Representing indoor humidity and air-conditioning dehumidification in CLMU BEM

## Scientific Relevance

Urban air conditioning provides both sensible cooling and dehumidification. However, the original Community Land Model Urban (CLMU) building energy model assumes that indoor air is dry and therefore represents only the sensible energy required to reduce indoor temperature. This prevents the model from explicitly simulating indoor humidity, latent heat load, condensate production, and the associated humidity-driven component of urban air-conditioning (AC) energy demand. This limitation is important for climate and energy applications because humidity can substantially increase building cooling loads, alter the sensitivity of AC demand to temperature, and produce demand peaks that do not necessarily coincide with the hottest days.

In this project, we add a physics-based indoor humidity and dehumidification parameterization to the CLMU building energy model. The new scheme:

- prognoses indoor specific humidity;
- calculates sensible cooling and latent dehumidification loads separately; and
- diagnoses condensate production and routes the condensate into urban runoff.

With this new scheme, we find that global urban dehumidification energy demand increases under climate change and that its relative contribution to total AC demand varies strongly across regions. We also find that AC demand can be two to three times higher on mildly hot, highly humid days than on days with similar temperatures but lower humidity. This development extends the urban climate–energy feedback represented by CLMU to include atmospheric moisture and the terrestrial water cycle. It enables CTSM and CESM to investigate humidity-driven urban energy demand, humid-heat risk, and interactions among air conditioning, anthropogenic heat, and climate.

Details of the scientific formulation and evaluation are described in Li et al. (2026), *Divergent humidity-driven growth in urban air-conditioning energy demand under climate change*.

## Plan of Implementation

The development is done in the forked repository and branch: https://github.com/cathyxinchangli/CTSM/tree/dehumidification_pr. It is based on merged changes of ctms5.2.001. It would need to be updated to the latest tags before attempting a merge.

- [x] **Represent indoor humidity and humid-air thermodynamics**

Add prognostic indoor specific humidity and account for both sensible and latent heat exchange through infiltration and exfiltration.

- [x] **Calculate sensible and latent AC loads**

Retain the existing temperature-based trigger for AC use. When cooling is active, calculate the sensible load required to meet the indoor temperature setpoint and the latent load required to meet an indoor relative-humidity setpoint.

- [x] **Update indoor conditions and diagnose condensate**

Update indoor temperature and humidity after AC energy removal. Calculate condensate produced by dehumidification and route it to urban surface runoff to maintain water conservation.

- [x] **Add model outputs and consistency checks**

Provide diagnostics for indoor humidity, sensible and latent AC loads, total AC demand, and condensate. Add checks that verify energy and water balances.

- [x] **Add a namelist toggle**

Add an integer namelist variable, provisionally named `building_humidity_mode`, with the following behavior:

- `0`: use the existing CLMU formulation without indoor humidity or AC dehumidification;
- `1`: enable indoor humidity and latent heat exchange, but no AC dehumidification; and
- `2`: enable indoor humidity, latent heat exchange, dehumidification load, and condensate production.

The default will be `0` to ensure bit-for-bit reproducibility with the original scheme.

## Key Assumptions

- Air conditioning is triggered by indoor temperature going over the temperature setpoint, only then would dehumidification needs be evaluated. Humidity exceeding the humidity setpoint alone will not tigger AC.
- Dehumidification is represented using a relative-humidity setpoint; the current implementation uses 65%, defined in src/main/clm_varcon.F90 as `rh_building_max`.
- Only outdoor moisture entering through infiltration and exfiltration is represented; no indoor humidity sources.
- Condensate is routed to urban runoff because sewer systems are not explicitly represented in CTSM.

## Collaborators

Xinchang “Cathy” Li (@cathyxinchangli), Lei Zhao (@Face2sea), Zhiwen Luo, Keith Oleson (@olyson), Yifan Cheng (@yifanc17), Xiaoxiong Xie, Alvin C. G. Varquez, Mitsuna Sekiya

## References

Li, X., Zhao, L., Luo, Z., Oleson, K., Cheng, Y., Xie, X., Varquez, A. C. G., and Sekiya, M. (2026). Divergent humidity-driven growth in urban air-conditioning energy demand under climate change. *Nature Cities*. https://doi.org/10.1038/s44284-026-00474-4