Understanding The Long-term Precipitation Trend Under Global Warming Through The Water Cycle

It is certain that globally the troposphere has warmed since the industrial revolution because of the greenhouse gas emissions.The water vapor content of air increases as the climate warms.The precipitation may possibly increase since the precipitable water increases.However,there is non-significant long-term trend in global-scale precipitation,as implied by the available globally records with short time span or incomplete spatial coverage or the model simulations.Based on the data of the historical simulations(1850-2005)from 30 CMIP5 models,this study aims to understand the uncertainty of the long-term precipitation trend under global warming from the perspectives of the overall water cycle and each link of the water cycle.The most notable conclusions can be summarized as follows:(1)Precipitation is influenced by the chain effect of the evaporation,the transport of water vapor,and the condensation.In other words,the precipitation is affected by the chain effect of the near-surface specific humidity,the specific humidity at the condensation level,and the relative humidity at the condensation level.The water-vapor-related variables are linked tightly in the water cycle and the warm surface air temperature favors the evaporation.Over the globe,the near-surface specific humidity increases significantly at most of the grid points while the increase of the specific humidity at the condensation level is significant at some,and,however,the long-term trend of the relative humidity at the condensation level is majorly non-significant.From the perspective of the water cycle,as assessed from the global grid points,the significance of the long-term trend of the water-vapor-related variable under global warming weakens,gradually,with the process in the cycle.Meanwhile,the uncertainty of their long-term trends increases step by step.The uncertainty,produced modestly in the process of the transport of water vapor and substantially in the process of condensation,leads to the insignificance(uncertainty)of the long-term precipitation trend through the chain effect in the water cycle.Therefore,the uncertainty of the long-term precipitation trend under global warming is mainly attributed to the condensation and slightly due to the transport of water vapor.(2)The warming trend in the troposphere varies with the location and with the altitudes over the globe.The latitudinal differences in the warming trends are visible.At high latitudes,the warming is great at lower levels,which gradually decreases with the height,and eventually becomes cooling near the tropopause.In the tropics and subtropics,the trend is small at lower levels,and increases with the altitude.The global-averaged warming trend increases gradually with the height,reaches its maximum around 300h Pa,and then decreases.Different warming amplitude in vertical direction leads to the change in the vertical difference of temperature(computed by using the lower layer to minus the upper layer),which further affects the atmospheric static stability.In the troposphere below 300h Pa,the vertical difference of temperature decreases in low and middle latitudes,and the static stability increases there;at high latitudes,the temperature difference increases and the static stability decreases.In the troposphere above 300h Pa,the vertical difference of temperature increases in middle and high latitudes,and the static stability decreases there;in the tropics,the temperature difference decreases and the static stability increases.In general,the static stability increases in most areas of the troposphere,especially in the low-mid latitudes,while decreases in a few regions.Most of the water vapor in the troposphere exits below 500h Pa,and the vapor content of the air in the tropics and subtropics is larger than that at high latitudes.The convection is also more active at lower latitudes.The static stability in low and middle latitudes of the troposphere below500h Pa increases,making the atmosphere more stable,which harms the convective activities,and thus the capacity of the vertical transport of water vapor weakens.Therefore,more water vapor evaporated by the warming cannot be transported into the upper levels as before.(3)Two indicators are constructed,through the derivation of the definition expression of the relative humidity,to quantitatively measure the contributions from air temperature(i.e.,the water holding capacity)and water vapor,because of their changes,to the change of relative humidity.The positive contribution of vapor-increase is larger than the negative contribution of warming near surface over the globe and at the midtroposphere over the tropical Pacific,thus the relative humidity increases there.The positive contribution of vapor-increase is accompanied with positive contribution of cooling at high latitudes near tropopause,and then the relative humidity also increases there.The relative humidity is characterized with an inverted-U-shaped decrease throughout the tropical upper troposphere,subtropics,and the middle latitudes,where the negative contribution of warming is stronger than the positive contribution of vapor-increase.The precipitation holds a very strong positive relation with the relative humidity at midtroposphere,especially over tropics and subtropics.The general pattern of precipitation trend there is very similar to the distribution of the change in relative humidity,and thus highly related to the relative contribution from water vapor and air temperature.Generally,the precipitation may increase(decrease)over regions where the positive contribution of vapor-increase is stronger(weaker)than the negative contribution of warming,which favors(harms)the condensation.The global-averaged long-term trend for the relative humidity is negative,while it for the precipitation is positive during 1850-2005,though both are not significant.At the global scale,the decrease in the frequency of precipitation,which also indicates the increase in the number of no-rain days,is the reason for the decrease in relative humidity.The increase in the number of extreme precipitation days and the increase in the rainfall intensity of extreme precipitation are the reasons why the rainfall amount still increases when the precipitation frequency decreases.(4)Through the derivation of the theoretical formula and empirical relationship,it is found that the change of water vapor in near-surface levels is not only affected by the magnitude of surface warming,but also related to the climatic vapor content there.To better understand the change of global-averaged near-surface water vapor,two indicators,i.e.,the“average term”and the“deviation term”are constructed.The average term measures the combined“contribution”from the global-averaged surface warming intensity and climatic vapor content.The deviation term measures the“contribution”from the spatial covariance of warming intensity and water vapor content.The global-averaged change of surface temperature has a weak influence on the deviation term,but a great impact on the average term.The global-averaged surface warming intensity determines the size of the average term,which further determines the change of the global-averaged near-surface water vapor,and then affects the change of the global-averaged precipitation.The spread of the changes in global-averaged precipitation during 1850-2005 among the 30 CMIP5 models originated from the differences of the simulated global-averaged warming intensity.However,under the condition of unit temperature increase,that is,when the global-averaged temperature increases by 1~oC,the change of global-averaged near-surface water vapor depends on the deviation term.When stronger warming occurs more in wetter regions,more water vapor will be produced.Then,the increased water vapor is more likely to offset the enhanced water-holding capacity of air caused by warming,and,consequently,the precipitation increases.(5)Two methods are used to estimate the relative importance.One is the physical method,which is based on the very tight relationship between the seasonal precipitation and relative humidity as well as the definition expression of the relative humidity.The other is the statistical method,which uses linear regression and can be applied generally for many of the relationship issues.Both methods reveal that for every season,precipitation is majorly dominated by water vapor over the globe,with the dominance being mostly over the middle-low latitudes.However,there are still many areas where precipitation is dominated by air temperature,which appears especially over the middle-high latitudes.