ATMOSPHERIC OBSERVATIONS

1
ATMOSPHERIC OBSERVATIONS S.K. Satheesh Centre
for Atmospheric Oceanic Sciences Indian
Institute of Science Bangalore.
2
METEOROLOGICAL MEASUREMENTS (Temperature, RH,
Pressure, Rainfall, Wind) RADIATION
MEASUREMENTS (Direct, Diffuse and Spectral
Flux) POLLUTANT MEASUREMENTS (Aerosols) CLOUDS
AND WEATHER (Cloud base height, Cloud droplet
concentration, Detection of weather
systems) UPPER AIR OBSERVATIONS (Radio sonde)
3
Physical Phenomena
Transducers
Signal Conditioning
A Typical Measurement System
Data Acquisition
PC
4
Part-1 METEOROLOGICAL MEASUREMENTS
TEMPERATURE Measurements
Mechanical and Electronic Thermometers
Thermistor
Infrared Temperature Sensor
5
Mechanical Thermometers
Bi-metallic strip
Liquid-in-glass thermometer
6
Hot junction
Electronic Thermometers
Thermo couples
Principle Thermoelectric effect The
thermoelectric potential can be expressed as a
non-linear function of temperature as, where
?, ?, ..etc. are constants depends on the
materials used, T is the temperature of the
measuring junction, and temperature of the cold
junction (reference junction) is kept at 0?C.
Cold junction
E ?T ?T2 ..
7
Thermistors
Principle Large negative resistance
coefficient. An increase in T by 1?C yields a
5 decrease in resistance.
The relation between electric resistance and
temperature is given by, where R(T) is
resistance at T, R(T0) is resistance at T0, B is
a constant depending on the material.
R(T) R(T0) expB(1/T-1/T0)
8
UNIT ?C
Infrared temperature sensor
9
HUMIDITY Measurements
Mechanical and Electronic
RH water content / water capacity at a given
temperature
Hygrometer or Psychrometer
10
Electronic Humidity Sensors Electronic
humidity sensors operate based on a capacitance
change of a polymer thin film capacitor.
Absorption of water vapour by the polymer alters
its capacitance. It responds to a 90 humidity
change in less than 1 second with accuracy of
?1. Advantages Suitable for aircraft and
radio sonde measurements.
11
UNIT Expressed in
12
Typical Variation of Temperature and RH within a
day
Noon
13
PRESSURE Sensor
Mechanical and Electronic
Principle The weight of the mercury column is
balanced by the pressure exerted on the dish of
mercury by the air above. If pressure decreases,
the column of mercury falls, if pressure
increases, the column of mercury will be more.
h
Mercury barometer
Disadvantages Mercury barometer is highly
inconvenient for mobile platforms such as
aircraft, radiosonde etc. and its response is
slow.
14
Electronic Pressure Sensors They are
liquid-free and called aneroid barometers. This
is a thin metal membrane that deforms in response
to changes in external pressure. Usually a
partially evacuated chamber is used. The chamber
compresses as pressure increases and expand as
pressure decreases.
UNIT mb or KPa
15
RAIN Sensor
Rain Gauge, Tipping bucket and Optical Rain
sensors
Rain Gauges
16
Tipping bucket
17
Optical Rain Sensors
Photodiode detector
diode laser
18
(No Transcript)
19
WIND Sensors
Sonic anemometer
Cup anemometer
Gill anemometer
20
Anemometry The science of measuring and
recording wind field is called anemometry. The
term wind field represents both wind speed and
wind direction. The speed is expressed in m/s
and direction is specified relative to North at
the palce of observation and is expressed in
degrees.
N
North ? 0? East ? 90?
South ? 180? West ? 270?
E
W
S
21
Anemometric devices are mainly three (1)
Mechanical (2) Thermodynamical and (3)
Electronic. Cup and Gill Anemometers Principle A
steady wind speed, u, causes a corresponding Cup
speed, U. Wind speed and Cup speed are related by
the power series expression, where a, b, c,
are calibration coefficients. Usually, the
coefficients of U2 and higher powered terms of U
are zero. Advantages Low cost Disadvantages
Mechanical degradation (friction), presence of
moving parts not favorable for long term
operation, Response is slow.
U a bU cU2 .
22
A
B
Wind Direction
23
Hotwire Anemometers Principle Works on the
principle that a given increase in wind speed
enhances the heat transfer to the environment,
which in turn decreases the temperature and
resistance of the wire. The resistance decrease
cause a change in current which is measure of
wind speed. Advantages Low cost Disadvantages
Response is slow.
Typical Hot-Wire Anemometer
24
Sonic Anemometers
Principle Operation is based on the interference
in the frequency of sound pulses sent across
short path length due to the wind. The time
difference, ?t, between the initial
transmission of sound pulses across the air
stream and their reception is a direct function
of mean air speed along the path. l path
length ? angle of wind (with speed u) with
respect to sound wave (with speed
c) Advantages Inertia-free, best for long term
operations, no moving parts, very slow and very
high winds (0.03 to 50 m/s) can be measured.
Best for air craft measurements. Disadvantages
Very expensive.
?t 2 l u cos(?) c2-u2-1
25
Laser Anemometer Principle Doppler effect It
consists of a laser beam which illuminates on
moving light scattering particles in the air. A
receiver unit detects the back scattered light.
The electronic unit measures the shift in
frequency due to Doppler Effect which can
be related to wind speed. Disadvantage Very
expensive Advantage Almost instantaneous,
suitable for air craft measurements and vertical
profiling.
26
Part-2 RADIATION MEASUREMENTS
Direct Flux directly from sun with out
interaction Diffuse Flux scattered light Global
Flux composite of direct and diffuse light
Global Flux Diffuse Flux Direct Flux cos
(?z)
27
A
B
Sun
?z
Surface
C
?z Solar Zenith Angle
28
Photodiode
Basic Terms Solar Energy is expressed in
Joules Energy per time (or power) is expressed
in Watts Flux density or Irradiance is Energy
per time per unit area (W m-2) Radiance is
Irradiance per unit solid angle (W m-2 sr-1)
29
Pyranometer
Measures Global Flux
30
Typical output from Pyranometer
noon
6 am
6 pm
31
Albedo meter
Albedo Solar Radiation incident / Solar
Radiation Reflected
32
Pyrheliometer
33
Shading balls
Radiation sensor
Sun tracker fixed with pyrheliometer
Sun tracker with shading ball arrangement
pyrheliometer
Sun tracker and shading ball arrangement
34
Part-3 POLLUTANT MEASUREMENTS
In situ or direct sampling (e.g., high volume
air samplers Remote measurements (sun
photometer)
35
Direct Sampling
High Volume Air Samplers
36
Aerosol Counter
37
Scanning Mobility Particle Sizer
38
1.0
10
0.1
Radius (?m)
39
(No Transcript)
40
Lambert-Beer Law
I I0 exp(-k dx) F F0 exp(-t m) m slant /
vertical V is proportional to I ln(V) ln(V0) -
t m
I0
I
dx
This is an equation for a straight line with
slope is m and y-intercept ln(V0)
41
tTotal tMolecules tAerosols
tMolecules tozone tRayleigh
42
Part-4 CLOUDS AND WEATHER
43
LIDAR in operation
44
Principle Electromagnetic signal is recorded by
a detector after it interacts with a target. By
interpreting the changes caused in the return
signal, the characteristics of the target can be
inferred.
S F(T)
T F-1(S)
45
LIDAR electronics
46
Simple Block Diagram of LIDAR
47

48
Aircraft Equipped with Optical Sensors for CLOUD
and AEROSOL studies
49
RADAR
50
RADAR measures the range and location of targets.
Targets can vary over a wide range. RADAR
consists of three parts (a) Transmitter (2)
Receiver (3) Electronics. Transmitter generates
short pulses of energy in the microwave region
of the EM spectrum and transmits as a narrow
beam. If the pulses intercept an object with
different refractive index than air, it causes
some of the energy to be scattered. Part of the
scattered energy will reach back the antenna.
The Power of return signal (P) is given by RADAR
equation which can be written in a simple form
as, where Radar term contains the transmitted
energy and Target term contains the returned
energy.
P C Radar term Target term
51
Weather Target
Weather Target
Background
52
(No Transcript)
53
(No Transcript)
54
(No Transcript)
55
Part-5 UPPER AIR OBSERVATIONS
56
1. Contamination Shield 2. Sensor Boom 3. Battery
Connector 4. Battery Wire Attachment Points 5.
Battery 6. Battery Container Cover
57
Altitude (m)
Temperature (?C)
RH ()
58
Thank you