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Aerosol Backscatter Cross Section Volumes Derived from Volume Imaging Lidar and High Spectral Resolution Lidar Data from August 24 to December 8, 1995

Antti K. Piironen, Paivi Piironen, Walter Wolf, Edwin W. Eloranta

Introduction

Three-dimensional aerosol backscatter cross section fields are derived from Volume Imaging Lidar (VIL) and High Spectral Resolution Lidar (HSRL) data. While the VIL is a scanning lidar providing high spatial resolution data of atmospheric scatterers, external information is needed for resolving the backscatter cross section from the data. Therefore, HSRL measured backscatter cross-sections are used to calibrate the VIL measurements. The method is discussed in detail later in this document.

Method

The high sensitivity of the VIL allows observations of inhomogeneities in the natural aerosol backscatter. The signal incident on the receiver is described by the single-scatter lidar equation [1,2]

There are two unknowns, backscatter cross section and extinction cross section in the equation (1) and only one measurement of backscattered signal power . Therefore, additional information is needed to solve the equation for backscatter.

This analysis uses backscatter cross sections measured with the HSRL for calibration of the VIL data. An aerosol layer that stays nearly constant with time is selected as a calibration target. Since HSRL operates at 532 nm wavelength while VIL at 1064 nm, aerosol and molecular backscatter values for the VIL are calculated from the HSRL values by dividing them by 2 and 16, respectively. We assume that the backscatter cross section varies as 1/wave_length for aerosol scattering. When this scaled backscatter cross section is used with HSRL measured average phase function, the equation (1) can be solved for the backscatter. This method provides an approximate calibration for the VIL data.

Construction of 3D Volumes

The data volumes have been produced from Volume Imaging Lida r (VIL) multiple cirrus cloud scans. These scans were aligned to be nearly perpendicular to the wind. Consecutive cirrus cloud scans enable a reconstruction of the cirrus clouds within a volume when the wind speed and direction are known. These reconstructions assume that the cirrus cloud structure does not change during the time it takes the cloud to drift across the volume. Each VIL profile is converted into an approximate backscatter profile. Then each data point is moved according to wind to provide snapshot volume at the chosen reference time. Local radiosonde launches are performed to provide wind information.

Provided Data Volumes

The provided backscatter volumes represent total backscatter cross section at 1064 nm wavelength. There are some features that the user should be aware of:

• Missing data points are assigned to zero. Such points consist of:
  • Unsampled areas due to scanning geometry.
  • Sampled points with signal below noise level. Note, that signal can go below noise level either if optical depth increases enough or if backscatter decreases.
• The calibration using HSRL data is an approximation. Further investigation is required before error estaimtes can be assigned to these data.

References

1

Schols, J.L. and E.W. Eloranta, ``The calculation of area-averaged vertical profiles of the horizontal wind velocity from Volume Imaging Lidar data,'' J. Geophys. Res., 97, D17, 18395--18407 (1992).

2

Collis, R.T.H. and P.B. Russell, ``Lidar measurement of particles and gases by elastic backscattering and differential absorption,'' in Topics in Applied Physics: Laser Monitoring of the Atmosphere, edited by E.D. Hinkley, Springer, New York, 71--151 (1976).

3

D. Wylie, P. Piironen, E. W. Eloranta, W. Wolf: ``Understanding satellite cirrus cloud climatologies with calibrated lidar optical depths'', accepted for publication Journal of Atmospheric Sciences, 1994.

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Aerosol Backscatter Cross Section Volumes Derived from Volume Imaging Lidar and High Spectral Resolution Lidar Data from August 24 to December 8, 1995

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