Changes In Convective Available Potential Energy And Convective Inhibition And Their Linkage With Precipitation Under Global Warming

Convective available potential energy(CAPE)has been widely used to quantify atmospheric instability or the positive buoyancy that would be experienced by a lifted parcel,while convective inhibition(CIN)represents the energy needed to lift a parcel to above the level of free convection.Both CAPE and CIN directly affect the occurrence frequency and intensity of atmospheric convection and thus convective precipitation.As air temperature and water vapor increase under greenhouse gas(GHG)-induced global warming,CAPE is expected to increase and CIN would also change.These CAPE and CIN changes would affect the changes in extreme precipitation and thus have important implications for the study of future changes in extreme weather and climate events,such as flooding and drought.Here,the 6-hourly reanalysis data are first analyzed to examine recent CAPE and CIN climatology and changes and the performance of a fully coupled global climate model in simulating the CAPE and CIN climatology is briefly evaluated.The future changes in CAPE and CIN and the underlying causes are then examined.Furthermore,we present the model-simulated future precipitation changes using 3-hourly data and their link to the future atmospheric thermodynamic changes is examined.1.Recent climatology and trends of CAPE and CINReanalysis data show that the climate annual-mean CAPE decreases poleward with maxima over the tropical oceans while the mean CIN is stronger over land than over ocean.CAPE show significant negative trends over most oceans(especially over the central tropical Pacific)and some low-latitude land areas(i.e.,over the central Africa)but positive trends over central America,Malaysia and Indonesia,and Eurasia from 1979-2018.The trends in CAPE are similar to the trends in low-level specific humidity but do not have an obvious relationship with the trends in low-level air temperature.The decreased CAPE is consistent with the increased level of free convection and the decreased level of neutral buoyancy,and verse vice.CIN changes are mostly insignificant while the trends in CIN are more comparable with the trends in the low-level relative humidity.Moreover,there exist a large proportion of absolute stable cases over the high latitudes in cold seasons.The climate model reasonably captures the mean CAPE and CIN patterns and the global distribution of the absolute stable cases in the reanalysis.In both the reanalysis and model,CAPE under the irreversible adiabatic process is stronger than that under the reversible process but with similar climatology mean and linear trend distributions.2.Model projected changes in CAPE and CIN and the underlying causesCAPE is expected to increase under GHG-induced global warming,but a recent regional study also suggests enhanced CIN over land although its cause is not well understood.Here we show that the climate model projects increased CAPE almost everywhere and stronger CIN over most land under global warming.Over land,the cases or times with medium-strong CAPE and/or CIN would increase while cases with weak CAPE and/or CIN would decrease,leading to an overall strengthening in their mean values.The CAPE increase results mainly from increased low-level specific humidity,which leads to more latent heating and buoyancy for a lifted parcel above the level of free convection and also a higher level of neutral buoyancy.The enhanced CIN over most land results mainly from reduced low-level relative humidity,which leads to a higher lifting condensation level and a higher level of free convection and thus more negative buoyancy.Over tropical oceans,the near-surface relative humidity increases slightly,leading to slight weakening of CIN.Over the subtropical eastern Pacific and Atlantic Ocean,the impact of reduced low-level atmospheric lapse rates overshadows the effect of increased specific humidity,leading to decreased CAPE.3.Model projected precipitation changes and its linkage with atmospheric thermodynamic changesPrevious studies indicated that light-moderate precipitation is projected to decrease whereas heavy precipitation would increase under GHG-induced global warming,but the underlying processes for these precipitation changes are not fully understood.In this study,we show that the climate model approximately captures the spatial patterns in the mean precipitation frequencies and the significant correlation between the precipitation frequencies or intensity and CAPE over most of the globe or CIN over tropical oceans seen in reanalysis.It also projects decreased light-moderate precipitation(0.01<P?1 mm/hr)but increased heavy precipitation(P>1 mm/hr)in a warmer climate.Most of the light-moderate precipitation events occur under low-CAPE and/or low-CIN conditions,which are projected to decrease greatly in a warmer climate as the increased temperature and humidity shift many of such cases into moderate-high CAPE or CIN cases.This results in large decreases in the light-moderate precipitation events.In contrast,increases in heavy precipitation result primarily from its increased probability under given CAPE and CIN,with a secondary contribution from the CAPE/CIN frequency changes.The increased probability for heavy precipitation partly results from a shift of the precipitation histogram towards higher intensity that could result from a uniform percentage increase in precipitation intensity due to increased water vapor in a warmer climate.