1. Atmospheric Sounding Systems
Atmospheric Instrumentation
M. D. Eastin

2. Outline Sounding Systems Review of Atmospheric Soundings
“Upsondes” (Weather Balloons)
Balloon
Sensor Package
Navigation / Telemetry System
Wind Estimation
Exposure Errors
“Downsondes” (GPS Dropsondes)
Experimental Sounding Systems
Atmospheric Instrumentation
M. D. Eastin

3. Review of Atmospheric Soundings
Definition and Concepts:
Goal: Obtain a near-vertical profile of atmospheric pressure, temperature, humidity,
and wind structure for use in both nowcasting and forecasting of convection,
as well as the assimilation into global and regional numerical forecast models
Methods: Balloon-based radiosondes **
Aircraft-based radiosondes **
Commercial Aircraft
Remote Sensing **
** Instrument: Radiosonde
Atmospheric Instrumentation
M. D. Eastin

4. Balloon Inflation Area
Review of Atmospheric Soundings
Latex Balloon
(helium)
Parachute
Sensor
Package
Components of a Radiosonde Sounding System:
1. Balloon (with gas) / Parachute
2. Atmospheric Sensor Package
3. Navigation / Telemetry (GPS, transmitter, receiver)
4. Analysis Software
Receiving
Antenna
Balloon Inflation Area
Helium
Tanks
Atmospheric Instrumentation
M. D. Eastin

5. Review of Atmospheric Soundings
Common Data Collected by Radiosondes:
1. Parameters: Pressure
Temperature
Humidity
Altitude / Time (determined from navigation satellites)
Latitude / Longitude (determined from navigation satellites)
Wind Speed / Direction (calculated from sequential locations)
2. Mandatory Levels: Atmospheric altitude/pressure levels at which all radiosondes
are required to report current observations
Tropospheric Levels (12) → Surface, 1000-mb, 925-mb, 850-mb,
700-mb, 500-mb, 400-mb, 300-mb,
250-mb, 200-mb, 150-mb, 100-mb
3. Significant Levels: Additional pressure levels at which a parameter experiences
a vertical gradient change (Ex: lapse rate changes sign)
Temperature begins to increase / decrease with height
Humidity begins to increase / decrease with height
Wind speed begins to increase / decrease with height
Atmospheric Instrumentation
M. D. Eastin

6. Review of Atmospheric Soundings
Display / Analysis of Radiosonde Sounding Data:
1. Skew-T Log-P diagram
Temperature plotted vs. pressure
(usually in red)
Dewpoint plotted vs. pressure
(usually in blue or green)
Wind speed and direction
plotted vs. pressure
(using barbs or a hodograph)
Atmospheric Instrumentation
M. D. Eastin

7. Review of Atmospheric Soundings
Display / Analysis of Radiosonde Sounding Data:
2. Lifted Parcels and Stability Indices
a. Surface Layer
b. Mixed Layer
Lifted Condensation Level (LCL)
Level of Free Convection (LFC)
Equilibrium Level (EL)
Convective Inhibition (CIN)
Convective Available Potential Energy (CAPE)
Lifted Index (LI)
Showalter Index (SI)
K-Index (K)
SWEAT Index T Td
LCL
LFC
Tp < Te
Tp > Te EL
CIN
CAPE
Thermodynamic
“path” of a
lifted parcel
from the surface
Atmospheric Instrumentation
M. D. Eastin

8. Review of Atmospheric Soundings
Display / Analysis of Radiosonde Sounding Data:
3. Calculation of Thermodynamic Quantities
Mixing ratio (w)
Saturation mixing ratio (ws)
Relative humidity (r)
Specific humidity (q)
Potential temperature (θ)
Wet-bulb temperature (Tw)
Given:
P = 800 mb
T = +9.5ºC
Td = –8.0ºC
Temperature at the LCL (TLCL)
Pressure at the LCL (PLCL)
Wet-bulb potential temperature (θw)
Pseudo-adiabatic equivalent
potential temperature (θe)
Td, w
P = 1000 mb
T, wsw
P = 800 mb θw
TLCL
θep θ Tw
PLCL
Atmospheric Instrumentation
M. D. Eastin

9. Review of Atmospheric Soundings
Uses of Radiosonde Sounding Data: Severe Weather Forecasting
Atmospheric Instrumentation
M. D. Eastin

10. Analysis of Radiosonde Observations
Review of Atmospheric Soundings
Uses of Radiosonde Sounding Data: Assimilation into Numerical Models
Analysis of Radiosonde Observations
Numerical Model Field
Atmospheric Instrumentation
M. D. Eastin

11. Upsondes – Weather Balloons
The Balloon:
Expandable latex weighing 200 – 1000 g
Inflated with a “lift gas” less dense than air:
1. Helium (inert but becoming rare)
2. Hydrogen (explosive but common)
Balloon (with a radiosonde package attached) ascends and
expands (volume increases) until it reaches burst altitude
(in the stratosphere) and then the radiosonde package
descends by parachute → very few are recovered
Balloon Diameter during Ascent:
Balloon volume, diameter, and lift gas density are inversely proportional air density
where: VBZ = balloon volume at height z (m3)
VBO = balloon volume at the surface (m3)
ρZ = air density at height z (kg m-3)
ρO = air density at the surface (kg m-3)
ρBZ = lift gas density at height z (kg m-3)
ρBO = lift gas density at surface (kg m-3)
DBZ = balloon diameter at height z (m)
DBO = balloon diameter at the surface (m)
Latex Balloon
(helium)
Parachute
Instrument
Package
Atmospheric Instrumentation
M. D. Eastin

12. Upsondes – Weather Balloons
Balloon Ascent Rate:
Balance between upward-directed free lift force (FL) and downward-directed drag force (FD)
where: DBZ = balloon diameter at height z (m)
ρZ = air density at height z (kg m-3)
ρBZ = density of lift gas at height z (kg m-3)
mB = balloon mass (kg)
mP = sensor package mass (kg)
g = gravity (m s-2)
CD = drag coefficient (none)
wB = balloon vertical velocity (m s-1)
If we solve for the balloon’s vertical velocity
If the vertical profile of air density is known (or can be reasonably estimated), then the
the balloon’s expected diameter and vertical velocity can be calculated as a function
of altitude (in a future homework)
Atmospheric Instrumentation
M. D. Eastin

13. Upsondes – Weather Balloons
Balloon Ascent Rate:
Tropospheric ascent rates are typically 5-6 m/s
The balloon’s vertical velocity deviates slightly
from this expectation for several reasons:
1. The balloon material is not perfectly elastic
2. A small portion of the lift gas will leak out
3. The internal lift gas temperature will become
warmer than then ambient air
4. Balloon shape is not perfectly spherical
5. Vertical air motions will impact ascent rate
6. Drag coefficient is not constant
Standard U.S. Weather Balloon Specifications:
Manufacturer / Model: Vaisala RS-92
Lift Gas: Helium
Balloon Mass: g
Sensor Package Mass: 280 g
Operating Time: minutes
Atmospheric Instrumentation
M. D. Eastin

14. Upsondes – Weather Balloons
Sensor Package: Vaisala RS-92
Barometer: Aneroid silicon capsule
Accuracy: ±0.4 mb
Resolution: mb
Dynamic Range: 3 – 1080 mb
Response Time: < 1 s
Thermometer: Platinum resistance thermistor
Accuracy: ±0.3°C
Resolution: °C
Dynamic Range: –90°C to +60°C
Hygrometers: Twin-heated film-capacitance hygristors
Accuracy: ±5.0%
Resolution: %
Dynamic Range: 0 – 100%
Response Time: < 20 s (due to heating cycle)
Temperature
Sensor
Humidity
Sensors
Pressure
(inside)
Atmospheric Instrumentation
M. D. Eastin

15. Upsondes – Weather Balloons
Navigation / Telemetry System: Vaisala RS-92
Sonde location is determined through simultaneous
triangulation with at least four GPS satellites
(more satellites increase position accuracy)
The sonde transmits the detected GPS signals
down to the ground station (which also receives
GPS signals directly from the satellites)
The ground station then computes the sonde
position relative to the ground station position
GPS Antenna: Code-correlating receiver
Channels: (maximum)
Position Accuracy: ±10 m (horizontal)
±20 m (vertical)
Wind Accuracy: 0.2 m/s
Telemetry Antenna: Synthesized digital multiplexer
Frequency: MHz
Data Downlink: bits/s
Data Cycle: 1 s
GPS
Antenna
Telemetry
Atmospheric Instrumentation
M. D. Eastin

16. Upsondes – Weather Balloons
Wind Profile Estimation:
Since balloons are characterized by large surface area and small mass, they usually follow
the mean wind flow, and thus horizontal winds can be determined by tracking the balloon’s
position during its ascent relative to the ground station’s position
where: x = east-west distance (m)
y = north-south distance (m)
z = altitude (m)
ϕ = elevation angle (degrees)
θ = azimuth angle (degrees)
Δt = time interval (s)
u = zonal wind component (m s-1)
v = meridional wind component (m s-1)
The vertical winds are rarely computed
since vertical position errors are larger
and precise ascent rate is unknown z R
Balloon
Ground
Station ϕ
Elevation y x θ
Azimuth
x = y = z = 0
t = 0
t = 1
t = 2
t = 3
t = 4
Atmospheric Instrumentation
M. D. Eastin

17. Upsondes – Weather Balloons
Exposure Errors: Radiation
Affects thermistors
Direct exposure to solar radiation can
produce a warm temperature error
depending on the following:
1. Sensor Diameter (D)
2. Sensor Reflectivity
3. Ascent Rate (wB)
The RS-92 platinum resistance thermistor
is highly reflective with a 0.2 mm diameter
Exposure Errors: Wet-bulbing
Affects thermistors and hygristors
Sensors remain wet for some time after emerging from a cloud.
Cool temperature error due to evaporation
Moist humidity error until heating cycle removes the excess moisture
The RS-92 sensors are coated with a thin film of water-shedding material (like Rain-ex)
that limits any sensor wetting (both in cloud and after emerging from cloud)
Atmospheric Instrumentation
M. D. Eastin

18. Downsondes – GPS Dropsondes
Concept:
Special radiosondes launched from
high-altitude aircraft and descend at
a controlled rate using a parachute
Only deployed over oceans
Only used during “targeted” observing
efforts designed to improve high-impact
forecasts for severe weather events or
natural disasters
Parachute
Instrument
Package
Dropsonde
Launch
Tube
in a
NOAA
Research
Aircraft
Atmospheric Instrumentation
M. D. Eastin

19. Downsondes – GPS Dropsondes
The Parachute:
Square-cone chute deployed upon exit from launch tube
Provides controlled and stable sonde descent without the
“pendulum motion” common with circular parachutes
Descent rate varies from 11–20 m/s in the troposphere
Standard Dropsonde Specifications:
Manufacturer / Model: Vaisala RD-94
Sensor Package Mass: 350 g
Transmitter frequency: MHz
Max Telemetry Range: km
Max Deployable Airspeed: 250 knots
Transmitter Antenna
GPS Antenna
PTH Sensors
Temperature
Sensor
Humidity
Sensors
Pressure
Atmospheric Instrumentation
M. D. Eastin

20. Downsondes – GPS Dropsondes
Standard Dropsonde Specifications:
Sensor Type Accuracy Resolution Range Response
Barometer Silicon Aneroid ±0.4 mb mb to 1080 mb < 1 s
Thermistor Platinum Resistance ±0.2 °C °C °C to +60°C < 2 s
Hygristors Heated Capacitance ±2.0 % % 0% to 100% < 20 s
Winds GPS Positions ±0.5 m/s m/s to 150 m/s N / A
Data Cycle: 2 Hz (PTH) Position Accuracy: ±5 m (horizontal)
4 Hz (winds) ±10 m (vertical)
Descent Time: 15 min (from 14 km) Wind Estimation → as with radiosondes, but
8 min (from 7 km) starting at drop location
2. Exposure Errors → same as for radiosondes
Transmitter Antenna
GPS Antenna
PTH Sensors
Temperature
Sensor
Humidity
Sensors
Pressure
Atmospheric Instrumentation
M. D. Eastin

21. Experimental Sounding Systems
Rocket Sondes:
Used to launch radiosondes into the upper level atmosphere
(up to 120 km) where balloons cannot ascend
Once the rocket reaches its maximum altitude, the sensor
package descends using a square-cone parachute
Used to study: Ozone Volcanic Emissions
Radioactivity Earth’s Magnetic Field
Cosmic Rays Mesospheric Chemistry
Atmospheric Instrumentation
M. D. Eastin

22. Experimental Sounding Systems
Glider Sondes:
Weather balloon transports instrumented glider
to burst altitude and then the plane glides back
to a designated point using an onboard GPS
and flight navigation computer
Reusable
Low-cost alternative to the standard weather
balloon packages often lost after each launch
(total cost of $115,000 per year)
“DataBird” manufactured by GPS-Boomerang:
Still in development (field tests with prototypes)
Expected to be useable for 50 flights
Expected cost → $1000 per glider
Expected savings → $100,000 per year
Ascends to km altitude in ~1 hour
Descends in ~30 minutes at 3-4 m/s
Measures pressure, temperature, humidity, winds
using proprietary sensors
Major remaining “hurdle” is air-space restrictions
Atmospheric Instrumentation
M. D. Eastin

23. Experimental Sounding Systems
Student Sonde:
Still in development (field tests in progress)
Inexpensive (volunteers – no health insurance)
Major remaining “hurdle” is getting the student
to survive at upper altitudes in the cold and
low-oxygen environment
Atmospheric Instrumentation
M. D. Eastin

24. Summary Sounding Systems Review of Atmospheric Soundings
“Upsondes” (Weather Balloons)
Balloon
Sensor Package
Navigation / Telemetry System
Wind Estimation
Exposure Errors
“Downsondes” (GPS Dropsondes)
Experimental Sounding Systems
Atmospheric Instrumentation
M. D. Eastin

25. References Atmospheric Instrumentation M. D. Eastin
Boire, G., D. C. Sutter, S. P. Pryor, and A.K. Brown, 1993: A low cost GS rawinsonde system. Preprints 8th Symposium on Meteorological Observations and Instrumentation, Anaheim, CA, American Meteorological Society,
Brock, F. V., and S. J. Richardson, 2001: Meteorological Measurement Systems, Oxford University Press, 290 pp.
Finger, F. G., and F. J. Schmidlin, 1991: Upper-air measurements and instrumentation workshop. Bulletin of the American Meteorological Society, 72,
Harrison, R. G., 2015: Meteorological Instrumentation and Measurements, Wiley-Blackwell Publishing, 257 pp.
Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS dropwindsonde. Bulletin of the American Meteorological
Society, 80,
Leurs, J.K., 1991: Estimating the temperature error of the radiosonde thermistor under different environmental conditions.
Journal of Atmospheric and Oceanic Technology, 7,
Olsen, R. O., R. J. Okrasinski, and F. J. Schmidlin, 1991: Inter-comparison of upper air data derived from various
radiosonde systems. Preprints 7th symposium on Meteorological Observations and Instrumentation,
New Orleans, LA, American Meteorological Society,
Pratt, R. W., 1985: Review of radiosonde humidity and temperature errors. Journal of Atmospheric and Oceanic Technology, 2,
Reynolds, R. D., 1966: The effect of atmospheric lapse rates on balloon ascent rates. Journal of Applied
Meteorology, 5,
Atmospheric Instrumentation
M. D. Eastin