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# Ceilometer

## Instrumentation

The first Ceilometer from JenOptik (CHX090102) was installed with a special optical unit in 2010, but this setup proved to suffer from signal loss. As a result, a standard CHM15k was installed in 2011, and the original unit was removed in 2012. The CHM15k setup has been in operation from 2011 until now with time-varying serial numbers (CHM090102, then CHM140102, CHM170159, and CHM200113).

One near infrared laser probes the sky vertically from the instrument to about 15 km above it. Targets like aerosol layers and clouds show up as echoes with certain backscatter intensity and signal extinction. Molecular absorption and Rayleigh scattering by air molecules is negligible at a wavelength of 1064 nm, so the CHM 15k works based on a photon counting method. Photon counting is more accurate and sensitive than analog measurement techniques, which makes it well suited for applications such as this one with a small count rate. The distance from ground is calculated from the travelling time of the laser pulses.

## Data Availability

The data is available as `.zarr` files in the catalog as:
 - `BCO.ceilometer_c1_v1` (first setup with special optical unit: 2010-02-10 to 2012-03-10)
 - `BCO.ceilometer_c2_v1` (second setup with standard unit: 2011-01-22 to 2011-10-01)
 - `BCO.ceilometer_c3_v1` (second setup with standard unit, but providing high resolution signal as well: 2015-07-26 to now, pending discovery of other configurations)

## Sample Plot

Plot cloud base height for each cloud layer for the first period of the instrument's operation in 2011:

```{code-cell} python
import intake
cat = intake.open_catalog("https://tcodata.mpimet.mpg.de/catalog.yaml")
ds = cat.BCO.ceilometer_c2_v1.to_dask()
ds.sel(time="2011-01").cbh.plot(x='time', y='layer')
```

The full dataset:

```{code-cell} python
ds
```