Ceilometer absolute calibration to calculate aerosol extensive properties Giovanni Martucci Alexander Marc de Huu Martin Tschannen Alain

WHY DO WE NEED AN ABSOLUTE CALIBRATION OF CEILOMETERS? THEY ARE ROBUST AND EASY TO INSTALL THEY CAN BE RUNNED UNATTENDED THEY CAN DERIVE THE ATTENUATED BACKSCATTER THEY ARE ROBUST AND EASY TO INSTALL THEY CAN BE RUNNED UNATTENDED THEY CAN DERIVE THE ATTENUATED BACKSCATTER If an absolute calibration could be performed the potential would be unprecedented. WE MUST THINK BIG, CEILOMETERS ARE GROWING IN NUMBER, VERY FAST! BUT NO ABSOLUTE BACKSCATTER CAN BE CALCULATED DUE TO LACK OF AN ABSOLUTE CALIBRATION

Different networks already exist that gather several of these ceilometers into national and trans-national networks. Examples of such networks are: The national German network of CHM15K created by the Deutscher Wetterdienst (DWD) counting today ~100 ceilometers most of which are profilers (storing the full profile of backscattered signal). The new EUMETNET programme E-PROFILE that is setting up the largest European network of national networks of ALC (Automatic LIDAR and Ceilometers) devices ( profile). If combined in a single network such a high number of ceilometer has the potential to provide a dense and 24H/7D information about the status of aerosol vertical distribution over Europe.

x ov x full x0x0 R T A ceilometer is based on the same principle of a LIDAR. i.e. Light Detection And Ranging. A ceilometer differs from a research LIDAR by the fact that: is normally single-wavelength with reduced power Normally emitting in the near infrared spectrum. Thanks to its reduced dimension and the robustness of the emitter-receiver units, it can be run 24H/7D with little or no maintenance. The LIDAR equation applies to the ceilometer signal exactly like for a normal LIDAR. Over an optical path x 0 – x full, where x 0 is the first range bin and x full is the first range bin of full emitter-receiver overlap, the LIDAR equation is: C, LIDAR constant β, backscatter coefficient, [sr -1 m -1 ] α, extinction coefficient, [m -1 ] O, overlap function When the extinction α can be neglected, β can be calculated only by determining two unknowns: C and O When the extinction α can be neglected, β can be calculated only by determining two unknowns: C and O

Ceilometer absolute calibration feasibility test

Relevant quantities to be calibrated In order of relevance: -The sensitivity to the reflected optical intensity, C -The overlap function between the receiver’s FOV and the Laser beam over the first km (until 100% overlap), O

Working Principle of a Ceilometer /Alain KüngMeeting at MeteoSuisse in Payerne -1-5ns laser pulse of 7-9µJ, 5-7 KHz at =1064nm, Ø=90mm, <0.3mrad -The pulse is scattered on particles (water particles mainly) -The light is collected by the field of view of the telescope (0.45mrad) and analyzed in the time domain (resolution 5-15m) overlap 0% overlap 100% Laser beam FOV Laser pulse

Testbed to determine the sensitivity and the spatial resolution Idea: simulate a reflection of given intensity. - Coupling loss given by the difference in surface area between the laser beam and the optical fiber. 1m Y X Laser beam Ø90mm calibrated photodiode emitter receiver

Testbed to determine the sensitivity Coupling efficiency: for a single mode fiber= (90mm/4.5µm) 2 = -86dB loss → 0.16nW for a multi-mode fiber = (90mm/90µm) 2 = -60dB loss → 65nW If more is needed, a collimation lens with a pinhole in front can be used. 1m Y X Laser beam Ø90mm, 65mW calibrated photodiode emitter receiver

Testbed to determine the overlap function Idea: map the intensity distribution of the laser beam and the FOV at 2 or more planes (1m and 50m) and extrapolate the propagation until 100% overlap. 1m50m1km Y X Y X emitter receiver

Testbed to determine the overlap function Difference between the beam and FOV angle: 150mm/1km=0.15mrad Over 0-50m the measurement needs to be 20x more precise and the uncertainty needs to be 10x smaller. Thus a 0.75µrad resolution is required. Spatial resolution must be 30µm with a 30dB dynamic for the photodetector 1m50m1km Y X Y X 20x 150mm

Design by Martin Tschannen And all this becomes real….

Benefit of the design -The calibration is independent of scattering processes which are not well modeled. -Separates the overlap function (geometry) and the sensitivity -Provides additional calibration of the spatial resolution -Simple design (No high speed electronics, no controlled atmosphere)

What we are looking for -Partners to set up a JRP on remote sensing and aerosols