Previous: Instrumentation Next: Results Up: Ext. Abst.

 

Analysis

A scattering ratio (SR) is calculated from the sum of Rayleigh and Mie (aerosol) scattering coefficients, ( $\beta_{aerosol}+ \beta_{Rayleigh}$, respectively), divided by the Rayleigh backscatter coefficient:

\begin{displaymath}SR = \frac{\beta_{aerosol}+ \beta_{Rayleigh}}{\beta_{Rayleigh}}.\end{displaymath}



 

The numerator corresponds to the raw lidar signal corrected for the background and the altitude squared dependence. The denominator is obtained using a fourth degree polynomial fit to the backscattered profile after the removal of the Mie component. When no aerosols are present, the SR equals one. Typically, for clear-sky conditions, the SR standard deviation at low altitudes (5 - 6 km) is $\sim 0.04$ and increases to $\sim 0.4$ at high altitudes (18 - 19 km). This rise in the standard deviation is due to the exponential decay in the signal with altitude. The minimum detectable optical depth for the OHP lidar is 1 x 10-3.

The presence of cirrus is determined when the following two criteria are met: the SR is greater than the defined threshold (SRt) and the cloud layer is situated in an air mass with a temperature of - 25 ° C or colder. The SRt is defined as the sum of the nightly mean SR from 18 - 19 km plus three times the standard deviation for this altitude range. Because the SRtis defined for each nightly determination, it is sensitive to the signal to noise of the particular observation. The - 25 ° C threshold, as determined from the radiosondes, has been recognized (Heymsfield, private communication) as an indicator of cirrus. Cirrus occurrence frequencies are calculated from the number of cirrus occurrences divided by the total number of measurements.

An optical thickness for a cirrus cloud is calculated from the integral of the extinction coefficient, $\alpha (z)$, :

\begin{displaymath}\tau_{cirrus}= \int_{z_{min}}^{z_{max}} \alpha (z) dz,\end{displaymath}



 

where zmin and zmax represent the minimum and maximum cirrus altitude, respectively. Using the SR and the phase function, $\phi_p =(\alpha / \beta_{aerosol}$), we can derive the relationship used to calculate the optical thickness of cirrus:

\begin{displaymath}\tau_{cirrus}= \phi_p \sigma_{Rayleigh} \int_{z_{min}}^{z_{max}} n_{air}(z) (SR(z)-1) dz,\end{displaymath}



 

where, $\sigma_{Rayleigh}$ = Rayleigh backscattering cross section, $\beta_{Rayleigh}$ = $\sigma_{Rayleigh}$ nair(z), and nair(z) = density of air, as calculated by the MSIS-E-90 atmosphere model [http://nssdc.gsfc.nasa.gov/space/model/models/msis.html]. A phase function of 18.2 sr [Platt and Dilley, 1984] is used, and$\sigma$Rayleigh (532 nm) = 5.7 x 10-32 m2 sr-1. No corrections for multiple scattering are made.

Previous: Instrumentation Next: Results Up: Ext. Abst.