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Understanding precipitation fog : fundamental research using field observations and numerical modeling Robert Tardif Météo-France CNRM Motivation Need for fundamental research on fog to: Increase our understanding of important physical mechanisms & interactions driving fog variability

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Understandingprecipitationfog:fundamentalresearchusingfield observations and numericalmodeling

Robert Tardif

Météo-France

CNRM

motivation
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
an example
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…
precipitation fog climatology
Precipitation fog - climatology
  • Fog type frequency
  • Diurnal/seasonal variability
  • f

Weak diurnal signal

Most common!

in NYC region

Strong

seasonal

signal

precipitation fog climatology5
Precipitation fog - climatology

Synoptic-scale disturbances

Stratiform rain falling in inversions

temperature

SO

S

NE

cold

warm

SST gradient

precipitation fog field observations
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
precipitation fog field observations7
Precipitation fog – field observations

Cloud base

  • Case study

Feb. 6-7

2004

Visibility

Fog onset @ 30 m

Fog onset @ sfc

Precip. rate

precipitation fog field observations8

2004-02-07 00 UTC

Precipitation fog – field observations
  • Lower tropospheric structure

2004-02-06 12 UTC

Appearance of low-level inversion @ fog onset

precipitation fog rain evap model
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…
precipitation fog rain evap model10
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)

precipitation fog rain evap model11
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
precipitation fog rain evap model12
Precipitation fog – rain evap. model
  • Departure from equilibrium & supersaturation
precipitation fog rain evap model13
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)
conclusions
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)
perspectives
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…