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Sistema LIDAR
estatus, funcionamiento y controlReunión Septiembre 28 2007
Agenda
Fundamentos y definiciones
Dispersión de Rayleigh y Mie
Dispersión inelástica
Operación
Transmitancia: Medida de turbulencia
Fundamentos
,0
)(FF
HT F=Flujo radiante
drrFrrdF t )()()( ,
,tProbabilidad por unidad de longitud de
remover un fotón del haz primario. Extinction coefficient
Sea
H
t drr
eHT 0
)(
)(
)(
)(
)()()(
r
r
rrr
A
At
Coeficiente de dispersión inelástica
Coeficiente de absorción
H
t drr0
)( Optical depth
Dependencia angular de la luz dispersada un
ángulo dado
=
P
= Phase function
Índice de refracción (es un número complejo):
Parte real = Velocidad de fase relativa.
Parte imaginaria = Capacidad de absorción del medio
Ejemplo, aire Parte real (Edlen 1953)
228
9.3815997
1302406030
13.8342)1(10
sm
Dependencia de la presión y temperatura (Pendorf 1957)
S
ss P
PT
Tmm )
00367.01
00367.01)(1()1(
Ts=15oC, Ps=101.325 kPa
Dispersión de la luz por moléculas
(Dispersión de Rayleigh)
Ignorando efectos por depolarización y ajustes por cambios en la presión y temperatura
)cos1(2
)1( 242
22
,
S
m NNm
m = parte real del i. de refrac.
N = densidad
Ns = 2.547 10 19 cm-3 para Ts = 288.15 K, Ps = 101.325 kPa
42
23
3)1(8
Sm N
Nm Integrando sobre ángulo
))()(7636
(3
)1(842
23
T
T
PP
NNm S
SSm
Si no se contaran los efectos de la T y P habría errores de hasta el 10%. (Bohren-Huffman, 1983)
= Factor de depolarización = 0.0279
= recomendado por Young 1981
42
223
3)1(8
S
mm N
mN
Dispersión por partículas (Dispersión de Mie)
Aproximación por Monodispersión
Las partículas dispersoras tienen la misma
Composición y tamaño
ppp N
Eficiencia de dispersión = 2 p
scQ
Size parameter = 2
scpp QN 2
Little moisture is condensed.
Condensation nuclei accumulate large cuantities of water. Droplets in a fog or cloud.
1
5040
251
Partículas pequeñas (atmósfera clara)
Partículas grandes (heavy fogs and clouds)
Partículas en las partes bajas de la atmósfera
Aproximación por dispersión múltiple
22
2
4
65
)21
(3
128
mm
p
dnlQ scscp )(2
1
2
Dispersión inelástica
Ecuación del LIDAR
r
tmp dxx
r
rrcFCrF
02
,,01 )(2exp
)()(
2)(
r
an dxxrr
CrFrP0
20 )(2exp)(
)()(
r
t dxxrr
TCrP0
2
200 )(2exp
)()(
Range corrected signal2)()( rrPrZ r
)()(
brZrZ
S
Para los casos de atmósfera homogénea constr
constr tt
)(
)(
))(2exp()( 02
200 rrr
TCrP t
rCrZr tr 2)ln()(ln)( 0
)(ln21
rZdrd
rt
Operation
The lidar is active between the end of astronomical twilight of one day andthe beginning of twilight the following day. In this way, a good signal-to-noise ratio is assured for the whole lidar dataset.
Following initialization, the system enters an operational mode called AutoScan. In AutoScan mode, the telescope performs a cycle of steering scripts, unless otherwise interrupted until the end of the night. When the laser is fired, the telescope position is determined by the coordinates contained in these scripts. There are four main steering strategies: three making up the AutoScan pattern and a fourth, shoot-the-shower, that periodically interrupts the AutoScan. These strategies are discussed below:
After the telescope cover is opened, an initialization procedure is executed to calibrate the incremental encoders used to determine the telescope position.A webcam located in the interior of the telescope cover is used to supervisethat these tasks are executed correctly. In this way, before starting a run, theoperator has information about the status of the telescope in real time and about the weather conditions of each site through the information being sentto the lidar web site.
Continuous scans: In this scan, the telescope is moved between two extreme positions with a fixed angular speed while the laser is shot. The telescope sweeps the sky along two orthogonal paths, each of those with an aperture angle of 90~. The purpose of these scans is to provide useful data for simple cloud detection techniques and to probe the atmosphere for horizontal homogeneity. An example of the data produced by this kind of scan is
Discrete scans: The telescope is positioned at a set of particular coordinates. The angular distance between two subsequent points increments with a fixed step in θ (zenith angle of the telescope position). The purpose of this angular distribution is to supply a constant step in height at a given horizontal distance from the lidar every time the telescope moves between two positions. Because the discrete shots increment in steps of equal height, and the telescope remains at the same coordinates for longer time periods than on the continuous scans, the data obtained from discrete scans are very useful to determine the vertical distribution of aerosols in the atmosphere.
Shoot the Shower: This rapid response mode is used to measure the atmospheric attenuation in the line of sight between the FD telescopes and a detected cosmic ray shower. This scanning mode suspends any of the previously mentioned sweeps.
The length of the lidar run depends on the length of astronomical twilight, which varies over the course of the year from less than five hours during the summer to almost fourteen hours during the winter. This has a direct impact on the amount of data generated by each station during a data acquisition run.
Shoot the shower
A primary design requirement of the lidar system is that it probes the atmosphere along the tracks of cosmic rays observed by the FDs.
This function, called shoot-the-shower (StS), exists to provide the FDs with atmospheric backscattering and absorption coefficients for showers of particular interest. StS is meant to compensate for unusual and highly localized atmospheric conditions that can affect FD observations at different times of the year.
These include the presence of low and fast clouds, and low-level aerosols due to fog, dust, or land fires.
Continuos scan
Discrete scan
StS