Toulouse IHOP meeting 15 June 2004 Water vapour variability within the growing convective boundary layer of 14 June 2002 with large eddy simulations and observations OUTLINE  Methodology and Objectives  Observational data: several scales of variability  LES simulations: comparisons obs/model  Conclusions & Perspectives Fleur Couvreux Françoise Guichard, Jean-Luc Redelsperger, Cyrille Flamant, Jean-Philippe Lafore, Valéry Masson r v (g/kg) W (m/s) ENE WSW Time (UTC)

Toulouse IHOP meeting 15 June 2004 Methodology & Objectives  LES : Is such a high resolution model able to represent the observed water vapour variability ?  first goal : from observations and simulations, to explain the mecanisms responsible for the water vapour variability  second goal: to understand the role of such variability on cloud formation and maintenance  IHOP observations: What are the fluctuations of water vapour mixing ratio observed within the growing convective boundary layer? 14 june 2002: BLE case What are the scales involved ? Is this day typical?

Toulouse IHOP meeting 15 June 2004 Time 12h 13h 14h 15h 16h 17h 18h 19h  (K) Zi (km) Time 12h 13h 14h 15h 16h 17h 18h 19h 2 morning Early afternoon Boundary layer mean value of  & r v Time in UTC=local time+5h r v (g/kg) A classic convective BL growth … but with large fluctuations of r v Zi (km)

Toulouse IHOP meeting 15 June 2004 Main characteristics : « Relatively » simple case of a growing boundary layer: a well mixed boundary layer reaching 1.5 km High Pressure system, homogeneous temperature field Weak subsidence constant whole day Low shear and weak wind (< 5m/s) from N to NE Existence of thermals (cf Cloud radar) Small cumuli developed after 1500 UTC NE/SW moisture gradient Heavy precipitation the day before, coherent distribution with moisture gradient The 14 june 2002 case Data : Soundings (35) Lidars (DLR-DIAL, LEANDRE et SRL) In-situ aircraft data (NRL-P3 et UWKA) Surface flux measurements (ISSF)

Toulouse IHOP meeting 15 June 2004 Different scales of variability: evidence in soundings 2 soundings separated from less than 10 km 1g.kg UTC Soundings : Different variability at different scales rvrv  Soundings in a 200 km wide domain 1130 UTC 1700 UTC 3 g/kg 5 g/kg rvrv rvrv

Toulouse IHOP meeting 15 June 2004 EW Different scales of variability : aircraft data and lidars + rvrv 3g.kg -1 rv’rv’ 1.5 g.kg -1 Aircraft r v measurement 1710Time (UTC) Time (UTC) 1745 rvrv r v measured by the DLR lidar Height (m) Height (m) WSW ENE Height (m)

Toulouse IHOP meeting 15 June 2004 Modelling: LES with Méso-NH (Lafore et al. 1998) Simulation :  x=  y=100m,  z streched (< 50m in BL) 3D turbulence scheme (Cuxart et al. 2000) early morning to early afternoon (duration 7h) a ‘realistic’ simulation: ISFF2 surface fluxes (prescribed) homogeneous initial sounding = composite of soundings at 1130 UTC large scale advection estimated from MM5 simulations and soundings Initial profile observations  rvrv

Toulouse IHOP meeting 15 June 2004 Mean profiles Temporal evolution of mean profiles  rvrv Comparison obs/LES at 1800 UTC  rvrv

Toulouse IHOP meeting 15 June 2004 Time variations of boundary layer characteristics Sensitivity to surface fluxes : +Bo -> +z i Sensitivity to large- scale advection : +ADV -> +z i +Bo -> +  m +Bo -> + qm Cf Bo ie SSH et SLH +ADV -> +  m +ADV -> + qm Several factors : Ws -> z i -> , q Adv  ->  -> zi Adv q -> q … Validated reference simulation, quantification of sensibility to different forcings rvrv  zi

Toulouse IHOP meeting 15 June 2004 Horizontal cross sections Z/zi=0.3 Z/zi=0.8 Z/zi=1. Z/zi=0.8 Z/zi= W (m/s)  v (K) r v (g/kg) 10 km

Toulouse IHOP meeting 15 June 2004 Characteristic length scale Los C(  )= Reference simulation at 17h r v length scale is larger than length scale of  v, , w from Lohou et al. (2002) (m) z/zi Rv   v w

Toulouse IHOP meeting 15 June 2004 LEANDRE and LES horizontal cross-sections Measurements from LEANDRE LES Simulations At 1600 UTC r v -r v  rv ~10 km

Toulouse IHOP meeting 15 June 2004 Vertical cross sections Evidence of dry downdrafts Several thermals in one humid zone LES r v & w DLR-DIAL r v

Toulouse IHOP meeting 15 June 2004 Evaluation of histograms of , r v, w P3 aircraft KA aircraft Z=0.4z i model … max --- min w’ ’’ rv’rv’ Dry downdraft Overshooting updrafts zi Equivalent gaussian

Toulouse IHOP meeting 15 June 2004 Boundary layer precipitable water

Toulouse IHOP meeting 15 June 2004 Second order moments __ simulation __ lidar DIAL o P3 o KA Strong relation between The inversion strength and the variance Max(  rv ) (g.kg -1 )  q à z i (g.kg -1 ) MNH 4.4 MNH 4.3 DIAL 3.9 DIAL 4.1 DIAL 3 lidar 10km-long Cross-section at 1730 UTC LES 1700 & 1800 UTC rvrv

Toulouse IHOP meeting 15 June 2004 Joint probability distribution At z/zi=0.3 + z/z i + larger spectrum of w + and q - At z/zi=0.8

Toulouse IHOP meeting 15 June 2004 Conclusions:  Evaluation of the LES Able to represent the variability observed at scales lower than 10 km (comparisons to soundings, lidars (DIAL et SRL), aircraft time-series) Quantification the impact of scales > 10 km on variability at scales < 10km  At first order, the boundary layer dynamics explain the observed variability at scales lower than 10 km even without surface heterogeneities and variability in the initial atmospheric state  Dry narrow downdrafts as a signature of the BL growth (via dynamics at the top) [Crum et al. (1987) and Weckwerth et al. (1996)]  impact on length scale, skewness, vertical transport..  Negative skewness is common (cf other IHOP days)  Observations from 14 juin 2002 during IHOP_2002 : Several scales of variability ( 10 km)

Toulouse IHOP meeting 15 June 2004 Perspectives:  Systematic analysis of IHOP data : - objective : to identify pertinent parameters controlling the water vapour variability in the boundary layer (such as strength of inversion ( ,q), fluxes…) from more idealised simulations -> 1D parameterizations  Understand the impact of such a variability on cloud formation and maintenance  Quantify time scales concerned by mechanisms involved in the water vapour variability: dry intrusion life time, transient state

Toulouse IHOP meeting 15 June 2004 F I N

Toulouse IHOP meeting 15 June 2004 Development of the CBL ( courtesy of Bart Geerts) aspect ratio: 1: UTC 1415 UTC 1530 UTC 1630 UTC 1730 UTC

Toulouse IHOP meeting 15 June 2004 Surface fluxes Sensible heat flux Latent heat flux Bo~1. Bo~0.5

Toulouse IHOP meeting 15 June 2004 Large scale forcings (advection) Large-scale forcings Deduced from MM5 Horizontal advection of  Horizontal advection of r v Subsiding w