Water Vapour Intercomparison Effort in the Frame of the Convective and Orographically-Induced Precipitation Study: Airborne-to-Ground-based and airborne-to-airborne Lidar Systems R. Bhawar, P. Di Girolamo, D. Summa, C. Flamant, D. Althausen, A. Behrendt, P. Bosser, M. Cacciani, C. Champollion, T. Di Iorio, G. Ehret, C. Kiemle, C. Herold, S. D. Mueller, S. Pal, M. Radlach, A. Riede, P. Seifert, M. Shiler, M. Wirth, V. Wulfmeyer Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Water vapour lidar inter-comparison effort error estimates for the different water vapour lidars based on mutual inter-comparison. main objective Different Instruments airborne and ground-based water vapour lidar systems 2 airborne DIALs: DLR DIAL & CNRS DIAL 4 ground-based: 3 Raman (BASIL, BERTHA, ING) 1 DIAL (UHOH) Simultaneous and co-located data for all lidar pairs to compute relative bias and root-mean square (RMS) deviations Need complete and comprehensive inter-comparison tables

Sample from the intercomparison table for IOP-9c on 20 July 2007

Approach to identify the airborne lidar profiles to be compared with the ground-based lidar profiles Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009 Distance between the aircraft footprint and the ground-based system not exceeding 10 km.

Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Comparisons between DIAL systems have been performed in terms of water vapour number concentration (water vapour molecules per m³), while comparisons between DIAL and Raman lidars are carried out in terms of water vapour mixing ratio. In order to do so, DIAL results expressed in terms of water vapour number concentration must be converted into water vapour mixing ratio. where m H2O is the water molecular mass ( molecular mass units), R is the gas constant of dry air ( J/(gK)), p is pressure and T is temperature. Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Relative bias Relative root-mean square deviation Absolute bias and root-mean square ( obt. through the multiplication by the mean of the data of the two instr.) Overall mean bias and RMS over the whole inter-comparison range (weighted mean)

A total of 25 profile-to-profile inter-comparisons between BASIL and CNRS DIAL 18 inter-comparisons possible with a minimum distance not exceeding 5 km 7 inter-comparisons with a minimum distance between 5 and 10 km Example of comparison at 20:08 UTC on 31 July 07 minimum distance between the two sensors of 1.8 km The two profiles show a very good agreement, with deviations not exceeding 0.25 g/kg. Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

All profiles of bias and RMS deviation between BASIL and CNRS DIAL With the exception of a few points, bias within ± 1 g/kg (or ± 20 %), RMS < 1.5 g/kg (or 30 %). High bias values are often found in coincidence with high values of RMS, especially in daytime comparisons, which may be associated with the large random error affecting BASIL measurements.

Profiles of mean bias and mean RMS deviation between BASIL and CNRS DIAL obtained considering the 25 profile-to-profile intercomparisons Mean bias increase with altitude from -1 % (-0.1 g/kg) to +10 % (+0.1 g/kg), with an intermediate maximum of +5 % (+0.25 g/kg) around 2 km (i.e. up to the top of the boundary layer). Larger bias values are found at the top of the boundary layer, where the effect of in- homogeneities may be larger. Overall mean bias: 2.1 % (or 0.12 g/kg) Overall RMS deviation: 13.7 % (0.82 g/kg) in the altitude region 0.5–3.5 km a.s.l. Overall mean bias: 1.41 % (or g/kg) Overall RMS deviation: % (0.78 g/kg) in the altitude region 0.5–3.0 km a.s.l. Overall mean bias: 1.13 % Overall RMS deviation: 10.1 % in the alt. region 0.5–3.0 km a.s.l. (cut-off dist. 5 km) 8 night-time and 17 daytime. When considering only the night-time comparisons, the analysis can be extended up to higher heights, i.e. up to the CNRS DIAL flight altitude (~4.5 km) Overall mean bias: 1.3 % (or g/kg) Overall RMS deviation: % (0.636 g/kg) in the altitude region 0.5–4.5 km a.s.l.

N.DateTime (UTC)Min. distance (km)BIAS (%)RMS dev. (%) 114 July : July 20076: July : July : July : July : July : July : July : July : July : July : July : July : July : July 20079: July : July : July : July : July : July : August : August : August :

A total of 3 profile-to-profile inter-comparisons between BASIL and DLR DIAL Example of comparison at 16:05 UTC on 18 July 07 minimum distance between the two sensors of 8.9 km The two profiles show a very good agreement, with deviations not exceeding 0.25 g/kg. Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

The mean bias is found to be within ± 3 % up to 3 km or ± 0.3 g/kg all the way up to 3 km. Profiles of mean bias and mean RMS deviation between BASIL and DLR DIAL All profiles of bias and RMS deviation between BASIL and DLR DIAL Overall mean bias: % (or g/kg) Overall RMS deviation: 15.2 % (0.59 g/kg) in the altitude region 0.5–3.5 km a.s.l. Overall mean bias: % (or g/kg) Overall RMS deviation: % (0.4 g/kg) in the altitude region 0.5–3.0 km a.s.l.

A total of 6 profile-to-profile inter-comparisons between BERTHA and CNRS DIAL (only night-time) Example of comparison at 19:26 UTC on 31 July 07 minimum distance between the two sensors of 9.89 km The two profiles show a very good agreement, with deviations not exceeding 0.25 g/kg. Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Profiles of mean bias and mean RMS deviation between BERTHA and DLR DIAL All profiles of bias and RMS deviation between BERTHA and CNRS DIAL Overall mean bias: % (or g/kg) Overall RMS deviation: 23 % (0.662 g/kg) in the altitude region 0.5–4.5 km a.s.l. The mean relative bias is found to vary with altitude from 20 % in the lower height interval to -30 % in the km interval (i.e. up to the top of the boundary layer). The mean absolute bias shows a lower altitude variability, with values in the range -0.3÷0.2 g/kg. Here and above negative values indicate that CNRS DIAL is dried than BERTHA. (Only night-time)

A total of 9 profile-to-profile inter-comparisons between ING Raman Lidar and CNRS DIAL (6 night-time, 3 daytime) Example of comparison at 20:56 UTC on 31 July 07 minimum distance between the two sensors of 0.8 km The two profiles show a very good agreement, with deviations not exceeding 1 g/kg.

Profiles of mean bias and mean RMS deviation between ING Raman Lidar and DLR DIAL All profiles of bias and RMS deviation between ING Raman Lidar and CNRS DIAL Mean relative bias: max. variability in the 2-3 km interval (i.e. up to the top of the boundary layer), up to ± 15 %. Mean absolute bias: -0.2÷1 g/kg. 9 inter-comparisons Overall mean bias: % (or 0.69 g/kg) Overall RMS deviation: % (1.74 g/kg) in the altitude region 0.5–2.0 km a.s.l. 6 night-time inter-comparisons Overall mean bias: 3.18 % (or 0.27 g/kg) in the altitude region 0.5–4.5 km a.s.l.

A total of 4 profile-to-profile inter-comparisons between UHOH DIAL and DLR DIAL Example of compariosn at 07:22 on 15 July 2007 min. distance between the two sensors of 1.5 km The two profiles show a very good agreement, with deviations not exceeding ± 2x10 22 m inter-comparisons Overall mean bias: 2.9 % in the alti

UHOH DIAL CNRS DIAL , 13:46 UTC

A total of 5 profile-to-profile inter-comparisons between CNRS DIAL and DLR DIAL Example of compariosn at 11:53 on 30 July 2007 min. distance between the two sensors of 2.48 km The two profiles show a very good agreement, with deviations not exceeding ± 0.25 g/kg or ± 1.5x10 22 m -3. N.N. DateTime (UTC) Min. distance (km) BIAS (%) RMS dev. (%) tracks 1 18 July : coinciden t, opp. direction 2 18 July : cross 3 30 July : cross 4 30 July : Parallel, same direction 5 30 July : % 19.63parallel, same direction Water vapour heterogeneity plays a major role in the interpretation of the airborne-to- airborne inter-comparisons, with effects being generally more marked than for the ground-based-to-airborne lidar inter-comparisons illustrated before. These effects are mainly due to horizontal averaging and are more severe for isolated crossing points of the flight tracks. 6 night-time inter-comparisons Overall mean bias: 3.93 % (or g/kg or 1.1x10 22 m-3 ) in the altitude region 0.5–4.0 km a.s.l. Smaller bias values are present when comparing data collected along parallel flight tracks and coincident directions for the two air-borne systems

0 5,89%-4.77%-0.05% BASIL BERTHADLR DIALCNRS DIAL 1.52% UHOH DIAL % ING % Derive the overall bias values for all the lidar systems from mutual bias values. This is possible when there is at least one instrument that carried out measurements that are comparable with those of all other lidar systems. This was the CNRS DIAL that, thanks to the several flights performed in the frame of the EUFAR Project H2OLidar, was able to guarantee multiple overpasses over all Supersites equipped with ground-based lidar systems. We attribute equal weight on the data reliability of each instrument and impose the summation of all mutual bias between lidar pairs to be zero. Overall relative values for UHOH DIAL, DLR DIAL, IGN Raman lidar, BASIL, CNRS DIAL and BERTHA are found to be %, %, %, 0.05 %, 1.52 % and 5.89 %, respectively. All sensors are found to be characterized by a overall bias not exceeding 5 %.

An intensive inter-comparison effort involving 6 water vapour lidar systems was carried out in the frame of COPS with the goal of providing accurate error estimates for these systems. A total of 57 profile-to-profile inter-comparisons involving all possible lidar pairs were considered. Results reveal the presence of low systematic errors (bias) – not exceeding 5 % - in the measurements carried out by all lidar systems operated during COPS. Specifically, overall relative values for the involved lidar systems are found to be: %, %, %, 0.05 %, 1.52 % and 5.89 % for UHOH DIAL, DLR DIAL, IGN Raman lidar, BASIL, CNRS DIAL and BERTHA, respectively. For what concerns the airborne-to-ground-based inter-comparisons, there appear to be no evident dependency of the bias and RMS deviation on spatial distance between the different lidar pairs. Concerning to the airborne-to-airborne inter-comparisons, results indicate that smaller bias values are present when comparing data collected along parallel flight tracks and coincident directions for the two air-borne systems. Summary Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Airborne DIAL vs Ground-based Lidar In order to reduce statistical fluctuations, for the purpose of the present inter- comparison, we considered: for the CNRS DIAL an integration time of 80 sec, corresponding to an horizontal integration length of km (vert. res.: 250 m). for the DLR DIAL an integration time of 50 sec, corresponding to an horizontal integration length of km (vert. res.: 300 m; step: 25 m). for BASIL an integration time of 1 min for night-time measurements and of 5 min for daytime measurements (vert. res.: 150 m; step: 30 m). for the UHOH DIAL an integration time of 5 min. for the BERTHA Raman Lidar an integration time of 3 min (vert. res.: 150 m; step: 60 m). for the ING Raman Lidar an integration time of 15 min. Joint 8th COPS Workshop and CSIP Meeting 2009, October 2009

Coming to the airborne-to-airborne inter- comparisons, results indicate that smaller bias values are present when comparing data collected along parallel flight tracks and coincident directions for the two air- borne systems. Comparison of data from airborne and ground-based lidars has the potential to allow assessing the representativeness error of vertically pointing ground-based lidar systems, and in general of ground- based remote sensors, used for satellite data validation, and assess the sub-grid scale variability of water vapour usually parameterized in mesoscale atmospheric models.