- The atmospheric boundary layer (ABL) is the lowest part of the troposphere directly influenced by the Earth's surface. It exhibits diurnal temperature variations and responds rapidly to surface forcings. - GPS radio occultation (RO) technique uses GPS signals passed through the atmosphere to determine properties like temperature, pressure, and water vapor in the ABL. Data from the COSMIC satellite constellation has provided global observations of the boundary layer. - GPS RO is a useful remote sensing technique for studying characteristics of the ABL like refractivity and temperature profiles, with advantages of all-weather capability and global coverage.

1. Atmospheric Boundary Layer
and
GPS RO Technique
M. S .Vani
Junior ResearchFellow,
Research Institue

2. Outline of talk:
• Earth’s Atmosphere
• Atmospheric Boundary Layer (ABL) Definition
and importance
• Diurnal variations of ABL
• Mechanisms or processes taking place in ABL
• GPS RO technique
• GPS RO technique usage for ABL

3. Atmosphere:
• Atmosphere is a layer of gases, aerosols and hydrometeors that covers the
earth and protects the earth from ultraviolet radiation of sun and the
temperature extremes are changing from day to night.
• Sun is the main source of the atmosphere. Gases, aerosols and
hydrometeors in the atmosphere absorbs some amount of solar radiation
which leads to the variation of thermal structure in atmosphere with
altitude.
• Atmosphere consists of 4 layers depending on its thermal structure.
1.Troposphere
2.Stratosphere
3.Mesosphere
4.Thermosphere

4. •Solar radiation (ie., Short Wave radiation) falls on the earth’s surface, the
surface gets heated up and it reradiates or reemits Long Wave radiation
(LWR).
• The LWR and convection plays a major role in the weather and climate of
lower atmosphere.
Long wave radiation reemission by earth

5. Atmospheric Boundary Layer (ABL):
• ABL is the lowest part of the troposphere, that is directly
influenced by the surface of the earth which responds to the surface
forcings with in a time scale of 1 hour or less. (Stull) or
• ABL is the layer of air which is directly above the earth’s surface in
which the effects of the surface are felt directly (Garratt,1992).
•ABL shows diurnal temperature variation
and this layer is influenced by surface
forcings like friction, viscosity, heat fluxes,
convection, turbulence.
• Thickness of ABL will be vary from
a height of 100m to 3km.
•This is the layer which mostly influence
the human lives. It is important to study about the layer for local
forecasting of weather, clouds, pollution dispersion, climate change.
Atmospheric boundary Layer in troposphere

6. Diurnal variations of ABL:
Fig 1: Diurnal variation of ABL showing different layers (1)

7. •Earth usually responds to changes of the radiative forcing which leads
to the diurnal variation of ABL.
•The solar heating will creates the thermal plumes, this plumes rises
and expands adiabatically up to reaching an equilibrium state.
•This rising of plumes and sinking of cool air will transport the
moisture, heat and aerosols.
•ABL has again divided into sub layers:
•Stable layer
•Surface layer
•Mixed layer and residual layer.
•ABL is driven by various mechanisms like radiative forcings, friction,
turbulence. The physical quantities like wind flow velocity, temperature
and moisture rapid fluctuations are strongly seen in the ABL

8. •The surface of the atmosphere encounters obstacles like trees, buildings
etc., which will reduce the velocity of the flow of air molecules which will
be stable called as stable layer.
•The layer in which the wind, temperature and humidity rapidly varying
with altitude due to turbulence will starts is known as ‘Surface layer’. It
will be up to few tens of meters.
•Due to the thermal or convective eddies rising from the surface layer and
associated turbulence, causes the well mixing of fluid (air) which is known
as ‘Mixed layer’.
•The flow of fluid near the surface will be changed by the interaction
earth’s topography in the form of Viscosity ie., molecular viscosity, which
transfers momentum due to random motions of molecules. The molecular
interaction/motion will creates disturbance, this triggered disturbance will
form the ‘Turbulence’.
•During day time Wind shear turbulence and convective generated
turbulence will exists. During Night time only wind shear will be
available.

9. •This random motions causes turbulence in ABL. This turbulent flow is
generated due to wind and temperature variations of eddies. Turbulent flow
fluid is responsible for mixing in the ABL.
•Aerosol concentration will be more in the atmospheric boundary layer then it
gradually decreases.
• Potential temperature showing in the figure is maximum at ground level
where the heat is delivered and then shows a uniformity across the Convective
boundary layer which indicates the higher mixing.
Fig: Typical vertical profile of Potential temperature and Aerosol
concentration on day time Boundary layer (Garratt, 1992)

10. Importance of ABL and different instruments for ABL studies:
• Study of ABL is important because of these processes affect our lives directly
or indirectly (influencing the weather).
• Most of the weather processes will occur in ABL only like turbulence,
Friction, Winds, Vertical transport of momentum and energy.
• Different instruments will be used for the ABL studies, which are insitu
measurements and Remote measurements.
• Insitu measurement instruments like cup anemometer, Hygrometer,
resistance thermometer etc., these sensors will be mounted on a tower to
collect the data. Remotely measuring instruments like SODAR, Lower
Atmospheric Wind profiler, LIDAR etc., Satellite based measurements such
GPS RO technique is widely used to study the boundary layer characteristics.

11. GPS RO Technique:
• GPS satellites are primarily using for positioning and navigation. The GPS
satellites emits radio signals at L1(1.57 GHz) and L2(1.22 GHz).
• A Low Earth Orbit satellite contains an RO instrument and observes GPS
satellites in the limb. As the density varies with height in atmosphere which
will refract or bent the GPS signal. This bending signal magnitude and angle
depends on the refractivity gradient of the atmosphere.
GPS Radio occultation. (photo : PlanetiQ)
• A constellation of satellites COSMIC using GPS RO technique provides radio
refractivity information with global coverage. Radio Occultation technique
based on Satellite, gives a global studies on ABL.

12. • On 15 April 2006, the joint Taiwan - U.S. Constellation Observing System
for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite
Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a
constellation of six microsatellites, was launched into a 512-km orbit.
•GPS RO technique needs no active calibration, it is weather independent
and global coverage is possible.
Features of GPS Radio Occultation: (COSMIC website)
• Assured long-term stability
• All-weather operation
• Global 3-D coverage: 40 km to the surface
• Vertical resolution: ~100 m in the lower troposphere
• Independent height, pressure, and temperature data
• A compact, low-power, low-cost sensor
• High accuracy: Averaged profiles to < 0.1 K

13. • Studies using GPS RO technique has been carried out by Kursinski et
al(1996), ware etal (1996), Rocken etal., (1997), Ratnam etal(2010)
previously.
•From GPS Ro gives the information of Phase delay which provides,
 Temperature
 Pressure
 Water vapor
 Ionic density
 Refractivity
 Bending angle
Atmospheric refractive index where is the light velocity
in a vacuum and v is the light velocity in the atmosphere
Refractivity
P=Pressure, T= Temperature, ew= water vapor content
)1(106
−= nN
vcn /= c
2
5
1073.36.77
T
e
T
P
N w
×+=

14. •GPS RO provides a valuable global view of the height-resolved refractivity
or moisture structure of ABL. The information about the refractivity from
which one can derive the atmospheric properties in the Boundary layer.
• Atmospheric Boundary Layer studies using satellite based measurements
and ground based measurements like Lidar, radiosondes etc.,
comparatively provides an accurate information about the properties of the
ABL.
• GPS RO and Lidar techniques are used to know the information of the
most reliable characterization of the ABL has to be carry out.

15. References:
• R.B.Stull (1999), An introduction to boundary layer meteorology, Kluwer Aca
demic Publishers, London.
•Garratt, J. R. (1994), The Atmospheric Boundary Layer, 315 pp., Cambridge
Univ. Press, New York.
• Air Pollution Training Institute website (APTI)
www.shodor.org/os411/courses/411c/module06/unit01/page01.html.
• Lidar.ssec.wisc.edu/papers/akp_thes/node6.html
• S. Sokolovskiy, Monitoring the atmospheric boundary layer by GPS radio
occultation signals recorded in the open-loop mode, Geophysical research letters,
Vol:33, L12813, 2006.
•COSMIC website http://www.cosmic.ucar.edu/ro.html
•Kursinski et al. 1996: Initial results of RO observations of Earth’s atmosphere using the
GPS. Science, 271, 1107-1110.
• Ware et al. 1996: GPS sounding of the atmosphere from low earth orbit: preliminary results.
Bull. Amer. Meteor. Soc., 77, 19-40.
• Rocken et al. 1997: Analysis and validation of GPS/MET data in the neutral atmosphere. J.
Geophys.Res., 102, 29849-29866.

16. Thank you for the attention