A climatology of cirrus clouds from ground-based lidar measurements over Lille

Transcription

1 A climatology of cirrus clouds from ground-based lidar measurements over Lille Rita Nohra, Frédéric Parol, Philippe Dubuisson Laboratoire d Optique Atmosphérique université de Lille, CNRS UMR 8518

2 Objectives To develop a methodology of detection and characterization of cirrus clouds from the micropulse lidar measurements To establish a climatology of macrophysical and optical properties of cirrus cloud during the period 2008/2013 To compare cirrus cloud properties over Lille to other sites and to spaceborne lidar Caliop 2

3 Introduction High level clouds, composed entirely of ice crystals Cirrus clouds cover about 30 % of the earth s surface Cirrus clouds play a major role in the climate system, they can result in a warming or a cooling effect Active remote sensors such as lidars or radars can be used to quantify the cloud properties 3

4 Ground-based lidar Lille (Cimel) Light detection and ranging (lidar) Active remote sensing information is obtained from a signal which is sent from a transmitter, reflected by a target, and detected by a receiver back at the source Lidar in LOA (since 2006) A pulse ND : YAG laser (532 nm), Q Switch 24 hours a day, 7 days a week Energy : 20 μj, frequency : 4,7 KHz Zmin = 250 m Altitude resolution = 15 m Evaluate the potential of a micropulse lidar for the study of high altitude clouds: - less expensive - Mobile - Possibility multi sites 4

5 Methodology backscattered power lidar: P z = K β(z) Z z α(z)dz 2 e 2 0, Three unknowns β : backscattering coefficient α : extinction coefficient k : system constant β(z)= β m (z) + β p (z) et α (z) = α m (z) + α p (z), m : molecule and p : particle Solution?? Assumptions in the lidar equation β m et α m are calculated theoretically 5

6 Methodology 1- Calculation of the system constant K 2- Calculation of the theoretical molecular signal 3- Determination of cirrus cloud boundaries (base altitude, top altitude, geometrical thickness) 4- Restitution of effective cloud optical thickness (COT) and effective lidar ratio (LR) 5- Multiple scattering parameter 6- Retrieving COT and LR 6

7 Methodology : case study 2 septembre 2011 Altitude (km) Reference altitude z 0 between 2 et 6 km, (absence of cloud and aerosol) calculate the theoretical molecular signal Determination of cloud boundaries Platt et al., 1994 lidar signal theoretical molecular signal Linear fit COT : transmittance method COT = z(sommet) z(base) α(z) dz LR : forward klett inversion (solution for z>z 0 ) LR= α p /β p Base=9.95 km top=11.29 km COT=0.5 LR = 19 sr Ln (p(z).z 2 ) Case study 2 septembre 2011 (20h) 7

8 % cas traitables Results : climatology of cirrus clouds over Lille during the period nuit jour detected cases treated cases Day % Night % sub-visible cloud (SVC ) : COT <0.03 visible cloud (VC) : 0.03 < COT< 0.3 opaque : COT > 0.3, (Sassen et Cho, 1992) Max : 1.2 Min : SVC VC opaque COT Class jour nuit Distribution of different classes of cirrus clouds 56 % of observed cirrus clouds are "visible" cirrus detected during the night month Number of cirrus occurrences for 2008/2013 as a function of month A maximum of observed cirrus clouds in Lille in autumn (35%) and a minimum in winter (19%) and intermediate in spring (22%) and summer (24%). 8

9 Results : relation between optical and geometrical thickness 87% of the studied cases have a thickness between 0.5 km and 2.5 km 0.6 A linear relation between the geometric and optical thickness COT The geometric thickness increases with the optical thickness Homogeneous cirrus cloud: mean extinction coefficient, 0.15 km-1 Correlation 0.33 close to the value 0.38 obtained during ICE, 89 in Norderney (53.72 N, 7.22 E ), (Ansmann et al., 1993) thickness (km) Dependance of optical depth on 0.5 km intervals of thickness 2ème journée scientifique CaPPA 9

10 Caliop products Ground-based lidar Lille (51.61 N, 3.14 E) 51 N 2 E 5 E 49 N 130 overpasses/year (day & night ) 3 channels lidar : 532 nm (// and ), 1064 nm Caliop : level 2 cloud product Horizontal resolution: 1/3, 1 and 5 km Vertical resolution : 30 m (altitude < 8.2km ), 60 m (between 8.2 and 20.2 km) 10

11 Comparison of Macrophysical cirrus cloud parameters derived from ground-based and caliop lidar during the period 2008/2013 % Day Lille : 9.8 ± 1.3 km Caliop : 9.3 ± 1.2 km % Night Lille : 9.7 ± 1.3 km Caliop : 9.3 ± 1.1 km CALIOP Ground-based lidar Lille Similar distribution Mean difference : 0.1 to 0.5 km Altitude (km) Day Altitude (km) Distribution of cloud base height Lille/Caliop Night Lille : -55 ± 7 C Caliop : -54 ± 6 C Night Mean difference temeprature of 1 C Lille : 11.1 ± 1.3 km Caliop : 10.7 ± 1.3 km Lille : 11.1 ± 1.3 km Caliop : 11.0 ± 1.2 km % % % Altitude (km) Altitude (km) Distribution of cloud top height Lille/Caliop Temperature ( C) Ditribution of Cloud Mid-temperature 11

12 Base height (km) Top height (km) Comparison of Macrophysical cirrus cloud parameters derived from ground-based and caliop lidar during the period 2008/2013 Average difference (Lille/Caliop) night day altitude base (km) altitude top (km) Lille Caliop Tropopause altitude 11.3 ± 1.4 Km 11.3 ± 1.1 Km Average temperature -59 ± 6 C -60 ± 5 C of tropopause Cirrus clouds are higher in summer, and lower in winter month month Monthly variation of base and top altitude of cirrus cloud over Lille 12

13 Conclusion An algorithm for detecting and characterizing of cirrus cloud over Lille from micropulse lidar measurements has been developed Optical and geometrical characteristics of cirrus clouds over Lille have been determined Comparison of results obtained from caliop and the ground-based lidar : - similar results for geometrical properties: base altitude, top altitude, geometrical thickness, temperature -Slightly difference for the optical properties due to the multiple scattering parameter, algorithm caliop: Ƞ = 0.6 Comparison of cirrus cloud properties over Lille to other sites Evaluate the potential of micropulse lidar to study cirrus clouds with some limitations: -limit value of cirrus optical thickness of no more than 4 thin cloud layers - attenuation of the lidar signal in the presence of low thick layers - very sensitive to the solar background noise 13

14 Thank you 14

15 Summary of the methodology Determination of different cloud layers, (4 layers) Selection of cirrus clouds Lidar signal (averged over 3 min) Refernce altitude (between 2 and 6 km) Calculation of theoretical molecular signal Determination of cloud boundaries Base altitude > 6km Cloud temeprature <-38 C Atmospheric sounding at Brussels at midnight (university of wyoming) Linear fit Cirrus Linear fit Geometrical properties It is limited to determining the altitude base of cirrus cloud and apparent top altitude Geometrical and optical properties Calcuationn of optical thickness (COT) Restitution of lidar ratio (LR) Inversion de klett 15

16 Slides supplémentaires: Caractéristiques techniques Lidar Cimel Emetteur Qswitch actif Nd YAG laser Longueur d onde 532 nm Puissance laser 38 mw Energie en sortie laser 14 μj Répétition du pulse 4.7 KHz Largeur du pulse laser <15 ns Largeur du pulse laser imposé Ti 100 ns Nombre d intégration des tirs 4096 Ouverture télescope 200 mm Divergence du faisceau 55 μrad Résolution verticale (CTi/2) 15 m Altitude maximal de visée 30 km Bande passante filtre réception 0.2 nm détecteur Photodiode avalanche Mode de détection Compteur de photons Temps d acquisition (2048 portes*100 ns*4096 >0.8s Masse Taille télescope Electronique Puissance électrique 30 Kg 220*1000 mm Rack 6U 230 V (+-20%) 50 Hz (+-10%) 300 W Transfert PC USB 17 february

17 Caractéristiques techniques : Caliop 17 february