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