 |
| Titel |
Calibration of the Raman Lidar at the Barbados Cloud Observatory |
| VerfasserIn |
Johannes Kiliani, Ilya Serikov, Björn Brügmann, Holger Linné, Bjorn Stevens |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144409
|
| Publikation (Nr.) |
 EGU/EGU2017-8229.pdf |
|
|
|
|
|
| Zusammenfassung |
| The Max Planck Institute for Meteorology has been conducting atmospheric measurements
on the Caribbean island of Barbados with Raman lidars starting in 2010. We present our
methods for calibrating temperature and water vapor retrievals from these measurements.
Raman lidars measure water vapor by the number density ratio of water vapor to nitrogen
molecules, from the amount of vibrational Raman scattered light by both H2O and N2. As
there are instrumental uncertainties in the lidar backscatter signal, this data must
be calibrated using reference measurements such as radiosondes or ground-based
weather stations. We use a combined approach including local balloon soundings,
regular soundings at the Barbados airport (11 km distant), and multiple weather
stations at the Barbados Cloud Observatory (BCO). The most stable calibration
method fits the Lidar profiles to a combination of ground stations, with an adjustment
for the typical boundary layer water vapor gradient. Since the weather stations
show systematic differences in measured H2O at the same location, their values
have to be adjusted to match a trusted source. For this, we consider the small set of
BCO balloon soundings as most reliable since they include a pre-launch calibration
procedure. This method yields calibrations with low variance and continuous data
availability.
The Raman lidar can also measure temperatures by Rotational Raman scatter of air
molecules. We define the Pure Rotational Raman Scatter (PRRS) ratio as the intensity ratio of
PRRS lines with opposite temperature sensitivity. The PRRS ratio links to the air temperature
with two dependent calibration constants, which are derived by comparing to balloon
soundings. Further, we use the dependence of the two constants to solve for a single
independent calibration constant, which we show to be sufficiently stable in time to model
the calibration for time periods where no soundings are available. The calibrated
temperature can be used to calculate relative humidity, with errors within 10%
common for up to 6-8 km altitude. The Lidar also gives backscatter coefficients by
calibrating to expected aerosol scatter in clear-sky volumes, and extinction coefficients
as the range derivative of molecular backscatter coefficients. Finally, we measure
depolarization ratios which allow distinguishing water droplets from ice crystals. |
|
|
|
|
|
|