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The doubled carbon dioxide reduces the model domain mean surface rain rate from PSR to PSRCO2 (Table 2). The comparison in model domain mean cloud budgets between PSR and PSRCO2 shows that two-thirds and one-third of the reduced mean rain rate are associated with the weakened mean net condensation and decreased mean hydrometeor loss, respectively. Since the doubled carbon dioxide affects radiation and the change in latent heat corresponds to the change in net condensation, the model domain mean heat budgets are analyzed to explain the reduction in net condensation by doubled carbon dioxide. The differences in the time and model domain mean heat budget, Eq. (1), between PSRCO2 and PSR (PSRCO2-PSR) in Fig. 3a show a reduced mean latent heat from 2 km to 5.5 km that is associated with the weakened mean net condensation. The reduced mean latent heat corresponds to the decreased mean infrared radiative cooling (Fig. 4a) in the thermal balance. The reductions in mean latent heat and infrared cooling are linked by the increase in the saturation mixing ratio and stable thermal stratification. The change in the saturation mixing ratio by the diurnal variation of radiation is a major factor that is responsible for the diurnal variation of the net condensation and rainfall (e.g., Tao et al., 1996; Gao and Li, 2010; Li and Gao, 2011). The stable thermal stratification is caused by the upward increase in magnitude of the weakened mean infrared radiative cooling. The weakened mean infrared radiative cooling between 13 km and the reduced mean solar radiative heating below 10 km are related to the doubled carbon dioxide. The weakened mean latent heat above 2 km tends to destabilize the surface layer, which enhances the mean latent heat. The weakened mean latent heat from 2 km to 5.5 km is largely offset by the enhanced mean latent heat near the surface, which leads to the small reduction in mean net condensation shown in Table 2. The enhanced mean local atmospheric warming from 6 km to 14 km is primarily associated with the weakened mean infrared radiative cooling (Figs. 3a and 4a).
To generalize the rainfall responses to the doubled carbon dioxide, we carried out four additional pairs of sensitivity experiments during June and July 2008, the same period as PSR and PSRCO2. The integration periods were 0800 LST 10 June to 0800 LST 15 June 2008, 0800 LST 12 June to 0800 LST 17 June 2008, 0800 LST 24 June to 0800 LST 29 June 2008, and 0800 LST 6 July to 0800 LST 11 July 2008. Like PSR and PSRCO2, the four additional sets of sensitivity experiments show the decreases in the mean rain rate in response to the doubled carbon dioxide. The reduction in the mean rain rate is mainly associated with the weakened net condensation.
The doubled carbon dioxide increases the mean rain rate from COARE to COARECO2 (Table 2). The enhanced mean rainfall is associated with the strengthened mean net condensation. The analysis of the mean heat budgets (Fig. 3b) reveals an enhanced mean latent heat above 6 km and near the surface associated with the intensified mean net condensation. Note a large difference in heat budget between 3 km and 6 km in Fig. 3b. Such a large heat-budget difference may result from the vertical shift of heat divergence and latent heat in response to the large vertical wind shear (Fig. 2b) and the vertical shift of the melting level. This can be demonstrated by the similar averages of the heat budgets at these vertical levels in COARECO2 and COARE. The enhanced mean latent heat above 6 km corresponds to the strengthened mean infrared radiative cooling (Fig. 4b) through the reduced saturation mixing ratio and unstable thermal stratification. The increased mean latent heat corresponds to the weakened surface sensible heat. The strengthened mean local atmospheric warming above 6 km is associated with the enhanced mean latent heat that corresponds to the intensified mean infrared radiative cooling (Figs. 3b and 4b).
Figure 3. Vertical profiles of differences between (a) PSRCO2 and PSR (PSRCO2-PSR) and (b) COARECO2 and COARE (COARECO2-COARE) for local temperature change (black), condensational heating (red), convergence of vertical heat flux (green), vertical temperature advection (blue), and radiation (orange) averaged for 5 days and model domain. Units are #cod#x000b0;C d-1.
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The net condensation is largely associated with water vapor convergence. Convective and stratiform rainfall corresponds to water vapor convergence and divergence, respectively. Convective-stratiform rainfall separation relies on magnitudes in radar reflectivity in observational studies (e.g., Churchill and Houze, 1984) or surface rain rate in numerical modeling (e.g., Tao et al., 1993). The partitioning analysis of convective-stratiform rainfall is compared with the analysis of the rainfall separation based on the surface rainfall budget, Eq. (2), developed by (Shen et al., 2010) using COARE simulation data. The comparison study shows that a considerable portion of the convective rainfall is associated with water vapor divergence (Shen et al., 2012a). Thus, in this section we conduct a rainfall separation.
During the pre-summer torrential rainfall event, the doubled carbon dioxide increases the rainfall and the rainfall area of DDL from PSR to PSRCO2 (Table 3). The reduction in rainfall of DDL is associated with the decreases in local atmospheric drying and hydrometeor loss/convergence. The rainfalls of two other rain types associated with water vapor divergence (DDG and MDL) are lower than the rainfall of other rain types, and are generally insensitive to the change in carbon dioxide. For the four rain types associated with water vapor convergence, the rain rates of DCG, MCL, and MCG are higher than that of DCL, while the fractional coverage of DCG, MCL, and MCG is larger than that of DCL. The doubled carbon dioxide reduces the rainfall through the enhanced hydrometeor gain/divergence for DCG and the weakened water vapor convergence for MCG. The doubled carbon dioxide increases the rainfall of MCL via the suppressed local atmospheric moistening. The increased rainfall in MCL is offset by the weakened rainfall in DCG and MCG. These rain types have no contribution to the decrease in the mean rainfall. The doubled carbon dioxide increases the rain rate of DCL through the enhanced local atmospheric drying and the strengthened hydrometeor loss/convergence. The calculations of rain intensity with the rain rate divided by the fractional coverage reveal that PSRCO2 (35.1 mm h-1) has a larger rain intensity than PSR (28.6 mm h-1). (Feng et al., 2011) used a global AGCM to project future precipitation change over China under the A1B scenario (increased greenhouse gas emissions). They found that extreme precipitation increases significantly over southeastern China and that the percentage increase in extreme precipitation is larger than that of mean precipitation. The results here reveal that the increase in extreme rainfall is associated with the doubled carbon dioxide in the PSR case. The partitioning analysis of the PSR case shows that the reduction in the mean rain rate results from the decrease in the rainfall of DDL primarily through the weakened local atmospheric drying. The relationship between rainfall, local atmospheric drying, and infrared radiative cooling in the PSR case is similar to the diurnal rainfall theory developed by (Gao and Li, 2010) in which the enhanced nocturnal infrared radiative cooling leads to the nocturnal rainfall peak through weakened local atmospheric drying associated with the reduced saturation mixing ratio.
(a) Fractional coverage and surface rainfall budget [(b) P S, (c) Q WVT, (d) Q WVF, and (e) Q CM] of seven rain types averaged for 5 days over the model domain in PSR, PSRCO2, COARE, and COARECO2. Surface evaporation (Q WVE) is not shown because it is negligibly small compared to other terms in surface rainfall budget. Units are % for fractional coverage and mm h-1 for surface rainfall budget. (a) PSR PSRCO2 COARE COARECO2 DCL 0.35 0.37 0.17 0.16 DCG 1.81 1.81 1.11 1.51 MCL 7.44 6.88 7.16 9.30 MCG 5.21 4.74 4.65 5.55 DDL 11.86 10.60 9.67 11.62 DDG 2.60 2.31 2.29 3.34 MDL 1.27 1.29 1.02 1.23 (b) PSR PSRCO2 COARE COARECO2 DCL 0.10 0.13 0.07 0.06 DCG 0.25 0.24 0.13 0.15 MCL 0.24 0.27 0.20 0.22 MCG 0.22 0.20 0.13 0.13 DDL 0.46 0.42 0.32 0.32 DDG 0.05 0.04 0.03 0.03 MDL 0.03 0.03 0.03 0.03 (c) PSR PSRCO2 COARE COARECO2 DCL 0.03 0.04 0.02 0.02 DCG 0.20 0.20 0.11 0.14 MCL -0.80 -0.71 -1.02 -1.14 MCG -0.64 -0.56 -0.71 -0.72 DDL 1.38 1.23 1.55 1.61 DDG 0.34 0.25 0.38 0.44 MDL -0.02 -0.03 -0.03 -0.02 (d) PSR PSRCO2 COARE COARECO2 DCL 0.04 0.04 0.02 0.02 DCG 0.39 0.40 0.32 0.35 MCL 0.75 0.69 0.92 1.01 MCG 1.41 1.27 1.33 1.32 DDL -1.57 -1.37 -1.80 -1.89 DDG -0.20 -0.15 -0.28 -0.33 MDL -0.03 -0.03 -0.03 -0.03 (e) PSR PSRCO2 COARE COARECO2 DCL 0.03 0.05 0.03 0.02 DCG -0.34 -0.37 -0.30 -0.35 MCL 0.29 0.28 0.28 0.32 MCG -0.55 -0.51 -0.50 -0.48 DDL 0.64 0.55 0.54 0.56 DDG -0.09 -0.07 -0.08 -0.09 MDL 0.08 0.08 0.09 0.08 During COARE, the rainfall of DDL is insensitive to the change in carbon dioxide, while the rainfall area of DDL expands from COARE to COARECO2. The insensitivity of the rainfall of DDL is caused by the offset between the enhanced local atmospheric drying and strengthened water vapor divergence. Like PSR and PSRCO2, the doubled carbon dioxide barely changes the rainfalls of DDG and MDL during COARE. Among the four rain types associated with water vapor convergence, the doubled carbon dioxide barely changes the rainfall of MCG, increases the rainfalls of DCG and MCL, and slightly reduces the rainfall of DCL. The enhanced rainfall is associated with the increases in local atmospheric drying and water vapor convergence for DCG and water vapor convergence and hydrometeor loss/convergence for MCL. The weakened rainfall of DCL corresponds to the suppressed hydrometeor loss/convergence. Unlike those in the PSR case, the rain intensity decreases from COARE (41.2 mm h-1) to COARECO2 (37.5 mm h-1). The rainfall separation analysis suggests that the enhanced mean rain rate results from the increases in the rainfall of DCG and MCL.