Understanding precipitation fog : fundamental research using field observations and numerical modeling

1. Understandingprecipitationfog:fundamentalresearchusingfield observations and numericalmodeling Robert Tardif Météo-France CNRM

2. Motivation • Need for fundamental research on fog to: • Increase our understandingof important physical mechanisms & interactions driving fog variability • Targeted (process-oriented) verification of models • Improve parameterizationsin NWP models • Identify what needs to be measured & assimilated • For improved forecasts through better statistical& NWP guidance and human integration of information

3. An example… • Precipitation fog: • Common occurrence! (in certain areas -> NE US) • Physics not well understood…what drives fog formation? • Parameterizations in NWP models adequate? • Integrated “zooming in” approach: • Climatology -> overall characterization • Use of dedicated observations -> observe as much as possible @ higher frequency • Use of modelsfor what cannot be measured…

4. Precipitation fog - climatology • Fog type frequency • Diurnal/seasonal variability • f Weak diurnal signal Most common! in NYC region Strong seasonal signal

5. Precipitation fog - climatology Synoptic-scale disturbances Stratiform rain falling in inversions temperature SO S NE cold warm SST gradient

6. Precipitation fog – field observations • Instrumented site – Brookhaven Natl. Lab. • 90-m tower (T, Hum, wind) • Ceilometer • Visibility (3) • Rain gauge • Droplet spectrometer • Microwave profiler • Sonic anemometers (2) • Radiation

7. Precipitation fog – field observations Cloud base • Case study Feb. 6-7 2004 Visibility Fog onset @ 30 m Fog onset @ sfc Precip. rate

8. 2004-02-07 00 UTC Precipitation fog – field observations • Lower tropospheric structure 2004-02-06 12 UTC Appearance of low-level inversion @ fog onset

9. Precipitation fog – rain evap. model • Evidence of warm raindrops evaporating into low-level cold air as mechanism for fog formation (Dolezel 1944, Byers 1959, Petterssen, 1969) • But how can evaporating raindrops be warmer than ambient air? • Inconsistency with widely used assumption of drops at equilibrium…

10. Precipitation fog – rain evap. model • Raindrop temperature • Energy budget of falling raindrop Equilibrium: Balance between latent and sensible heat fluxes Net flux = 0 (steady state) (assume Fv=Fh)

11. Precipitation fog – rain evap. model • Numerical model: • Drop ↔ environment energy exchanges in Lagrangian ref. frame • Evolution of drop temperature and size • Evolution of ambient temperature and water vapor → supersaturation

12. Precipitation fog – rain evap. model • Departure from equilibrium & supersaturation

13. Precipitation fog – rain evap. model • Real case simulation: • Rain microphy. model: • Bins + Marshall-Palmer • Supersaturation → condensation • Fog droplet gravitational settling • Forced by observations: • Precip. rate • Evolution of temperature & humidity profiles • (horizontal advections)

14. Conclusions • Precipitation fog -> common phenomena! • Evaporation of non-equilibrium raindrops an important process • Equilibrium is an exception rather the rule! • Deprature from equilibrium is small, but significant near saturation • Depends on raindrops sizeand vertical gradients of temperature and humidity • Weak supersaturationstypical -> sensitivity to CCN character (size & chemical composition)

15. Perspectives • This work paved the way for: • Need to develop parameterization of rainfall evaporation including non-equilibrium effects • Targetedevaluation of numerical models (ex. representation of inversions + stratiform rain) • Development of nowcastingsystem based on local observations (precip. + lower tropospheric structure) • Radar • UHF profiler • Microwave profiler • TAMDAR profiles • etc…