radiosounding experiment · turn on the radiosonde (using the same frequency as the receiver)....

Climatological and Hydrological Field Work Radiosounding

Radiosounding Experiment

Climatological and Hydrological Field Work

Rietholzbach research catchment, 17th to 21st of June 2019

Lecturer: Ronny Meier

1 Introduction

Upper air measurements of temperature, relative humidity, pressure and wind speed andwind direction have been taken since the early 1900s. Early measurements were obtainedwith recording instruments flown on kites or aircrafts. With the development of radiocommunications in the 1930s, the first radiosonde was developed by Soviet meteorologistPavel Molchanov.

This lab will collect data from radiosondes attached to balloons. A meteorological bal-loon, carrying instruments and transmitting equipment is released, untethered, rises untilit bursts, and falls back using a parachute. The radiosonde is equipped with sensorsmeasuring temperature, relative humidity, and pressure. Additionally, the radiosondetransmits GPS data to a radio receiver which allows to track the path of the sounding.

Radiosondes are launched twice daily at 12 UTC and 00 UTC at about 900 stationsworldwide. In Switzerland the radiosondes are launched in Payerne. They provide valu-able information regarding the vertical structure of the atmosphere which is ingested intonumerical weather prediction models. The term ‘sonde’ or ‘radiosonde’ refers to the in-strument package itself, while ‘sounding’ is normally used for the entire sonde-balloonlaunch and the data collected by a launch.

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2 Theory

2.1 Hydrostatic Balance

The height of a point in the atmosphere can be calculated by starting with the hydrostaticbalance:

dp

dz= −ρg (1)

where p is pressure, z is height, ρ is the air density, and g is gravitational acceleration.Combining the hydrostatic balance with the ideal gas law pV = mRT and R = 287 J

kg·Kwe find:

dp

p= − g

RTdz (2)

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 withheight:

p(z) = p0 e− g

RTz (3)

where p0 is the surface pressure.

2.2 Skew-T Log-P Diagram

The temperature profile of the atmosphere gives information about the stability of theatmosphere. An unstable atmosphere can lead to large scale convection and the devel-opment of thunderstorms. Other features, such as inversions or large vertical shear, canalso be detected. An often-used tool to analyze the stability of the atmosphere and toestimate cloud conditions from sounding measurements is the skew-T log-P diagram. Insuch a diagram pressure is plotted logarithmically on the vertical axis (log-P) againstisothermal lines on the horizontal axis which are skewed by 45◦ (skew-T). There are fivekey variables in the diagram:

• Temperature in ◦C on the x-axis. The skewed isotherms (red), lines of constanttemperature are going to the upper right.

• The pressure in hPa is on the y-axis in logarithmic units. The isobars are thehorizontal lines in black.

• Dry adiabats (yellow), moist adiabats (green). For low temperatures on the topatmosphere both adiabats are parallel. (Why?)

• The mixing ratio in g/kg (blue), from the left to the upper right.

In a skew-T log-P diagram different levels of the atmosphere can be determined:

• Lifting Condensation Level (LCL)

• Level of Free Convection (LFC)

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• Level of Neutral Buoyancy/Equilibrum Level (LNB)

What do the different levels mean? Determine the different levels in the sounding ofPayerne from the 5th of August 2013 (see Figure 2).

Figure 1: Skew-T-log p Diagram.

2.3 Ascent Rate

When the balloon reaches its equilibrium velocity the buoyancy force from the balloonis balanced by the drag force and the gravitational force. We will fill the balloon withhelium until it has a lift of 800 g. It can also be calculated as follows:

L = V (ρAir − ρHe) (4)

, where L is the lift in kg, V the volume of the filled balloon in m3 (= 43πr3), ρAir =

1.205 kg m−3 and ρHe = 0.166 kg m−3 the densities of air and helium under standardconditions (T = 0 ◦C, p = 1013.25 hPa). From this lift we can subtract the payload Pincluding the mass of the radiosonde, tether, parachute, balloon (see ?? for the mass ofthe sonde) to get the residual lift R:

R = L− P (5)

When the balloon reaches its equilibrium velocity v the remaining buoyancy force (Rg)is balanced by the drag (0.5 ρair v

2CD A) resulting in the following equation for v:

v =

(Rg

0.5CdAρAir

)1/2

(6)

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Figure 2: Sounding in Payerne of 5th of August 2013 at 12 UTC(http://weather.uwyo.edu/upperair/sounding.html).

, where g = 9.81 m s−2 the gravitational constant, Cd = 0.25 the drag coefficient for asphere in the atmosphere and A = πr2 the cross sectional area of the balloon in m2.

a) What will be the equilibrium ascent rate of our balloon at surface conditions (youcan derive an estimate of the balloon volume from Equation 4)? Keep in mind thatthe balloon itself also has a weight of 600 g.

b) Previously, it was shown that atmospheric pressure decays exponentially in anisothermal and hydrostatically balanced atmosphere. What does this imply forthe volume of the balloon (assume that the pressure in the balloon equilibrates withthe surrounding pressure)?

c) Explain with Equation 6 why we can assume the ascent rate to be approximatelyconstant even though the volume of the balloon changes with height (Also see Dennyet al., 2016).

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3 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 at-mosphere, sampling data at frequent (approximately one second) intervals. Soundingstypically reach 30 km in height before the balloon bursts due to the cold temperatures inthe stratosphere. In addition to temperature, RH, and pressure, the sounding also mea-sures wind speed and direction by tracking the balloon’s location with either a theodoliteor embedded GPS receiver. Soundings that include wind data are sometimes called raw-insondes.

This year’s lab uses both iMet-1 radiosondes, which we have left from the last year, andthe iMet-4 radiosonde. They both have the same temperature sensor, which is a glassbead thermistor (resistance changes with temperature). The humidity is measured by acapacitive thin-film moisture sensor (porous polymer material that acts as a hydro-activesponge between two electrodes: the capacitance changes with relative humidity). Thissensor was updated in the iMet-4 sonde and should now have a faster response time (cps.Figs. 7 and 8). The pressure sensor is a piezoresistor (semiconductor membrane whose re-sistance varies with mechanical stress) and exhibits also slightly different features betweenthe two sondes. Besides, the differing sensors, the iMet-4 radiosonde is lighter than theiMet-1 radiosonde and designed differently, which can further influence the measurements.

Data is transmitted via an FM radio signal at frequencies of 401-405 MHz and is detectedby the portable antenna which is connected to a commercial radio receiver. Finally, thesignal is converted to digital data and read into the laptop. For this and the recordingof the launch we are using the program SkySonde Server developed at NOAA. Thesoftware is downloaded from: http://www.esrl.noaa.gov/gmd/ozwv/wvap/sw.html.

4 Radiosonde Errors

Since the beginning, upper atmosphere data has been prone to many errors. Some of thesecontinue to plague the data, reducing the quality. Additionally, the frequent redesign andnumerous manufacturers of radiosondes means that historical records are often inhomo-geneous.

Temperature measurements can be affected by slow or inaccurate sensors, or throughspurious heating by solar radiation or nearness to other warm bodies. The latter areusually 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?

It is often assumed that sensors respond instantaneously to the surroundings. In reality,the sensors take varying amounts of time to equilibrate with the air surrounding them(check the specifications in the appendix). This is called the ”lag error”.

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• How would the amount of lift in the balloon affect the sounding measurements?

Humidity is also notoriously difficult to measure. In the past, sensors included ”hairhygrometers”, where a piece of hair is mounted under tension and the length of it changeswith 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 withchanges in humidity. Most sensors have a lower temperature limit below which they willreport erroneous values (Hint: check the specifications in the appendix).

5 Procedure

This lab has two parts. 1) Sampling of the surface layer with the DJI Mavic 2 Pro drone(see manual on drones). 2) A routine radiosonde launch which will capture a full atmo-spheric profile. An approximate time-line follows:

00:00 Short repetition of theory00:15 Begin first sampling of surface layer with drone00:45 Begin setting up for balloon sounding01:30 Launch of balloon sounding02:00 Second sampling of surface layer with drone02:30 Short final discussion on already gathered data

5.1 Preparation for Routine Sounding

Prepare the sonde, the balloon, and ground station as described below. Label the sondewith the address of ETH so that people potentially finding it can return it to ETH (onesonde costs more than 200 CHF). Additionally, you will have to prepare the balloon forthe routine sounding yourself as described below.

5.1.1 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.

• Build up the mobile antenna and place it on a stable place. Connect it to to theantenna 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 radiofrequency 403 MHz then press enter. This frequency must match the one that willbe broadcasting from the radiosonde.

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• 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 theancillary 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

5.1.3 Data Collection

Data receiving and collection is handled by the program SkySonde.

Figure 3: Screenshot of SkySonde Server.

• Turn on the radiosonde (using the same frequency as the receiver).

• Start the program SkySonde Server (only when radio receiver and laptop are con-nected).

• The iMet by itself will send either a PTU or PTUX packet, and a GPS or GPSXpacket. Make sure that the lights for PTU/GPS are blinking. (see Figure 3).

• While the server is running, open SkySonde Client (program contains detailedflight setup information, parses and calculates instrument data from packets, andstores several output file types)

• In the Acquisition tab select Data Source: Sky Sonde Server (real time), fill inRadiosonde 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).

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• Monitor the data on the SkySonde Client and tick the variables (Pressure, Temper-ature 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"

Figure 4: Screenshot of SkySonde Client

5.1.4 Balloon

Prepare the balloon following the the steps below. Important: Where gloves whentouching the balloon. Oils, especially from skin, will damage the rubber causing theballoon to burst prematurely.

• Carefully remove a balloon from its packaging.

• 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 inflatesstabilize 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 byattaching the 800 g weight to the nozzle and waiting until it just lifts the weightoff,the ground. This will give an ascent rate which you calculated with Equation 6.

• 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 cordas 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 returningdetails on the sonde.

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Figure 5: Screenshot of SkySonde Client Monitoring

Figure 6: Sounding set-up.

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5.1.5 Launch

Position yourselves well away from any trees or buildings, taking the wind conditions intoaccount. Have one person hold the balloon and the second hold the sonde. It is bestto simply lay the sonde in the open palm of your hand. Once the balloon is releasedthe sonde will lift out of your hand. Clutching the sonde has been known to break theline 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.

6 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. Denny, M.: Weather Balloon Ascent Rate, The Physics Teacher, 54, 268271,doi:10.1119/1.4947151, 2016.

5. Wernli, H. and Peter T. Skript of Leture ”Atmosphaere”, Fall term 2015.

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7 iMet Specifications

Specifications subject to change without notice 1 Compatible with DMT ECC Type Ozone Sensor * Telemetry Receiver System (iMet-2000)

3854 Broadmoor SE, Grand Rapids, MI phone: 616-285-7810, fax: 616-957-1280 e-mail: [email protected]

iMet-1 Radiosonde GPS/RDF Technology

1680/403 MHz Flexibility

Features

System Overview

Meteorological Sensors

Available in Four Models: Operating Principle GPS or RDF Pressure (optional)

– 1680 MHz RDF Nominal Frequencies 1680 / 403 MHz Type Piezoresistive

– 1680 MHz GPS Range > 250 km with TRS * Range 2 to 1070 hPa

– 403 MHz GPS Altitude > 42 km with TRS * Accuracy 0.5 hPa < 400 hPa

− 403 MHz Research 1 Battery Alkaline Dry Cell 0.5 hPa > 400 hPa

Operating Time > 2 Hours Resolution < 0.01 hPa

Advanced Sensor Technology: Weight 260 Grams Response Time < 1.0 Sec

– Thin Polymer Humidity Sampling Rate 1 / Second

– Bead Thermistor Temperature Case Expanded Polystyrene

– Optional Solid State Pressure

– 12 Channel C/A Code GPS Transmitter Temperature

Tuning Range 1668.4 – 1700 MHz

Type Bead Thermistor

Simple to Use: 400.15 – 406 MHz Range - 95 to + 50 Deg

– Dry cell batteries Output Power 300 mW Accuracy 0.2 Deg C

– Switchable power on / off Transmission 2400 baud, FM Resolution < 0.01 Deg

– No pre-flight temp & humidity Bandwidth 120 kHz (1680 MHz) Response Time 2.0 Sec

recalibration required 20 kHz (403 MHz) @ 1000 hPa

– Switch controlled frequency Stability Crystal Controlled

– Compact and light weight

GPS Receiver Humidity

Type C/A code, 12 Channel

Type Capacitive

Tracking Continuous Range 0 to 100% RH

Update Rate 1 Hz Accuracy 5% RH

Acquisition Time 50 sec (cold start) Resolution < 0.1% RH

Position Accuracy 10 m Response Time 2 Sec @ 25 Deg C

Wind Velocity Accuracy 1.0 m/s 60 Sec @ - 35 Deg

Altitude Accuracy 15m

Figure 7: http://intermetsystems.com/ee/pdf/iMet-1-ABxn_Data_150316.pdf

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* Subject to ground station, balloon size and atmospheric conditions ¹ All uncertainties expressed at a 95% confidence level ² Primary atmospheric pressure derived by GPS altitude 3 GECOS Reference Upper-Air Network

Specifications subject to change without notice, Rev 10 171208

Grand Rapids, MI USA

www.intermetsystems.com

iMet-4 Radiosonde 403 MHz GPS Synoptic Technical Data Sheet

Temperature and Humidity

The iMet-4 measures air temperature with a

small glass bead thermistor. Its small size

minimizes effects caused by long and short-

wave radiation and ensures fast response times.

The humidity sensor is a thin-film capacitive

polymer that responds directly to relative

humidity. The sensor incorporates a temperature

sensor to minimize errors caused by solar

heating.

Pressure and Height

As recommended by GRUAN3, the iMet-4 is

equipped with a pressure sensor to calculate

height at lower levels in the atmosphere. Once

the radiosonde reaches the optimal height,

pressure is derived using GPS altitude combined

with temperature and humidity data.

The pressure sensor facilitates the use of the

sonde in field campaigns where a calibrated

barometer is not available to establish an

accurate ground observation for GPS-derived

pressure. For synoptic use, the sensor is bias

adjusted at ground level.

Winds

Data from the radiosonde's GPS receiver is used

to calculate wind speed and direction.

Radiosonde Data Transmission

The iMet-4 radiosonde can transmit to an

effective range of over 250 km*.

A 6 kHz peak-to-peak FM transmission

maximizes efficiency and makes more channels

available for operational use. Seven frequency

selections are pre-programmed - with custom

programming available.

Calibration

The iMet-4’s temperature and humidity sensors

are calibrated using NIST traceable references to

yield the highest data quality.

Benefits

• Superior PTU performance

• Lightweight, compact design

• No assembly or recalibration required

• GRUAN3 qualified (pending)

• Status LED indicates transmit frequency

selection and 3-D GPS solution

• Simple one-button user interface

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iMet-4-AB Radiosonde Document 202084, Rev 10

* Subject to ground station, balloon size and atmospheric conditions ¹ All uncertainties expressed at a 95% confidence level ² Primary atmospheric pressure derived by GPS altitude 3 GECOS Reference Upper-Air Network

Specifications subject to change without notice, Rev 10 171208

Grand Rapids, MI USA

www.intermetsystems.com

MEASUREMENTS GEOPOTENTIAL HEIGHT Pressure derived Measurement cycle 1 Hz Measurement range SFC to 40 km Resolution 0.1 m TEMPERATURE SENSORS Glass Bead Combined Uncertainty/Reproducibility ¹ Manufacturer Shibaura 1080 - 400 hPa 15 m / 10 m Measurement range +60°C to -90°C 400 - 10 hPa 200 m / 150 m Resolution 0.01°C Response time: still air/ 5 ms-1 (1000 hPa) 2 / < 1 sec Repeatability in Calibration 0.2 C GEOPOTENTIAL HEIGHT GPS derived Combined Uncertainty/Reproducibility ¹ Measurement range SFC to 40 km > 100 hPa 0.5 C / 0.3 C Resolution 0.1 m < 100 hPa 1.0 C / 0.75 C Combined Uncertainty/Reproducibility ¹ Night flight 0.3 C / 0.3 C 1080 - 400 hPa 30 m / 15 m Solar correction ≤ 1.2 C 400 - 3 hPa 60 m / 20 m HUMIDITY SENSOR Capacitive Polymer WIND SPEED AND DIRECTION Manufacturer IST Resolution 0.1 m/s / 1 degree Measurement range 0-100 % RH Speed Resolution 0.1% Combined Uncertainty/Reproducibility ¹ 0.5 / 0.25 m/s Response time Direction @ 25C 0.6 seconds Combined Uncertainty/Reproducibility ¹ 1 degree @ 5C 5.2 seconds @ -10C 11 seconds @ -40C 61 seconds Repeatability in Calibration 5 % TELEMETRY Uncertainty/Reproducibility ¹ Transmission type Synthesized > 0 C 5% / 3% Maximum Range > 250 km -40 to 0 C 5% / 5% Frequency stability ± 2 kHz Deviation, peak to peak 6 kHz PRESSURE ² Sensor Output Power 30 – 500 mW Manufacturer Measurement Specialties Modulation GFSK Measurement range 1200 hPa - 10 hPa Data Rate 1200 Baud Resolution 0.01 hPa Standard Frequencies 402, 402.5, 403, 403.5 404, 404.5, 405

Response time 0.5 milliseconds Custom Frequencies Available

Uncertainty/Reproducibility ¹

Whole range 2.0 / 1.5 hPa GPS RECEIVER 1200 - 400 hPa 1.0 / 0.75 hPa Manufacturer / Type U-Blox CAM-M8 400 hPa - 10 hPa 2.0 / 1.5 hPa Cold Start Time < 60 seconds (typical)

PRESSURE GPS derived Measurement range SFC to 3 hPa OPERATIONAL DATA Resolution 0.1 hPa Battery Lithium Uncertainty/Reproducibility ¹ Operating time > 135 minutes 1080 - 400 hPa 2.0 / 1.5 hPa Weight 120 grams 400 hPa - 3 hPa 0.5 / 0.25 hPa Dimensions Body (LWH): 139x67x31 With boom (LWH): 235x67x31 Calibration Stability 2 years

Figure 8: intermetsystems.com/ee/pdf/202084-10_iMet-4_Technical_Data_Sheet.

pdf

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