David J. Brinker and Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

Jack Power

Meteorological Service of Canada, Downsview, Ontario, Canada

Comparison of Balloonsonde and Remote

Sensing Atmospheric Measurements

NASA/TM—2006-214118

February 2006
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David J. Brinker and Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

Jack Power

Meteorological Service of Canada, Downsview, Ontario, Canada

Comparison of Balloonsonde and Remote

Sensing Atmospheric Measurements

NASA/TM—2006-214118

February 2006

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

11th Conference on Aviation, Range, and Aerospace Meteorology

sponsored by the American Meteorological Society

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1. INTRODUCTION

The most effective deterrent to the hazardous

build up of airframe ice is the avoidance of those

atmospheric regions where meteorological

conditions make its accretion possible. This

avoidance is only possible through forewarning the

pilot, which today is accomplished through the use

of pilot encounter reports or weather analysis. A

better method is the measurement of actual

atmospheric conditions, which requires either the

widespread deployment of in-situ measurement

devices or the development of remote sensing

instrumentation. As part of its aircraft icing

research program, the NASA Glenn Research

Center is conducting a program to develop

technologies for the remote sensing of

atmospheric conditions that are conducive to icing

(Reehorst, 2003). The goal of the program is to

develop an integrated package of instruments,

suitable for incorporation into the aircraft, which

would provide the flight crew with sufficient

warning to avoid a region of potential danger

(Reehorst, 2001). A suite of instruments, currently

ground based, is used to identify a region of

supercooled liquid water which is labeled as

hazardous if its water content is sufficiently high.

During the recently completed Alliance Icing

Research Study (AIRS II), these instruments were

deployed in conjunction with those of other U.S.

and Canadian researchers at the Mirabel Airport

near Montreal, Quebec. As part of the study,

balloonsondes were employed to provide in-situ

measurement of the atmospheric conditions that

were being concurrently remotely sensed.

Balloonsonde launches occurred daily at 1200

UTC to provide AIRS forecasters with local data,

and, additionally when research aircraft were

present in the airspace. Data from the soundings

are compared with the processed data from the

NASA remote sensing instruments, which include

an X-band radar, a ceilometer and radiometers.

2. EXPERIMENTAL INSTRUMENTATION

2.1 Balloonsonde System

The balloonsonde system used in the AIRS II

study is a commercially available product from the

VIZ Meteorological Systems Group of Sippican,

Inc. The Model W-9000 Meteorological Processing

System is an integrated hardware and software

package that provides meteorological profiles

through the use of telemetered radiosonde data

and Global Positioning Satellite (GPS) location

data.

The balloon-borne radiosonde sensors,

transmitters and GPS receiver are contained in a

molded foam case, approximately 500 grams in

weight, including a water-activated battery that is

typically good for two hours of flight. Temperature

and humidity are directly measured, while

pressure is calculated from GPS data and the

Standard Atmosphere. Temperature is obtained

using a standard rod thermistor fabricated from

mined iron oxide, positioned at the end of an arm

to remove it from the close proximity of the sonde

case. Heating from solar radiation, as well as the

wet bulb effect from water retention, is minimized

by the application of reflective, then surfactant,

coatings on the sensor. The range of

measurement is +50  ̊C to –100  ̊C with an

accuracy of 0.2  ̊C A deposited carbon film on an

acrylic substrate is used for humidity

measurement. This configuration is been well

known for superior performance at higher humidity

levels (>20%). Recent development has improved

the performance of the sensor at lower humidity

levels; after calibration at both 33% RH and 11%

COMPARISON OF BALLOONSONDE AND

REMOTE SENSING ATMOSPHERIC MEASUREMENTS

David J. Brinker and Andrew L. Reehorst

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Jack Power

Meteorological Service of Canada

Downsview, Ontario, Canada M3H 5T4

NASA/TM—2006-214118 1
RH a linear interpolation is used for this range.

The sensor is installed in a tube integral to the

foam sonde case. This location prevents direct

solar radiation from warming the element and rain

from impinging on the carbon film. Response time

is typically less than one second.

Direct measurement of the ambient pressure

is not employed in the sondes used in this study.

The sondes are instead equipped with a GPS

receiver, which acquires the signal from 5-7

satellites prior to launch. The base station also

uses GPS to determine its location. The resulting

comparison of positional data is used to calculate

sonde altitude, latitude and longitude, its velocity

with respect to the launch site and rate of ascent.

An additional spare data channel was available on

the sonde, which was not utilized.

The meteorological data, calibration values

and GPS parameters are sampled and transmitted

to the ground station once per second, with the

data set retransmitted a second time for reliability.

The transmitter operates at 403 MHz, tunable over

a range of 400 to 406 MHz. The sonde transmitter

has a nominal radiated power of 250 milliwatts,

sufficient to allow for data collection over a range

of 200 km when using the directional, six element

Yagi antennas. The ground station includes the

403 MHz receiver and antenna controller, a GPS

receiver and an serial interface for initializing the

radiosonde. System control and data display and

storage is accomplished through the use of laptop

computer. Data can be viewed during the flight in

either a graphical or tabular mode. WMO coding

is available for processing the data into coded

messages for transmission, with all flight data

Figure 1, Balloonsonde system ready for launch,

ground station and data acquisition system is on

table at left.

stored on the hard drive of the computer. The

ground station and a balloon sonde ready for

launch is seen in Figure 1. Figure 2 shows a

launch of the balloon sonde. The multi-element

direction antenna is visible on the roof of the

building.

2.2 Remote Sensing System

Four instruments currently comprise the

NASA icing remote sensing system: two

radiometers, one that measures multiple

frequencies between 22 and 59 GHz and another

that measures at 89 and 150 GHz; a vertically

staring X-band radar and a lidar ceilometer.

Instrument control and data acquisition for all

instruments is through individual personal

computers.

Two Radiometrics Corporation microwave

radiometers were deployed at Mirabel Airport.

The TP/WVP-3000 profiling radiometer provides

temperature and water vapor profiles from the

surface to 10 km, and low-resolution liquid water

profiles. Observations using seven frequencies

Figure 2, Launch of balloonsonde at Mirabel

Airport, multi-element direction antenna is visible

on the building roof.

NASA/TM—2006-214118 2
from 52 to 59 GHz are used to determine

temperature measurement as a function of

altitude. Cloud water vapor content is determined

through the observation of five frequencies in the

22 to 30 GHz range. Information on the liquid

water content of the cloud is obtained by

measurement of microwave energy as a function

of frequency near 22 GHz and by scanning on

either side of 60 GHz. The second radiometer,

operates at 89 and 150 GHz with elevation

scanning capability and dual polarization at 89

GHz. Its purpose is to discriminate ice from liquid

water in meteorological conditions that may be

associated with aircraft icing. Both radiometers

are free standing, as is the ceilometer.

The X-band radar is a Honeywell WU-870

airborne weather radar unit which utilizes a 24-in

diameter, flat-plate antenna operating at 9375

MHz. The radar is configured in a vertical staring

mode. It has been modified to allow the use of a

Figure 3, Remote sensing system deployed at

Mirabel Airport. The radar dome is seen on the

roof of the trailer in the foreground, with the

ceilometer on the ground directly behind it. The

two radiometers are at the center rear of the

photograph.

PC running under the Windows XP operating

system for data acquisition instead of the standard

Honeywell flight control/display unit. The radar

transmitter/receiver antenna is mounted in the roof

of the trailer, protected from the weather by a

radome.

Cloud base and height as well as vertical

visibility are measured using a lidar ceilometer.

The instrument, a Vaisala Model CT25K, employs

a pulsed diode laser operating in the infrared

wavelengths (905 nm) and is able to detect up to

three cloud layers simultaneously. The data

acquisition systems for all of the remote sensing

instruments are located in a trailer which is also

used for equipment transportation. The trailer and

instruments, Figure 3, were situated about 1 km

from the balloonsonde launch site.

3. RESULTS AND DISCUSSION

Although the AIRS II field test program ran

from November, 2003 through February, 2004, the

NASA remote sensing system was operational onsite only during the 2003 portion of the study.

During this period, 70 of the 89 balloonsonde

launches took place, with nine of these coincident

with the optimum datasets from the remote

sensing system. Optimum conditions in this case

encompass rain free conditions and dry

radiometer windows. In this paper the results of

two representative flights are examined.

Temperature and relative humidity as a function of

altitude through 10 km from the TP/WVP 3000

radiometer and balloonsonde are compared.

Comparisons of this data for a November 11,

2003, 15:35 UTC launch are shown in Figures 4.1

(temperature) and 4.2 (relative humidity). Sonde

initial ascension rates were about 5 m/s, so the

data covering the first 10 km of altitude were

collected over a 30-35 min period of time. The

radiometer data were taken from a single scan

during the middle of the sonde ascent through 10

km. The temperature profile of the radiometer

agreed well with the balloonsonde data. The

average difference in values over the data set is

1.7  ̊C with the greatest difference 4.45  ̊C at about

2 km altitude. Above 6 km, the radiometer read

consistently colder by about 2.5  ̊C, which agrees

well with earlier results (Reehorst, 2001A). A

probable cause is the training of the artificial

neural network used in retrieving water vapor,

cloud liquid water and temperature profiles from

the radiometric data. Although it was trained using

NASA/TM—2006-214118 3
Nov.11, 2003; 15:35Z Launch at YMX

0

2

4

6

8

10

-60 -50 -40 -30 -20 -10 0 10

Temperature ( ̊C)

Altitude (km)

Radiometer Balloonsonde

Figure 4.1, Temperature comparison for November 11, 2003, 15:35 UTC launch.

Nov.11, 2003; 15:35Z Launch at YMX

0

2

4

6

8

10

0 20 40 60 80 100

Relative Humidity (%)

Altitude (km)

Radiometer Balloonsonde

Figure 4.2, Relative humidity comparison for November 11, 2003, 15:35 UTC launch.

NASA/TM—2006-214118 4
Dec. 10, 2003 17:03Z Launch

0

2

4

6

8

10

-60 -50 -40 -30 -20 -10 0 10

Temperature ( ̊C)

Altitude (km)

Radiometer Balloonsonde

Figure 5.1, Temperature comparison for December 10, 2003, 17:03 UTC launch.

Dec. 10, 2003; 17:03Z Launch

0

2

4

6

8

10

0 20 40 60 80 100

Relative Humidity (%)

Altitude (km)

Radiometer Balloonsonde

Figure 5.2, Relative humidity comparison for December 10, 2003 17:03 UTC launch.

NASA/TM—2006-214118 5
data from the Montreal area, accuracy is directly

dependent upon the close match of the training

data with those conditions encountered during

the field study. Other possible sources of error

are the gradual decay of radiometer accuracy

with increasing altitude and the continuous drift

of the sonde during flight; horizontal wind

speed reached 40 m/s above 6 km. Similar

results are seen in Figure 5.1, the temperature

profiles from a December 10, 2003, 17:03 UTC

launch. In this case the average temperature

difference was 1.6  ̊C with a maximum

difference of 3.85  ̊C at 0.5 km. Above 5 km, the

difference was about 2  ̊C, similar to the

previous case. Winds were lighter in this case,

reaching 14 m/s near 10 km.

With regard to relative humidity, larger

discrepancies are seen between the radiometer

and balloonsonde data than were seen in the

temperature comparison. Figures 4.2 and 5.2

show relative humidity as a function of altitude

for the two launches. For the November 11

data, there was a dry zone from just above

ground level to 1.75 km. The radiometer is not

particularly good in tracking this rapid decrease

in humidity nor does it accurately track the

lowest humidity portions of the band.

Ceilometer data for this time period shows a

cloud deck with a lower boundary at 2.1 km,

which agrees well with both the radiometer and

sonde data. The radiometer generally tracks

well with the sonde up to 8 km. Above 8 km, the

radiometer data deviates again from the sonde,

as in the temperature cases, possible causes

are the training of the radiometer neural-net, drift

of the sonde from its launch position and

increasing inaccuracy of the radiometer with

altitude.

For the December 10 case, ceilometry

shows a layer of clouds with a base at about 1.2

km. Both the radiometer and sonde track this

well, although the radiometer deviates

significantly at the upper and lower boundaries

of the cloud, which are areas of rapid change of

humidity. Above this layer, the air becomes

quite dry, with dew points in the –60  ̊C range.

The radiometer consistently mis-measures the

humidity under these increasingly dry conditions.

4. CONCLUSION

Comparison of remotely sensed

(radiometer) temperature and relative humidity

with in situ measurements (balloonsonde) during

the AIRS II field program reveals strengths and

shortcomings. The ability of the radiometer to

accurately track temperature was good,

particularly at lower altitudes. The discrepancy

from sonde data above 5 km was generally

constant and could be due to mismatch between

the dataset used to train the radiometer neuralnet and actual conditions, drift of the sonde with

time or decreasing inaccuracy of the radiometer

with altitude. Agreement with regard to relative

humidity was not as good. The radiometer is not

able to track rapid changes in humidity and

appears to return consistently high humidity

values in area of very dry air. In all cases, both

systems accurately measured the boundaries of

cloud layers as delineated by the ceilometer.

Despite these deficiencies, the results from the

AIRS II field test demonstrated the soundness of

the basic concepts of the remote sensing

system.

Future work includes the automation of

methods of comparison of the large volumes of

data generated by these systems, which at

present are not easily compared. The

radiosonde system will be installed in the aircraft

hangar at NASA Glenn in Cleveland for the

upcoming icing flight season, in close proximity

to the remote sensing system. This will allow for

simultaneous remote and in situ (both the

balloonsonde system and the icing research

aircraft) sensing of the conditions of interest.

The development of a liquid water content

sensor for integration into the radiosonde is also

of interest and is under study.

5. REFERENCES

Reehorst, A.L. and Koenig, G.G., 2001: Groundbased Icing Condition Remote Sensing System

Definition. NASA/TM-2001-211102, 43 pp.

Reehorst, A.L., 2001A: Comparison of Profiling

Microwave Radiometer, Aircraft, and

Radiosonde Measurements From the Alliance

Icing Research Study (AIRS), 15 pp.

Reehorst, A.L. and Politovich, M.K., 2003:

Development of Icing Condition Remote Sensing

Systems and their Implication for Future Flight

Operations. SAE 2003-01-2096, 6pp.

NASA/TM—2006-214118 6
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E–15453

WBS–22–728–41–05

12

Comparison of Balloonsonde and Remote Sensing Atmospheric Measurements

David J. Brinker, Andrew L. Reehorst, and Jack Power

Aircraft icing; Aviation meteorology; Remote sensing; Radiosondes; Atmospheric

sounding; Radiometers

Unclassified -Unlimited

Subject Category: 03

Prepared for the 11th Conference on Aviation, Range, and Aerospace Meteorology sponsored by the American

Meteorological Society, Hyannis, Massachusetts, October 4–8, 2004. David J. Brinker and Andrew L. Reehorst,

NASA Glenn Research Center; and Jack Power, Meteorological Service of Canada, 4905 Dufferin Street, Downsview,

Ontario, Canada M3H 5T4. Responsible person, David J. Brinker, organization code RTI, 216–433–2236.

As part of its aircraft icing research program, the NASA Glenn Research Center is conducting a program to

develop technologies for the remote sensing of atmospheric conditions. A suite of instruments, currently ground-based,

are used to identify a region of supercooled liquid water which is labeled as hazardous if its liquid water content is

sufficiently high. During the recently completed Alliance Icing Research Study (AIRS II), these instruments were

deployed in conjunction with those of other U.S. and Canadian researchers at the Mirabel Airport near Montreal. As

part of the study, balloonsondes were employed to provide in-situ measurement of the atmospheric conditions that were

being concurrently remotely sensed. Balloonsonde launches occurred daily at 1200 GMT to provide AIRS forecasters

with local data and additionally when research aircraft were present in the airspace. In this paper, we compare the

processed data from the NASA remote sensing instruments, which included an X-band radar, lidar and two radiometers, to the data gathered from the 70 soundings conducted while the NASA instruments were active. Among the

parameters compared are cloud upper and lower boundaries, temperature and humidity profiles and freezing levels.