Probability of Cloud-Free-Line-of-Sight (PCFLOS) Derived From CloudSat and CALIPSO Observations

Don Reinke and Karen Milberger
Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State University

Statement of Problem/Summary of Research Objective

Cloud-Free Line of Sight (CFLOS), and the related PCFLOS (Probability of a Cloud-Free Line of Sight), refer to the ability to obtain a visible line of sight through the atmosphere unimpeded by Cloud. An application of CFLOS statistics might be to determine the altitude where an aircraft pilot has the highest probability of seeing the ground.

CFLOS from a passive (visible, infrared, or microwave) satellite sensor’s perspective can only be determined based on the highest cloud layer. CFLOS at a level underneath the highest layers cannot be determined directly (see figure below). CloudSat, for the first time, provides accurate layered cloud bases and tops — allowing us to determine CFLOS from levels below the highest cloud layers.

The figure below shows CFLOS for different vantage points, showing the complication imposed by multilayered clouds beneath an opaque upper level cloud.

A Proposed Solution

Above, a top-down view of simultaneous MODIS infrared imagery is shown below the corresponding CloudSat image. An analysis of the MODIS cloud field cannot determine whether there is a Cloud-Free Line of Sight beneath the cloud mass, while the CloudSat image clearly shows a cloud-free layer (on the left) below the opaque cloud top. Using the CloudSat vertical profile data, it should be possible to determine CFLOS at varying altitudes and view angles from nadir (0°) to 90° off nadir, along the CloudSat orbit track.

CloudSat Data

The CloudSat instrument is a 94 GHz (~3 mm wavelength), nadir-pointing, Cloud Profiling Radar (CPR).

CALIPSO Data

The CALIPSO instrument is a dual channel (532 and 1062 nm) near-nadir pointing lidar. It has a vertical resolution of 30 m (< 8 km) and 60 m (< 8 km), with an 80 m instantaneous footprint. The overlap with a CloudSat footprint is shown below.

In a similar manner, CALIPSO vertical profiles should provide information about CFLOS. However, the lidar is attenuated rapidly when viewing the atmosphere through optically opaque clouds, so the only additional information would be to show the location of thin cloud above the CloudSat-measured cloud top, or the occurrence of low level cloud that CloudSat misses (in the lowest 1.5 km).

Combined CloudSat / CALIPSO product (2B-GEOPROF-Lidar)

CFLOS from CloudSat

CloudSat offers a unique solution to the determination of CFLOS from space. By directly measuring opaque cloud bases and tops, we are now able to determine the CFLOS from any location within the CloudSat FOV.

Above, an opaque cloud at “B” hides the fact that there is no CFLOS at levels below the cloud top, while at “A” it hides a high probability of a CFLOS below the upper cloud layer.

Calculation of CFLOS

For each CloudSat profile, CFLOS is determined by looking at 10° intervals from nadir to 90° (horizontal) at each of 20 vertical levels. These levels extend from .96 km to 19.2 km altitude at ~1 km intervals. (Every 4 bins in the vertical = 4 x 240 m = .96 km.)

Calculation of “PCFLOS”

CFLOS statistics are gathered over a global 1-degree grid to produce the Probability of a Cloud-Free-Line-of-Sight (PCFLOS).

PCFLOS from CloudSat CPR (left) and CALIPSO Lidar (right). Viewing height is level 20 (TOA), and viewing angle is 0° (nadir). Data are for the month of July, 2006-2010.

PCFLOS from CloudSat
January 2007-2010 (varying height)

January 2007-2010 (varying viewing angle)

Future Work

Will examine the data to identify tropospheric “cloud-free vaults” and to look at methods for using CFLOS for parameterization of cloud layers in mesoscale models.

APPENDIX: Notes on the prototype CloudSat/ CALIPSO PCFLOS web viewer

http://www.cloudsat.cira.colostate.edu/pcflos/PCFLOS_prototype.php

This is a prototype product that is generated from CloudSat and CALIPSO cloud mask products (CloudSat 2B-GEOPROF and 2B-GEOPROF-Lidar). CFLOS observations are generated with the following assumptions/background information:

1. View angles are given as the angle displacement from nadir. For comparison with standard, passive, cloud fraction analyses, one can select the top level and a view angle of 0 (see note 2 below).
2. PCFLOS displays indicate the probability of a cloud-free-line-of-sight. This is the inverse of standard cloud fraction. From the highest level and a view angle of 0° (nadir), a PCFLOS of 30% will be equivalent to a cloud fraction of 70% (0.7).
3. A cloud-free-line-of-sight is flagged when one can view, along a line of sight of at least 25 km, without seeing an intervening cloud. This distance is arbitrary and can easily be modified for specific applications. 25 km was chosen because it is generally considered to be the cloud-free distance that is of interest for aviation applications.
4. In the case where the sight vector intersects the earth surface in less than 25 km, a CFLOS is indicated if no clouds are encountered between the view point and the earth surface.
5. CFLOS calculations are done in both the forward and rear directions along the CloudSat orbit track. This is done to both increase the number of observations, and to account for varying direction of cloud advection over the global domain (CloudSat and CALIPSO reach a maximum latitude of ± 82 degrees). In some domains the cloud advection will have a component in the direction of flight and in others it will be opposed to the direction of flight. The PCFLOS, thus, includes both the forward and backward views.