atmospheric boundary layer studies

16
Atmospheric Boundary Layer and GPS RO Technique M. S .Vani Junior ResearchFellow, Research Institue

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Page 1: atmospheric boundary layer studies

Atmospheric Boundary Layer andGPS RO Technique

M. S .VaniJunior ResearchFellow,Research Institue

Page 2: atmospheric boundary layer studies

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

Page 3: atmospheric boundary layer studies

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.Troposphere2.Stratosphere3.Mesosphere 4.Thermosphere

Page 4: atmospheric boundary layer studies

•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

Page 5: atmospheric boundary layer studies

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

Page 6: atmospheric boundary layer studies

Diurnal variations of ABL:

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

Page 7: atmospheric boundary layer studies

•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

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•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.

Page 9: atmospheric boundary layer studies

•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)

Page 10: atmospheric boundary layer studies

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.

Page 11: atmospheric boundary layer studies

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.

Page 12: atmospheric boundary layer studies

• 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

Page 13: atmospheric boundary layer studies

• 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 velocityin 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

251073.36.77

Te

TPN w

Page 14: atmospheric boundary layer studies

•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.

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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.

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Thank you for the attention