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Fire-related Weather and Climate in Theory, Observations and Climate Model Simulations

Wenchang Yang

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Group Meeting

Oct 29, 2025

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Key thermal variables from weather and climate

  • Temperature: tas, tasmin, tasmax
  • Humidity: vapor pressure (e), specific humidity (q), relative humidity (RH)
  • Vapor pressure deficit (VPD):

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Clausius-Clapeyron relationship

https://asclimateservices.org/code-and-equations/clausius-clapeyron/

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Wet gets wetter, dry gets drier

Held and Soden, 2006, J. Climate

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Obs. (e.g. ERA5) contradicts CMIP6 historical simulations in dry regions.

Simpson et al. 2024, PNAS

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CMIP6 historical

Obs

Vapor pressure (e)

increase

decrease

Relative humidity (RH)

no trend

decrease

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Vapor pressure in AM2.5C360: no trend

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CMIP6 historical

Obs

AM2.5C360

e

increase

decrease

no trend

RH

no trend

decrease

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RH in AM2.5C360: decreases but less than Obs.

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CMIP6 historical

Obs

AM2.5C360

e

increase

decrease

no trend

RH

no trend

decrease

decrease

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Spatial patterns also show large spread

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VPD in AM2.5C360: increases but less than Obs.

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Also large spread in spatial patterns

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Tas in AM2.5C360: increases and resembles Obs.

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Spatial patterns show less degree of spread

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VPD trend in AM2.5C360: JAS season

Chiodi et al. 2021, GRL

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VPD trend in AM2.5C360: impact from warming

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VPD in AM2.5C360: impacts from various forcings

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Summary

  • AM2.5C360 shows less bias than CMIP6 coupled models in simulating trends of fire-related weather and climate over the southwest US.
  • Different ensemble members show large spread.
  • Large internal variability or missing key processes especially from the land component?

Held, I. M., and B. J. Soden, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate, 19, 5686–5699, https://doi.org/10.1175/JCLI3990.1.

Chiodi, A. M., Potter, B. E., & Larkin, N. K. (2021). Multi-decadal change in western US nighttime vapor pressure deficit. Geophysical Research Letters, 48, e2021GL092830. https://doi.org/10.1029/2021GL092830

I.R. Simpson, K.A. McKinnon, D. Kennedy, D.M. Lawrence, F. Lehner, & R. Seager, Observed humidity trends in dry regions contradict climate models, Proc. Natl. Acad. Sci. U.S.A. 121 (1) e2302480120, https://doi.org/10.1073/pnas.2302480120 (2024).

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backup slides

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AM2.5 vs. AM2.5C360: multi-decadal trends

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AM2.5 vs. AM2.5C360: idealized experiments

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