The HWG experiments indicate that it was more likely for tropical cyclone numbers to decrease in the 2CO2 experiments than in the 2K experiments (Fig. 3a). Zhao et al. (2013a) show that, for several of the HWG models, decreases in mid-tropospheric vertical velocity are generally larger for the 2CO2 experiments than for the 2K experiments (Fig. 3b). For the 2CO2 experiment, the decrease in upward mass flux has previously been explained by Sugi and Yoshimura (2004) as being related to a decrease in precipitation caused by the decrease in radiative cooling aloft. This is caused by the overlap of CO2 and water vapour absorption bands, whereby an increase in CO2 will reduce the dominant radiative cooling due to water vapor. This argument assumes that tropical precipitation rates are controlled by a balance between convective heating and radiative cooling (Allen and Ingram 2002). The simulated decrease in precipitation was combined with little change in stability. In contrast, in their 2K experiment, precipitation increased but static stability also increased, which was attributed to a substantial increase in upper troposphere temperature due to increased convective heating. Yoshimura and Sugi (2005) note that these effects counteract each other and may lead to little change in the upward mass flux, thus leading to little change in tropical cyclone formation rates for the 2K experiment, as seen in their results. A thorough analysis of the HWG experiments along these lines has yet to be performed, however. The 2K and 2CO2 may also have different effects on the intensity of storms. If models are used, it is possible to simulate reasonably well the observed distribution of intensity (see below). The model resolutions of the HWG experiments are in general too coarse to produce a very realistic simulation of the observed tropical cyclone intensity distribution. Nevertheless, some insight into the overall effects of these forcings on intensity of storms can be obtained, particularly when compared with the almost independent PI theory. First, Held and Zhao (2011) showed that one of the largest differences between the results of the 2K and 2CO2 experiments conducted for that paper was that PI
The HWG experiments indicate that it was more likely for tropical cyclone numbers to decrease in the 2CO2 experiments than in the 2K experiments (Fig. 3a). Zhao et al. (2013a) show that, for several of the HWG models, decreases in mid-tropospheric vertical velocity are generally larger for the 2CO2 experiments than for the 2K experiments (Fig. 3b). For the 2CO2 experiment, the decrease in upward mass flux has previously been explained by Sugi and Yoshimura (2004) as being related to a decrease in precipitation caused by the decrease in radiative cooling aloft. This is caused by the overlap of CO2 and water vapour absorption bands, whereby an increase in CO2 will reduce the dominant radiative cooling due to water vapor. This argument assumes that tropical precipitation rates are controlled by a balance between convective heating and radiative cooling (Allen and Ingram 2002). The simulated decrease in precipitation was combined with little change in stability. In contrast, in their 2K experiment, precipitation increased but static stability also increased, which was attributed to a substantial increase in upper troposphere temperature due to increased convective heating. Yoshimura and Sugi (2005) note that these effects counteract each other and may lead to little change in the upward mass flux, thus leading to little change in tropical cyclone formation rates for the 2K experiment, as seen in their results. A thorough analysis of the HWG experiments along these lines has yet to be performed, however. The 2K and 2CO2 may also have different effects on the intensity of storms. If models are used, it is possible to simulate reasonably well the observed distribution of intensity (see below). The model resolutions of the HWG experiments are in general too coarse to produce a very realistic simulation of the observed tropical cyclone intensity distribution. Nevertheless, some insight into the overall effects of these forcings on intensity of storms can be obtained, particularly when compared with the almost PI theory. First, Held and Zhao (2011) showed that one of the largest differences between the results of the 2K and 2CO2 experiments conducted for that paper was that PI independent
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