1 Sistema LIDAR estatus, funcionamiento y control Reunión Septiembre 28 2007 Agenda Fundamentos y definiciones Dispersión de Rayleigh y Mie Dispersión inelástica Operación

2 Transmitancia: Medida de turbulencia Fundamentos F=Flujo radiante Probabilidad por unidad de longitud de remover un fotón del haz primario. Extinction coefficient Sea

3 Coeficiente de dispersión inelástica Coeficiente de absorción Optical depth Dependencia angular de la luz dispersada un ángulo dado = = Phase function

4 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) Dependencia de la presión y temperatura (Pendorf 1957) T s =15 o C, P s =101.325 kPa

5 Dispersión de la luz por moléculas (Dispersión de Rayleigh) Ignorando efectos por depolarización y ajustes por cambios en la presión y temperatura 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

6 Integrando sobre ángulo 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

7 Dispersión por partículas (Dispersión de Mie) Aproximación por Monodispersión Las partículas dispersoras tienen la misma Composición y tamaño Eficiencia de dispersión = Size parameter =

8 Little moisture is condensed. Condensation nuclei accumulate large cuantities of water. Droplets in a fog or cloud. Partículas pequeñas (atmósfera clara) Partículas grandes (heavy fogs and clouds) Partículas en las partes bajas de la atmósfera

9 Aproximación por dispersión múltiple

10 Dispersión inelástica

11 Ecuación del LIDAR

12 Range corrected signal Para los casos de atmósfera homogénea

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14 Operation The lidar is active between the end of astronomical twilight of one day and the 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 supervise that these tasks are executed correctly. In this way, before starting a run, the operator has information about the status of the telescope in real time and about the weather conditions of each site through the information being sent to the lidar web site.

15 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

16 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.

17 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.

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19 Continuos scan Discrete scan StS

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