Climate Sensitivity Parameter in the Test of the Mount Pinatubo Eruption

Abstract

The author has developed a dynamic model (DM) to simulate the surface temperature change (ΔT) caused by the eruption of Mount Pinatubo. The main objectives have been 1) to test the climate sensitivity parameter (λ) values of 0.27 K/(Wm-2) and 0.5 K/(Wm-2), 2) to test the time constants of a simple first-order dynamic model, and 3) to estimate and to test the downward longwave radiation anomaly (ΔLWDN). The simulations show that the calculated ΔT of DM follows very accurately the real temperature change rate. This confirms that theoretically calculated time constants of earlier studies for the ocean (2.74 months) and for the land (1.04 months) are accurate and applicable in the dynamic analyses. The DM-predicted ΔT values are close to the measured value, if the λ-value of 0.27 K/(Wm-2) has been applied but the λ-value of 0.5 K/(Wm-2) gives ΔT values, which are about 100% too large. The main uncertainty in the Mount Pinatubo analyses is the ΔLWDN flux, because there are no direct measurements available during the eruption. The author has used the measured ERBS fluxes and has also estimated ΔLWDN flux using the apparent transmission measurements. This estimate gives the best and most consistent results in the simulation. A simple analysis shows that two earlier simulations utilising General Circulation Models (GCM) by two research groups are depending on the flux value choices as well as the measured ΔT choices. If the commonly used minimum value of -6 Wm-2 would have been used for the shortwave anomaly in the GCM simulations, instead of -4 Wm-2, the ΔT values would differ from the measured ΔT values almost 100%. The main reason for this error seems be the λ-value of 0.5 K/(Wm-2).