How will precipitation change under global warming? Chia Chou Research Center for Environmental Changes Academia Sinica September 22, 2009 NCU

Figure 3.6 Figure 3.6. Global and hemispheric annual combined land-surface air temperature and SST anomalies (°C) (red) for 1850 to 2006 relative to the 1961 to 1990 mean, along with 5 to 95% error bar ranges, from HadCRUT3 (adapted from Brohan et al., 2006). The smooth blue curves show decadal variations (see Appendix 3.A). IPCC AR4

IPCC AR4

Figure 3.12 Figure 3.12. Time series for 1900 to 2005 of annual global land precipitation anomalies (mm) from GHCN with respect to the 1981 to 2000 base period. The smooth curves show decadal variations (see Appendix 3.A) for the GHCN (Peterson and Vose, 1997), PREC/L (Chen et al., 2002), GPCP (Adler et al., 2003), GPCC (Rudolf et al., 1994) and CRU (Mitchell and Jones, 2005) data sets. IPCC AR4

Figure 3. 14. Precipitation for 1900 to 2005 Figure 3.14. Precipitation for 1900 to 2005. The central map shows the annual mean trends (% per century). Areas in grey have insufficient data to produce reliable trends. The surrounding time series of annual precipitation displayed (% of mean, with the mean given at top for 1961 to 1990) are for the named regions as indicated by the red arrows. The GHCN precipitation from NCDC was used for the annual green bars and black for decadal variations (see Appendix 3.A), and for comparison the CRU decadal variations are in magenta. The range is +30 to –30% except for the two Australian panels. The regions are a subset of those defined in Table 11.1 (Section 11.1) and include: Central North America, Western North America, Alaska, Central America, Eastern North America, Mediterranean, Northern Europe, North Asia, East Asia, Central Asia, Southeast Asia, Southern Asia, Northern Australia, Southern Australia, Eastern Africa, Western Africa, Southern Africa, Southern South America, and the Amazon. IPCC AR4

  Global water vapor budget (Held and Soden 2006):    M: mass flux; q: PBL water vapor  thermodynamic dynamic     

Thermodynamic contribution   changes in moisture  easy part

Figure 3.21 Figure 3.21. The radiative signature of upper-tropospheric moistening is given by upward linear trends in T2−T12 for 1982 to 2004 (0.1 ºC per decade; top) and monthly time series of the global-mean (80°N to 80°S) anomalies relative to 1982 to 2004 (ºC) and linear trend (dashed; bottom). Data are from the RSS T2 and HIRS T12 (Soden et al., 2005). The map is smoothed to spectral truncation T31 resolution. IPCC AR4

 1-3% in P per 1ºC T (model simulations)   <0  <0  slowing of tropical circulation dynamic component   7.5% in q per 1ºC T (Clausius-Clapeyron) thermodynamic component

The Walker circulation is weakened more than the Hadley circulation Vecchi and Soden (2007) The Walker circulation is weakened more than the Hadley circulation

E ≈ LWs+SWs (assuming H is small) P ≈ LW+SW (assuming H is small) Vecchi and Soden (2007) In global average, P = E E ≈ LWs+SWs (assuming H is small) P ≈ LW+SW (assuming H is small)

less latent heat release (precipitation) 地表長波幅射 CO2, CH4, N2O, H2O… 太陽短波幅射 增溫效應 地表吸收 less cooling by LW less latent heat release (precipitation)

Observed precipitation Wentz et al. 2007 7.5% per 1ºC T

Changes in convective and non-convective zones Allan and Soden 2007

Changes in zonal averages Zhang et al. 2007

Chou et al. 2007

enhancement of seasonal cycle in tropical precipitation Chou et al. 2007

Dynamic contribution   changes in circulation  hard part

Classification of the Tropics

Area I (P΄<0)

The upped-ante mechanism

Area IIa and IIb (P΄>0)

The rich-get-richer mechanism (IIa) The deepening effect of convection (IIb)

 >0 or <0  <0   No constraint  7.5% in q per 1ºC T   <0  1-3% in P per 1ºC T (controlled by energy budget)

Chou and Chen 2009

The deeper (shallower) convection, shallower deeper ascent stability Chou and Chen 2009 The deeper (shallower) convection, the more stable, -, (unstable, +) the atmosphere

Area III (P΄>0)

Climate regime shift

Mechanisms of tropical precipitation changes Chou et al. 2009

Conclusion Thermodynamic component ( )  enhance rainfall over convective regions reduce rainfall over subsidence regions rich-get-richer (poor-get-poorer) Dynamic component ( )  reduce over convective margins (upped-ante) enhance in convective center (rich-get-richer) enhanced rainfall but weakened upward motion (deepening of convection)