Cloud Parcel Model – Part 0: Introduction and Context (A)

In this introductory part of the Cloud Parcel Modelling series, we clarify what conditions define the beginning of a cloud parcel simulation. When does the model start? What processes are included — and what’s left out? This foundational understanding is crucial before diving into growth equations and microphysical feedbacks.

So, the cloud parcel model is conditioned after CCN particles activate?

✅ A Cloud Parcel Model typically starts from the point when:

πŸ’§ The rising air parcel becomes supersaturated (RH > 100%)
➕ Cloud Condensation Nuclei (CCN) begin to activate into cloud droplets

πŸ”„ Step-by-Step View of What the Cloud Parcel Model Covers:

  1. Air parcel rises due to updraft

  2. Cooling occurs due to adiabatic expansion

  3. Relative humidity increases

  4. At RH ≈ 100%, the supersaturation (S > 0) triggers:

    • Some CCN → activated

    • Droplet formation begins

  5. From that point on, the parcel model:

    • Simulates condensation growth of activated droplets

    • Tracks latent heat release, supersaturation evolution, temperature, droplet size distribution, etc.

❌ The model does not explicitly simulate:

  • CCN behavior before activation (e.g. water uptake at RH < 100%)

  • Hygroscopic swelling of sub-saturated particles

  • Deliquescence point of salts (NaCl, (NH₄)₂SO₄, etc.)

πŸ” Terminology Clarification:

  • CCN = Cloud Condensation Nuclei
    → Aerosol particles that can potentially activate

  • Activated Droplets
    → CCN that have surpassed their critical radius and are now growing as real cloud droplets (typically >0.1 Β΅m radius)

🧠 Summary:

Yes, a standard cloud parcel model begins its detailed physics after CCN activation.
It assumes that only those CCN that activate will significantly interact with the supersaturated water vapor and affect cloud microphysics.

  • How about the large unactivated CCN particles that looked like as cloud droplet? 
  • Is it included or identified or tracked in cloud parcel model?

🧊 Large Unactivated CCN That Appear Like Cloud Droplets:

These are hygroscopic aerosol particles (like salts or organics) that:

  • Have not yet reached activation (critical supersaturation)

  • But have grown significantly due to water uptake at high RH (e.g. >95%)

  • May look like cloud droplets under a microscope or lidar, especially if their size > ~0.1 Β΅m

πŸ‘©‍πŸ”¬ Are They Included or Tracked in Cloud Parcel Models?

✅ Explicitly?

No, standard cloud parcel models do not explicitly track these unactivated but swollen particles.

❌ Why?

Because the parcel model:

  • Only tracks droplets that have passed the KΓΆhler activation point

  • Focuses on condensational growth after activation

  • Assumes that only activated droplets:

    • Contribute to latent heat release

    • Contribute to water vapor depletion

    • Contribute to cloud optical depth

πŸ”¬ However, some advanced models may:

  • Include hygroscopic growth of unactivated particles in separate aerosol dynamics modules (e.g., bin models or modal schemes)

  • Use this information to:

    • Refine optical property predictions (important in remote sensing)

    • Estimate activation fraction more accurately

But this is not part of the standard parcel model framework.

🌧️ In Observations:

  • Instruments like CCN counters or cloud probes may misclassify large unactivated particles as cloud droplets

  • This causes ambiguity in measured droplet number concentration (Nd) or cloud base


πŸ“Œ Summary Box

🧊 What about large unactivated particles that absorb water?

These particles may grow significantly at high RH, but they do not count as activated droplets in the parcel model.

Standard parcel models track only activated CCN, because these droplets:

  • 🌑️ Release latent heat
  • ⚡ Grow rapidly
  • ☁️ Dominate cloud optical and microphysical properties

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