Tropical Free‐Tropospheric Humidity Differences and Their Effect on the Clear‐Sky Radiation Budget in Global Storm‐Resolving Models. (2nd November 2021)

• Record Type: Journal Article
• Authors: Lang, Theresa; Naumann, Ann Kristin; Stevens, Bjorn; Buehler, Stefan A.
• Is Part Of: Journal of advances in modeling earth systems. Volume 13:Number 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-02
• Subjects: storm‐resolving models -- humidity -- free troposphere -- outgoing longwave radiation; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2021MS002514
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: Reducing the model spread in free‐tropospheric relative humidity (RH) and its response to warming is a crucial step toward reducing the uncertainty in clear‐sky climate sensitivity, a step that is hoped to be taken with recently developed global storm‐resolving models (GSRMs). In this study we quantify the inter‐model differences in tropical present‐day RH across GSRMs, making use of DYAMOND, a first 40‐day intercomparison. We find that the inter‐model spread in tropical mean free‐tropospheric RH is reduced compared to conventional atmospheric models, except from the tropopause region and the transition to the boundary layer. We estimate the reduction to ∼50%–70% in the upper troposphere and 25%–50% in the mid troposphere. However, the remaining RH differences still result in a spread of 1.2 W m − 2 in tropical mean clear‐sky outgoing longwave radiation (OLR). This spread is mainly caused by RH differences in the lower and mid free troposphere, whereas RH differences in the upper troposphere have a minor impact. By examining model differences in moisture space we identify two regimes with a particularly large contribution to the spread in tropical mean clear‐sky OLR: rather moist regimes at the transition from deep convective to subsidence regimes and very dry subsidence regimes. Particularly for these regimes a better understanding of the processes controlling the RH biases is needed. Plain Language Summary: Errors in the humidity and its change with global warming simulated by climate models limit our ability to predict how the climate system responds to an increase in greenhouse gas concentrations. In this study we investigate how large these humidity errors are in recently developed high‐resolution models. We focus on the relative humidity (RH), which measures the amount of moisture in the air compared to what air can hold at a given temperature. We find that the disagreement in the tropics is reduced compared to conventional climate models, but the RH errors still have a considerable effect on the radiation budget. We also investigate in which regions of the tropics a further reduction of errors would be most beneficial. In the vertical, it is the altitude region between about 1 and 10 km. In the horizontal, we find two tropical regimes that are particularly important: Dry regimes with very strong subsidence and moister regimes at the edge of deep convective regimes. Particularly for those regimes a better understanding of the processes that cause the model errors is needed. Key Points: A 40‐day comparison of storm‐resolving models indicates that free‐tropospheric relative humidity (RH) differs less than among conventional models The remaining RH differences still cause a non‐negligible (∼1.2 W m − 2 ) spread in tropical mean clear‐sky outgoing longwave radiation Reducing humidity biases is most beneficial in the lower and mid free troposphere of dry subsidence regimes and near deep convective regimes