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Held-Soden-Final - Robust Responses of the Hydrological...

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Unformatted text preview: Robust Responses of the Hydrological Cycle to Global Warming Issac M. Held and Brian J. Soden Presentation by: Eric, Kim, Matt, Patricia, Xiaofeng, and Zoe What is the hydrological cycle? The cycling of water between various reservoirs in the Earth system (i.e. the atmosphere, the ground, lakes and rivers, glaciers, ice sheets, oceans, and the crust and mantle) Mass of water in Residence Ame of water = _the atmosphere_ _30 kg m-2_ 10 days in the Mean rainfall rate 1 m per year atmosphere averaged over Earth's surface What are the dominant terms in the moisture budget? Averaged over the globe, EM PM = 0 For a limited region, EM PM = Tr As the surface temperature increases, EM and PM will increase as described in Held and Soden (2006) Held and Soden (2006) summarize that as the climate warms: Lower tropospheric water vapor increases (according to the Clausius-Clapeyron equaAon) The strength of the hydrological cycle increases (PM and EM) Exchange of mass between the boundary layer and the mid-troposphere decreases Atmospheric circulaAon slows down Increase in horizontal vapor transport Equatorward latent heat transport increases and is offset by an increase in poleward dry staAc energy transport Using the Clausius-Clapeyron (CC) EquaAon: ! !!! !"#!! ! !!!!!"! ! ! ! ! !!!!!! !! !" !" !! ! ! !!"!!"#! SaturaAon vapor pressure increases by 6.4% for each 1 K increase in temperature. Change in PrecipitaAon vs. Change in Water Vapor Unlike water vapor, change in precipitaAon does not follow the CC relaAonship - Every model shows an increase in water vapor and in precipitaAon, but models that show a larger increase in water vapor doesn't correlate to larger increases in precipitaAon. Change in precip is due to the increase of net radiaAve flux at the surface, along with the decrease in Bowen's raAo (B = QH/QL, where QH is sensible heat, and QL is latent heat) From these effects, precipitaAon is expected to increase by about 2% What happens to convecAon if precipitaAon doesn't increase as much as water vapor? P=Mq, P increases slightly, but Q increases with CC scaling. So M decreases rapidly, but less than CC scaling rate. ! !! ! !! ! !! ! ! ! !"!! ! !!!"!! ! !!!!!!!!! ! !" ! !" ! !" Greenhouse warming and CC Scaling l Key is to idenAfy aspects of the atmosphere that correlate with es 1) Increased column integrated water vapor 2) Enhanced pagern of E-P -More precip in areas of high precip, less in dryer regions 3) Mixing raAo P = Mq (P = precip, M = mass exchange per unit Ame, q = mixing raAo) Greenhouse warming and CC Scaling l Factors that do not scale with CC 1) P and E, which scale by 2% rather than 6% -Discrepancy likely due to cloud feedbacks 2) Zonal mean Hadley Cell does not weaken as much as CC scaling suggests -the redistribuAon of convecAon is such as to increase the variance of the zonal mean, thus not weakening the Hadley Cell as much 3) Decreased convecAve mass fluxes -scales by ~5% rather than 7% Why precipitaAon doesn't increase homogeneously everywhere? The result for P-E in the moisture transport is If we remove T from the derivaAve, assuming that P-E has more meridional structure than T, then P-E saAsfies the CC scaling: The pa/ern of P-E is simply enhanced, becoming more posi;ve where it is already posi;ve and more nega;ve where it is more nega;ve (i.e. we/er regions get we/er and drier region get drier). The modified version (6) has the accidental advantage in this regard that it predicts that P-E will simply remain small where it is already small. DistribuAon of (P-E) This simple thermodynamic constraint is clearly an important component of many regional changes, at least for subtropical to subpolar laAtudes. It might provide a useful first approximaAon outside of the deep Tropics. 2nd Example: Droughts and floods can be though of as produced by low-frequency variability in the flow field and therefore in the moisture transport. Using same analysis, then the intensity of both floods and droughts will increase, as more water is transported by regions of anomalous vapor convergence to the region of anomalous vapor divergence. Fig. 7. The annual-mean distribuAon of (P-E) from the ensemble mean of (a) PCMDI AR4 models and (b) the thermodynamics component predicted from (6) from the SRES A1B scenario. In addiAon... From Isaac Held's Blog: 13. The strength of the hydrological cycle. Regional precipitaAon changes are certainly sensiAve to regionally specific changes in circulaAon as well. Water Vapor ObservaAons Weather Balloon records over land can produce soundings Microwave satellite measurements EvaporaAon - PrecipitaAon Rain gauges at sites usually airports EvaporaAon using Eddy covariance instrument Greenhouse warming and CC Scaling (extra slides) l 1) Increased column integrated water vapor 2) Increased horizontal moisture transport 3) Enhancement of E P and it's temporal variance 4) Decrease in horizontal sensible heat fluxes in the extra-tropics 5) mixing raAo 6) Increasing of dry stability in the model Tropics 7) FracAonal reducAon in staAonary eddy component of the spaAal variance of the convecAve mass flux in the Tropics 8) When ignoring T in the P - E equaAon : (P-E) = - (TF ) becomes : (P-E) = T(P-E) 9) Oceanic differenAal storage plus transport Factors that do scale with CC Greenhouse warming and CC Scaling (extra slides) l Factors that do not scale with CC 1) P and E (strength of the hydrological scale) scale by ~2% 2) FracAonal reducAon in the zonal mean component of spaAal variance of convecAve mass flux in the Tropics 3) Zonal mean Hadley Cell does not weaken as much as CC scaling would suggest 4) Decreased convecAve mass fluxes Appendix: Figures ...
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