Drakkar Calibration The Drakkar microwave radiometer Calibrating Drakkar

Drakkar Calibration • The Drakkar microwave radiometer • Calibrating Drakkar • The Calibrated Data • Future Work

Drakkar • a microwave radiometer developed at CETP • sensitive to electromagnetic waves emitted at two frequencies, 23. 8 and 36. 5 GHz, by atmospheric water vapour and liquid water • equipped with a reception antenna fixed in the vertical with a field of view varying from 11 to 13 degrees depending on the frequency

Calibrating Drakkar • Using radiosounding observations, during clearsky periods, as the input of a radiative transfer model (Kummerow), we obtained simulated Brightness Temperatures (BTs) which were then compared to the measured BTs. • The Kummerow Model requires, as input, atmospheres including altitude, temperature & relative humidity • High Resolution Radiosoundings were employed (low resolution radiosoundings and ECMWF Data were also compared)

Calibrating Drakkar • The 23. 8 GHz and 36. 5 GHz channels of Drakkar had been calibrated in 2001 • Comparing the simulated to observed BTs during the Vapic Campaign (May-June 2004) we found the 23. 8 channel to still be calibrated while the 36. 5 BTs were 6 to 7 degrees too high • We therefore assume the 23. 8 channel has remained calibrated during the Cloud. NET period • In order to calibrate the 36. 5 channel over the Cloud. NET period we assume that the Liquid Water Path should be equal to zero during periods of clear sky

Drakkar Products • Liquid Water Path LWP = Aa + (Ab*ln(280 -TB 23. 8)) - (Ac*ln(280 -TB 36. 5)) • Water Vapour Path WVP = Ad - (Ae*ln(280 -TB 23. 8)) + (Af*ln(280 -TB 36. 5)) where: Aa = 1636. 36, Ab = 182. 41, Ac = 476. 42 Ad = 66. 13, Ae = 20. 72, Af = 8. 99

• LWP was summed over clear-sky periods (in January 2003) for all combinations of calibration constants varying over a range determined by comparisons made between simulated and observed BTs • This plot represents the solution space of LWP C 36. 5=-1. 8

• LWP was summed over clear-sky periods (in July 2003) for all combinations of calibration constants varying over a range determined by comparisons made between simulated and observed BTs • This plot represents the solution space of LWP • Calibration validated during the Vapic Campaign (May. June, 2004) C 36. 5=-4. 4

Prior to Calibration Post Calibration

The Calibrated Data • Net. CDF files of calibrated data including brightness temperatures, liquid water path, and water vapour path are now available • These files are daily starting in October of 2002 and ending mid-December of 2003 • Files, which will complete the dataset (ending September 2004) will soon be available

Future Work • Utilise the model WVP in order to refine the Drakkar calibration • Intercomparison of BT calibration methods (Nicolas Gaussiat and Anne Armstrong) • Automating the v&w, Rad. On, and Radar-Lidar methods for the three sites • Intercomparison of the Cloud. NET IWC Algorithms • Model Evaluation with the Cloud. NET products