NetCDF files - CL61

CL61 User Guide

Document code
English (United States)
Document type
User guide

Network Common Data Form (netCDF) is a set of software libraries and self-describing, machine-independent data formats that support the creation, access, and sharing of array oriented scientific data.

The netCDF file name format is <cl.sender.fileprefix><reporting.livefile.nameprefix>, where:

  • You can define 2 different types of prefixes to write in the netCDF filename.
  • The last part is the timestamp when the file was created.

NetCDF is a set of interfaces for array-oriented data access and a freely distributed collection of data access libraries for C, Fortran, C++, Java, and other languages.

The netCDF format used in CL61 is built on netCDF‑4 which is built on Hadoop Distributed File System (HDFS). You can open and read CL61 netCDF files with any HDFS or netCDF‑4 reader, for example, Panoply.

NetCDF file format allows the grouping of data types into a single file. A netCDF file is made of one or several variables. Each variable consists of:

  • Data: a multidimensional table or value
  • Metadata: data that characterizes the data

CL61 netCDF files are sent using SFTP protocol to a customer server. The ceilometer is defined as a secure client using a key-based scheme. The files contain the following data:

  • Time series of atmospheric backscatter and polarization profiles
  • Time series of cloud heights and vertical visibility
  • Time series of total cloud cover (sky condition)
  • Time series of cloud layer specific covers and heights (sky condition)
  • Metadata, including:
    • Interface version
    • Device identity and location
    • Device status and monitoring data
Table 1. NetCDF global attributes
Attribute Description
Title Type of the device
Institution User configurable with CL61 configuration parameters
Source Blank
Conventions NetCDF convention version

Device firmware version, the version of the netCDF schema

This attribute needs to be checked when parsing older files for backward compatibility. Instructions to be added when there are changes in the netCDF schema.
Comment User configurable with CL61 configuration parameters
Unit m
Temporal span of this file in minutes Default 1 min
Time between consecutive profiles in seconds Default 5 s
Table 2. Variables in output netCDF file
Variable name Type Dimension Description Unit
cloud_base_heights int time, layer Heights (range) of the detected cloud bases meters
vertical_visibility int time Visibility in the direction of the instrument beam meters
p_pol float time, range Parallel-polarized component of the backscattered light (PPOL)


x_pol float time, range Cross-polarized component of the backscattered light (XPOL)


beta_att float time, range Attenuated volume backscatter coefficient (ABS)


linear_depol_ratio float time, range Linear depolarization ratio of the backscatter volume (LDR) -
time double time Seconds since 1970-01-01 00:00:00.000 seconds
range double range Distance in the direction of the instrument beam meters
layer int layer Number of the observed cloud layer (1, 2, 3, 4, 5) -
longitude double time Longitude degrees east
latitude double time Latitude degrees north
elevation int time Measurement site height above or below a fixed reference point, most commonly a reference geoid meters
beta_att_sum double time Scaled integral of the attenuated volume backscatter coefficient 10-4sr-1
beta_att_noise_level double time A unitless number describing the noise level of the attenuated volume backscatter coefficient -
tilt_correction short time Use of tilt correction
tilt_angle float time Instrument tilt angle from the vertical
height_offset short time

Instrument height offset to reference level

Positive if the instrument is placed, for example, on the roof of a building.

Negative if the instrument is placed below the ground level altitude, for example, in a pit.

This value will be added to the cloud base height results.

sky_condition_total_cloud_cover short time Total amount of cloud cover oktas
sky_condition_cloud_layer_covers short time, layer Amount of cloud cover in different cloud layers oktas
sky_condition_cloud_layer_heights int time, layer Height of different cloud layers meters
Table 3. NetCDF dimensions
Dimension Description Value
Time Number of profiles in file Depends on file length
Range Length of single profile 3276
Layer Number of reported cloud base heights 5