde’ or ‘radiosonde’ refers to the instrument package itself, while ‘sounding’ is normally used for the entire sonde-balloon
launch and the data collected by a launch.
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2.1
Theory
Hydrostatic balance
The temperature profile of the atmosphere gives information about the stability of the
atmosphere. An unstable atmosphere can lead to large scale convection and the development of thunderstorms. Other features, such as inversions or large vertical shear, can
also be detected.
The height of a point in the atmosphere can be calculated by starting with the hydrostatic balance;
dp
= −ρg
(1)
dz
where p is pressure, z is height, ρ is density, and g is gravity. Combining this with
J
pV = mRT and R = 287 kg·K
we find:
dp
g
=−
dz
p
RT
1
(2)
which leads to:
RT
d(ln p)
(3)
g
If an isothermal atmosphere is assumed (i.e., temperature is independent of pressure),
this expression can be integrated, and the pressure takes the form of an exponential with
height.
dz = −
2.1.1
Skew-T-log p-Diagram
The information from a sounding can be used in a skew-T-log-p diagrams to analyze the
stability of the atmosphere and to estimate cloud conditions. There are five key variables
in the diagram:
• Temperature in ◦ C on the x-axis. The skewed isotherms (red), lines of constant
temperature are going to the upper right.
• The pressure in hPa is on the y-axis in logarithmic units. The isobars are the
horizontal lines in black.
• Dry adiabats (yellow), moist adiabats (green). For low temperatures on the top
atmosphere both adiabats are parallel. (Why?)
• The mixing ratio in g/kg (blue), from the left to the upper right.
Figure 1: Skew-T-log p Diagram.
In a skew-T-log-p diagram different levels of the atmosphere can be determined:
• Lifting Condensation Level (LCL)
• Level of Free Convection (LFC)
• Level of Neutral Buoyancy/Equilibrum Level (LNB)
What do the different levels mean? Determine the different levels in the sounding of
Payerne from the 5th of August 2013 (see Figure 2).
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Figure 2:
Sounding in Payerne of 5th of
(http://weather.uwyo.edu/upperair/sounding.html).
3
August
2013
at
12
UTC
2.1.2
Ascent rate
The following formulas can be used to determine the ascent rate of our balloon.
a) We will fill the weather balloon with around 800 g helium (carbalon). What will be
the approximate ascent rate v for the ascent? Use the following formulas:
L = V (ρAir − ρHe )
(4)
R=L−P
(5)
v=
Rg
0.5 Cd A ρAir
!1/2
(6)
L is the lift in kg, V the volume of the filled balloon in m3 , ρAir = 1.205 kg m−3
and ρHe = 0.166 kg m−3 the densities of air and helium under standard conditions
(T = 0 ◦C, p = 1013.25 hPa), P is the payload, including the mass of the radiosonde,
tether, parachute, balloon, R is the residual lift, g = 9.81 m s−2 the gravitational
constant, Cd = 0.25 the coefficient of friction of a sphere and A = πr2 the cross
sectional area m2 .
b) Explain with Equation 6 why we can assume the ascent rate to be approximately
constant.
http://championship.endeavours.org/2015/Resources/Tech/Balloon%20Rise
%20Rate%20and%20Bursting%20Altitude.pdf
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Equipment
A radiosonde is a small package containing sensors for temperature, relative humidity
(RH), and pressure. It is attached to a balloon and allowed to rise through the atmosphere, sampling data at frequent (approximately one second) intervals. Soundings
typically reach 30,000 m in height before the balloon bursts due to the cold temperatures in the stratosphere. In addition to temperature, RH, and pressure, the sounding
also measures wind speed and direction by tracking the balloon’s location with either a
theodolite or embedded GPS receiver. Soundings that include wind data are sometimes
called rawinsondes.
This lab uses a iMet-1 radiosonde. The temperature sensor is a glass bead thermistor
(resistance changes with temperature). The humidity is measured by a capacitive thinfilm moisture sensor (porous polymer material that acts as a hydro-active sponge between
two electrodes: the capacitance changes with relative humidity). The pressure sensor is
a piezoresistor (semiconductor membrane who resistance varies with mechanical stress).
Winds are not measured.
Data is transmitted via an FM radio signal at 403, 404 or 405 MHz and is detected by
the portable antenna which is connected to a commercial radio receiver. Finally, the signal is converted to digital data and read into the laptop. For this and the recording of the
launch we are using the program SkySonde Server developed at NOAA. The documentation can be found here: ftp://ftp.cmdl.noaa.gov/user/emrys/SkySondeThe software is
downloaded from: http://www.esrl.noaa.gov/gmd/ozwv/wvap/sw.html.
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Table 1: Response time of the temperature sensor.
pressure (hPa) τ (s)
1000
3
10
8
10
21
Table 2: Response time of the humidity sensor.
temperature (o C)
τ (s)
20
0.3
0
1.5
-20
9
-30
20
-60
unreliable
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Radiosonde Errors
Since the beginning, upper air data has been prone to many errors. Some of these
continue to plague the data, reducing the quality. Additionally, the frequent redesign
and numerous manufacturers of radiosondes means that the historical record is quite
problematic.
Temperature measurements can be affected by slow or inaccurate sensors, or through
spurious heating by solar radiation or nearness to other warm bodies. These are usually
termed “radiation errors”.
• Why is it important that the sonde is at least 10 m away from the balloon?
• How might day and night soundings have different errors?
Theoretically, sensors respond instantaneously to new surroundings. In reality, the
sensors take varying amounts of time to equilibrate with the air surrounding them. This
is called the “lag error”.
• How would the amount of lift in the balloon affect the sounding?
• What is the response times of the sensors are (see Table 1)?
Humidity is also notoriously difficult to measure. In the past, sensors included “hair
hygrometers”, where a piece of hair is mounted under tension and the length of it changes
with ambient humidity (frequently the source of unwanted frizzy hair in humans), and
“goldbeater’s skin”, a very thin sheet of parchment which will expand or contract with
changes in humidity. Most sensors have a lower limit (often 0o C or −40o C) and will
report erroneous values below this, skewing the record (see Table 2).
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5
Procedure
This lab has two parts. 1) There is a routine radiosonde launch which will capture a full
atmospheric profile. 2) A tethered balloon sounding, to collect detailed data about the
boundary layer. The radiosonde is lifted by a tethered balloon. An approximate time-line
follows:
00:00 begin setting up first radiosonde
00:15 launch radiosonde
00:45 check conditions for tethered sounding
01:30 terminate full sounding
01:45 begin tethered sounding—at least two ascents and descents
03:00 finish tethered sounding
5.1
5.1.1
Preparation for routine sounding
Ground Station
• Turn on the laptop.
– password: iaceth
• Attach a audio cable, from the receiver’s audio output (often labeled REC OUT)
to the computer’s microphone input
• Connect the antenna cable to the back of the radio receiver.
• Built up the mobile antenna and place it on a stable place. Connect it to to the
antenna cable.
• Turn on the radio receiver (button on the upper left corner of the receiver.
• Set the receiver to WFM (wideband frequency modulation) and enter the radio
frequency 405 then press enter. This frequency must match the one that will be
broadcasting from the radiosonde.
• Turn the ‘AF GAIN’ to one third of the maximum value. Check that the ‘SQUELCH’
knob is turned down completely.
5.1.2
Sonde
• Remove the sonde from the bag and follow the instructions (before step 6, tuck the
ancillary cable back into the top of the box).
• Record the serial number of your radiosonde, which is located on its removable
door. You will need this for your output data file name.
5.2
Data Collection
Data receiving and collection is handled by the program SkySonde.
• Turn on the radiosonde (using the same frequency as the receiver).
• Start the program SkySonde Server (only when radio receiver and laptop are connected).
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Figure 3: Screenshot of SkySonde Server.
• The iMet by itself will send either a PTU or PTUX packet, and a GPS or GPSX
packet. Make sure that the lights for PTU/GPS are blinking. (see Figure 3).
• While the server is running, open SkySonde Client (program contains detailed
flight setup information, parses and calculates instrument data from packets, and
stores several output file types)
• in the Acquisition tab: Select Data Source: Sky Sonde Server (real time), fill in
Radiosonde ID, Flight name for the output directory (see Figure 4)
• in the Station tab: Select Station Name Rietholzbach, tick Use First Radiosonde Packet.
Click OK (see Figure 4).
• Monitor the data on the SkySonde Client and tick the variables (Pressure, Temperature etc) to visualize the data in real-time (see Figure 5).
• The data is stored in the folder: Computer/Data not backupt(D:)
/BalloonSundings/SkySonde Data/"FLIGHTNAME"
5.2.1
Balloon
• While wearing gloves, carefully remove a balloon from its packaging.
• You will want to wear gloves because it is important not to touch (with your hands,
the ground, your head, etc.) any part of the balloon except for the thick rubber
right at the opening. Oils, especially from skin, will damage the rubber causing the
balloon to burst prematurely.
• Insert the nozzle from the gas tank into the opening of the balloon.
• Gently turn on the gas, while others hold the nozzle and the balloon. As it inflates
stabilize the balloon to keep it from hitting anyone’s head.
• You want to inflate the balloon until it has about 800 g of lift—estimate that by
attaching the 800g weight. weight to the nozzle and waiting until it just lifts the
weight off the ground. This will give an ascent rate which you calculated with
Equation 6.
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Figure 4: Screenshot of SkySonde Client
Figure 5: Screenshot of SkySonde Client Monitoring
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Figure 6: Sounding set-up.
• Turn off the gas and tie the balloon shut, keeping the string only on the thick
rubber.
• Attach the sonde to the parachute and the balloon using the prepared winded cord
as shown in figure 6. Loop the string through the sonde hole twice before tying it.
• Tape the knots and the opening of the sonde. Put a sticker with the returning
details on the sonde.
5.3
Launch
Position yourselves well away from any trees or buildings, taking the wind conditions into
account. Have one person hold the balloon and the second hold the sonde. It is best to
simply lay the sonde in the open palm of your hand—once the balloon is released the
sonde will lift out of your hand. Clutching the sonde has been known to break the line
connecting it to the balloon!
Make sure the data is being collected on the computer. While the sonde ascends you
can prepare for the boundary layer sounding.
5.4
Preparation for boundary layer sampling
We sample the boundary layer using a tethered balloon. Check the current wind at:
http://www.iac.ethz.ch/research/rietholzbach/datasets
Prepare a second sonde as above. Note that you do not need a parachute since there
will be now full sounding. Note that you can attach a smaller pilot balloon (200 g) since
the the ascend will only around 50 . Attach the sonde to the 50m cord. Attach also a
additional cord to the balloon so that you can hold it while preparing the sounding. Once
the first sounding is completed (balloon bursts and begins falling, the signal is lost, or
due to time constraints you need to move on to the boundary layer part) click the quit
button. Then follow the same procedure as above. However, change the receiver and
radiosonde frequencies to 403 Hz.
Aim to have three full ascents and descents from the ground up to 50 m.
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6
Analysis
Back at ETH, you will analyze the sounding data (both the full and the boundary layer
soundings) from all groups. You will compare the different weather conditions, analyze
the structure of the atmosphere and will present the results to the other groups. You can
structure your analysis along the following points:
• Compare the balloon tracks of the five different soundings
• Plot the profiles of all measured variables.
• Calculate the lapse rate.
• Determine the different levels of the atmosphere as LCL, LFC and LNB.
• Determine the stability of the atmosphere.
• Calculate energy fluxes for the boundary layer.
• Estimate errors of the sounding.
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References
1. http://championship.endeavours.org/2015/Resources/Tech/Balloon%20Rise
%20Rate%20and%20Bursting%20Altitude.pdf
2. http://weather.uwyo.edu/upperair/sounding.html
3.
ftp://ftp.cmdl.noaa.gov/user/emrys/SkySonde%20User%20Manual.pdf
4. Wernli, H. and Peter T. Skript of Leture ”Atmosphaere”, Fall term 2015.
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A
iMet Specifications
iMet-1 Radiosonde
GPS/RDF Technology
1680/403 MHz Flexibility
Features
System Overview
Available in Four Models:
Operating Principle
GPS or RDF
Pressure (optional)
Nominal Frequencies
1680 / 403 MHz
Type
Piezoresistive
Range
> 250 km with TRS *
Range
2 to 1070 hPa
Altitude
> 42 km with TRS *
Accuracy
0.5 hPa < 400 hPa
Battery
Alkaline Dry Cell
Operating Time
> 2 Hours
Resolution
< 0.01 hPa
Advanced Sensor Technology:
Weight
260 Grams
Response Time
< 1.0 Sec
– Thin Polymer Humidity
– Bead Thermistor Temperature
– Optional Solid State Pressure
Sampling Rate
1 / Second
Case
Expanded Polystyrene
– 12 Channel C/A Code GPS
Transmitter
–
–
–
−
1680 MHz RDF
1680 MHz GPS
403 MHz GPS
403 MHz Research 1
0.5 hPa > 400 hPa
Temperature
1668.4 – 1700 MHz
Type
Bead Thermistor
400.15 – 406 MHz
Range
- 95 to + 50 Deg
Output Power
300 mW
Accuracy
0.2 Deg C
Transmission
2400 baud, FM
Resolution
< 0.01 Deg
Bandwidth
120 kHz (1680 MHz)
Response Time
2.0 Sec
Tuning Range
Simple to Use:
– Dry cell batteries
– Switchable power on / off
– No pre-flight temp & humidity
recalibration required
– Switch controlled frequency
– Compact and light weight
Meteorological Sensors
20 kHz (403 MHz)
Stability
@ 1000 hPa
Crystal Controlled
GPS Receiver
Humidity
Type
Tracking
Update Rate
Acquisition Time
Position Accuracy
Wind Velocity Accuracy
Altitude Accuracy
C/A code, 12 Channel
Continuous
1 Hz
50 sec (cold start)
10 m
1.0 m/s
15m
Specifications subject to change without notice
1
Compatible with DMT ECC Type Ozone Sensor
* Telemetry Receiver System (iMet-2000)
Type
Range
Accuracy
Resolution
Response Time
Capacitive
0 to 100% RH
5% RH
< 0.1% RH
2 Sec @ 25 Deg C
60 Sec @ - 35 Deg
3854 Broadmoor SE, Grand Rapids, MI
phone: 616-285-7810, fax: 616-957-1280
e-mail: [email protected]
http://intermetsystems.com/ee/pdf/iMet-1-ABxn_Data_150316.pdf
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