Modeling of Instantaneous and Adjusted Radiative Forcing of the 2022 Hunga Volcano Explosion.

We used the regional meteorology-chemistry model WRF-Chem with bins sulfate aerosol microphysics to study the climate impact of the Hunga volcano eruption on January 15, 2022. We conduct simulations in the 45S-10N latitude band with periodic boundary conditions in longitude and lateral boundary conditions prescribed from ERA-Interim reanalysis that constrain meteorological fields. The spectral nudging of the horizontal wind components in the stratosphere imposes the QBO. To simulate the Hunga volcano eruption, we injected 150 Mt of water vapor (WV) and 0.45 Mt of SO2 into the middle stratosphere at 35 km. Because of the relatively high stratospheric temperature at that altitude, about 120 Mt of water was retained in the stratosphere. The volcanic water vapor cloud was cooled by thermal radiation and, therefore, descended to 25 km in about two weeks. Both the simulated mass of the remaining WV and the altitude of the volcanic water layer agree with observations. The zonal mean anomaly of volcanic WV mixing ratio averaged over the 30S-10N latitude belt at 25 km exceeded 10 ppmv for two weeks after the eruption. Still, it reduced to 3-4 ppmv in three months. The global water vapor instantaneous LW radiative forcing at the top of the atmosphere (TOA) appears negative, reaching -0.028 W/m2. At the surface, water vapor radiative forcing is two orders of magnitude smaller than at TOA. Volcanic SO2 was oxidized in 3-4 weeks. Sulfate aerosol's effective radius grows to 0.4 mm a month after the eruption but decreases to 0.2 m in 3-4 months. The instantaneous globally averaged radiative forcing of volcanic sulfate aerosols is about one order of magnitude stronger than TOAs water vapor forcing, reaching -0.15 W/m2 a month after the eruption at TOA and surface. Sulfate aerosols absorb SW and LW radiation, warming the stratosphere, but the loss of heat by thermal emission of water vapor cools the stratosphere by 1K. This cooling decreases the outgoing LW flux at TOA and the downward LW flux at the tropopause. As a result, WV's adjusted global LW radiative forcing becomes positive at TOA, reaching 0.04 W/m2 at TOA and the tropopause. The clear-sky SW forcing is not affected by the stratospheric temperature adjustment. A comparison of water vapor radiative forcing calculated using broadband and line-by-line stand-alone radiative transfer models shows that the RRTM broadband model widely used in global and regional models overestimates the WV radiative forcing almost twice. Thus, we found that sulfate aerosols dominate the radiative forcing generated by the Hunga volcano eruption for at least eight months after the explosion. By then, the sulfate aerosols and WV forcings decreased 3-5 times compared to their pick values. The direct Hunga aerosol radiative forcing is about 30 times smaller than that of the 1991 Pinatubo eruption. The direct WV radiative forcing at the surface is negligibly tiny all the time. It cannot activate the slow ocean feedback and, therefore, cannot cause long-term climate perturbations. However, strong stratospheric cooling, associated changes in stratospheric circulation, and ozone depletion might affect tropospheric climate indirectly.