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Abstract
The so-called “perturbation D” was a nondeveloping West African disturbance observed near Dakar (Senegal) during special observing period (SOP) 3 of the African Monsoon Multidisciplinary Analysis (AMMA) in September 2006. Its mesoscale environment is described with the dropsonde data obtained during flights on three successive days with the Service des Avions Français Instrumentés pour la Recherche en Environnement Falcon-20 aircraft. Processes involved in this evolution are studied qualitatively with ECMWF reanalyses and Meteosat-9 images. The evolution of perturbation D was the result of an interaction between processes at different scales such as the African easterly jet (AEJ), a midtropospheric African easterly wave (AEW), a series of mesoscale convective systems, the monsoon flow, dry low- to midlevel anticyclonic Saharan air, and a midlatitude upper-level trough. The interaction between these processes is further investigated through a numerical simulation conducted with the French nonhydrostatic Méso-NH model with parameterized convection. The growth of the simulated disturbance is quantified with an energy budget including barotropic and baroclinic conversions of eddy kinetic energy, proposed previously by the authors for a limited domain. The development of the simulated system is found to result from barotropic–baroclinic growth over West Africa and baroclinic growth over the tropical eastern Atlantic. It is suggested that these energy conversions were the result of an adjustment of the wind in response to the pressure decrease, presumably caused by convective activity, and other synoptic processes. A comparison with the developing case of Helene (2006) reveals that both perturbations had similar evolutions over the continent but were associated with different synoptic conditions over the ocean. For perturbation D, the anticyclonic curvature of the AEJ, caused by the intensification of the eastern ridge by a strong flow of dry Saharan air, prohibited the formation of a closed and convergent circulation. Moreover, a midlatitude upper-level trough approaching from the northwest contributed to increase the northward stretching and then weakened the perturbation. It is therefore suggested that at least as important as the intensity of the AEW trough and associated convection leaving the West African continent are synoptic conditions associated with the Saharan heat low, the subtropical high pressure zone, and even the midlatitude circulation, all of which are instrumental in the (non)cyclogenetic evolution of AEWs in the Cape Verde Islands region.
Abstract
The so-called “perturbation D” was a nondeveloping West African disturbance observed near Dakar (Senegal) during special observing period (SOP) 3 of the African Monsoon Multidisciplinary Analysis (AMMA) in September 2006. Its mesoscale environment is described with the dropsonde data obtained during flights on three successive days with the Service des Avions Français Instrumentés pour la Recherche en Environnement Falcon-20 aircraft. Processes involved in this evolution are studied qualitatively with ECMWF reanalyses and Meteosat-9 images. The evolution of perturbation D was the result of an interaction between processes at different scales such as the African easterly jet (AEJ), a midtropospheric African easterly wave (AEW), a series of mesoscale convective systems, the monsoon flow, dry low- to midlevel anticyclonic Saharan air, and a midlatitude upper-level trough. The interaction between these processes is further investigated through a numerical simulation conducted with the French nonhydrostatic Méso-NH model with parameterized convection. The growth of the simulated disturbance is quantified with an energy budget including barotropic and baroclinic conversions of eddy kinetic energy, proposed previously by the authors for a limited domain. The development of the simulated system is found to result from barotropic–baroclinic growth over West Africa and baroclinic growth over the tropical eastern Atlantic. It is suggested that these energy conversions were the result of an adjustment of the wind in response to the pressure decrease, presumably caused by convective activity, and other synoptic processes. A comparison with the developing case of Helene (2006) reveals that both perturbations had similar evolutions over the continent but were associated with different synoptic conditions over the ocean. For perturbation D, the anticyclonic curvature of the AEJ, caused by the intensification of the eastern ridge by a strong flow of dry Saharan air, prohibited the formation of a closed and convergent circulation. Moreover, a midlatitude upper-level trough approaching from the northwest contributed to increase the northward stretching and then weakened the perturbation. It is therefore suggested that at least as important as the intensity of the AEW trough and associated convection leaving the West African continent are synoptic conditions associated with the Saharan heat low, the subtropical high pressure zone, and even the midlatitude circulation, all of which are instrumental in the (non)cyclogenetic evolution of AEWs in the Cape Verde Islands region.
Abstract
The West African perturbation that subsequently evolved into Hurricane Helene (2006) during NASA’s African Monsoon Multidisciplinary Analysis (NAMMA), 15 August–14 September 2006, and AMMA’s third special observing period (SOP-3), 15–29 September 2006, has been simulated with the nonhydrostatic Méso-NH model using parameterized convection. The simulated disturbance evolved over West Africa and the adjacent eastern tropical Atlantic through interactions between different processes at the convective scale, mesoscale, and synoptic scale. The aim of this paper is to quantify the energetics of the simulated disturbance. A set of energy equations is first developed in the hydrostatic case to solve the limitations of Lorenz’s analysis when applied to a finite domain. It is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the Méso-NH results. Application to the simulated pre-Helene disturbance allows one to determine the most important terms in these equations. These simplifications are taken into account to derive an energy cycle including barotropic and baroclinic conversions of eddy kinetic energy. The development of the simulated system was found to result from barotropic–baroclinic growth over West Africa and barotropic growth over the tropical eastern Atlantic. It is suggested that most of these energy conversions were the result of an adjustment of the wind field in response to the pressure decrease, presumably caused by convective activity.
Abstract
The West African perturbation that subsequently evolved into Hurricane Helene (2006) during NASA’s African Monsoon Multidisciplinary Analysis (NAMMA), 15 August–14 September 2006, and AMMA’s third special observing period (SOP-3), 15–29 September 2006, has been simulated with the nonhydrostatic Méso-NH model using parameterized convection. The simulated disturbance evolved over West Africa and the adjacent eastern tropical Atlantic through interactions between different processes at the convective scale, mesoscale, and synoptic scale. The aim of this paper is to quantify the energetics of the simulated disturbance. A set of energy equations is first developed in the hydrostatic case to solve the limitations of Lorenz’s analysis when applied to a finite domain. It is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the Méso-NH results. Application to the simulated pre-Helene disturbance allows one to determine the most important terms in these equations. These simplifications are taken into account to derive an energy cycle including barotropic and baroclinic conversions of eddy kinetic energy. The development of the simulated system was found to result from barotropic–baroclinic growth over West Africa and barotropic growth over the tropical eastern Atlantic. It is suggested that most of these energy conversions were the result of an adjustment of the wind field in response to the pressure decrease, presumably caused by convective activity.
Abstract
Two West African disturbances observed in August and September 2006 during the National Aeronautics and Space Administration African Monsoon Multidisciplinary Analysis (NAMMA) and the Special Observing Period 3 (AMMA/SOP-3) have been simulated using the Méso-NH numerical model with explicit convection. The first disturbance spawned Hurricane Helene (2006) off the West African coast, and the second one, referred to as perturbation D, though relatively intense, failed to develop. Over the continent, each case was associated with a well-defined African easterly wave (AEW) trough with embedded growing and decaying convective activity of various size, duration, and intensity. The aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the AEW trough, with respect to the synoptic-scale processes. The absolute vorticity budgets are analyzed during the “continental” and “oceanic transition” stages of these AEW troughs in order to highlight the similarities and differences between the developing pre-Helene disturbance and the nondeveloping perturbation D. For the developing case, low- to midlevel cyclonic vorticity was produced by convective processes through tilting and stretching. Cyclonic vorticity was then transported upward through vertical advection associated with convection and outward through horizontal advection mostly induced by the large-scale midlevel diverging circulation related to the downstream AEW ridge. For the nondeveloping case, low- to midlevel cyclonic vorticity production through stretching and tilting, and its vertical transport were relatively similar over the continent but smaller over the oceanic transition because of weaker convective activity. The outward transport through horizontal advection was also weaker as there was little midlevel divergence induced by the downstream AEW ridge in this case.
Abstract
Two West African disturbances observed in August and September 2006 during the National Aeronautics and Space Administration African Monsoon Multidisciplinary Analysis (NAMMA) and the Special Observing Period 3 (AMMA/SOP-3) have been simulated using the Méso-NH numerical model with explicit convection. The first disturbance spawned Hurricane Helene (2006) off the West African coast, and the second one, referred to as perturbation D, though relatively intense, failed to develop. Over the continent, each case was associated with a well-defined African easterly wave (AEW) trough with embedded growing and decaying convective activity of various size, duration, and intensity. The aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the AEW trough, with respect to the synoptic-scale processes. The absolute vorticity budgets are analyzed during the “continental” and “oceanic transition” stages of these AEW troughs in order to highlight the similarities and differences between the developing pre-Helene disturbance and the nondeveloping perturbation D. For the developing case, low- to midlevel cyclonic vorticity was produced by convective processes through tilting and stretching. Cyclonic vorticity was then transported upward through vertical advection associated with convection and outward through horizontal advection mostly induced by the large-scale midlevel diverging circulation related to the downstream AEW ridge. For the nondeveloping case, low- to midlevel cyclonic vorticity production through stretching and tilting, and its vertical transport were relatively similar over the continent but smaller over the oceanic transition because of weaker convective activity. The outward transport through horizontal advection was also weaker as there was little midlevel divergence induced by the downstream AEW ridge in this case.
Abstract
It is well accepted that global circulation models equipped with stable water isotopologues help us better understand the relationships between atmospheric circulation changes and isotope records in paleoclimate archives. Still, isotope-enabled models do not disentangle the different processes affecting precipitation isotopic compositions. Furthermore, the relevance of this model-oriented approach relies on the realism of the modeled isotope results, which would support the interpretation of the proxy records in terms of modeled climate changes. To alleviate these limitations, the newly developed WRF-Hydro-iso-tag, a version of the isotope-enabled regional coupled model WRF-Hydro-iso enhanced with an isotope-tracing procedure, is presented. Physics-based WRF-Hydro-iso-tag ensembles are used to regionally downscale the isotope-enabled Community Earth System Model for southern Africa, for two 10-yr slices of mid-Holocene and preindustrial times. The isotope-tracing procedure is tailored to assess the origin of the hydrogen isotope deuterium contained in southern African precipitation, between the Atlantic and Indian Oceans. In comparison to the global model, WRF-Hydro-iso-tag simulates lower precipitation amounts with more regional details, as well as mid-Holocene-to-preindustrial changes in precipitation isotopic compositions that better match plant-wax deuterium records from two marine sediment cores off the Orange and Limpopo River basins. Linear relationships between mid-Holocene-to-preindustrial changes in temperature, precipitation amount, moisture source, and precipitation deuterium compositions are derived from the ensemble results. A deuterium enrichment in the Orange River-related sediment core may not be related to an aridification but rather indicate a summer circulation change enabling a larger contribution of more isotopically enriched moisture from the Atlantic Ocean.
Significance Statement
The knowledge of past climates is crucial for understanding our Earth system and apprehending future climate change. Plant materials in sediment archives contain atoms of hydrogen from past precipitation that allow paleoclimate reconstructions, using compositions of the hydrogen isotope deuterium. However, in the tropics, deuterium-depleted plant remains can either denote a wetter climate phase or a change in atmospheric circulation patterns with longer distances between ocean evaporation and land precipitation. This work provides an innovative dynamical downscaling method of global paleoclimate models to disentangle the effects of precipitation amount change and moisture source change on deuterium records, and ultimately to improve paleoclimate reconstructions. The interpretation of a deuterium enrichment in a marine sediment core as a marker for aridification is revised. The enrichment caused by an atmospheric circulation change bringing a larger amount of more isotopically enriched moisture flow from the Atlantic Ocean to southern African precipitation would be a more physically sound explanation.
Abstract
It is well accepted that global circulation models equipped with stable water isotopologues help us better understand the relationships between atmospheric circulation changes and isotope records in paleoclimate archives. Still, isotope-enabled models do not disentangle the different processes affecting precipitation isotopic compositions. Furthermore, the relevance of this model-oriented approach relies on the realism of the modeled isotope results, which would support the interpretation of the proxy records in terms of modeled climate changes. To alleviate these limitations, the newly developed WRF-Hydro-iso-tag, a version of the isotope-enabled regional coupled model WRF-Hydro-iso enhanced with an isotope-tracing procedure, is presented. Physics-based WRF-Hydro-iso-tag ensembles are used to regionally downscale the isotope-enabled Community Earth System Model for southern Africa, for two 10-yr slices of mid-Holocene and preindustrial times. The isotope-tracing procedure is tailored to assess the origin of the hydrogen isotope deuterium contained in southern African precipitation, between the Atlantic and Indian Oceans. In comparison to the global model, WRF-Hydro-iso-tag simulates lower precipitation amounts with more regional details, as well as mid-Holocene-to-preindustrial changes in precipitation isotopic compositions that better match plant-wax deuterium records from two marine sediment cores off the Orange and Limpopo River basins. Linear relationships between mid-Holocene-to-preindustrial changes in temperature, precipitation amount, moisture source, and precipitation deuterium compositions are derived from the ensemble results. A deuterium enrichment in the Orange River-related sediment core may not be related to an aridification but rather indicate a summer circulation change enabling a larger contribution of more isotopically enriched moisture from the Atlantic Ocean.
Significance Statement
The knowledge of past climates is crucial for understanding our Earth system and apprehending future climate change. Plant materials in sediment archives contain atoms of hydrogen from past precipitation that allow paleoclimate reconstructions, using compositions of the hydrogen isotope deuterium. However, in the tropics, deuterium-depleted plant remains can either denote a wetter climate phase or a change in atmospheric circulation patterns with longer distances between ocean evaporation and land precipitation. This work provides an innovative dynamical downscaling method of global paleoclimate models to disentangle the effects of precipitation amount change and moisture source change on deuterium records, and ultimately to improve paleoclimate reconstructions. The interpretation of a deuterium enrichment in a marine sediment core as a marker for aridification is revised. The enrichment caused by an atmospheric circulation change bringing a larger amount of more isotopically enriched moisture flow from the Atlantic Ocean to southern African precipitation would be a more physically sound explanation.
Abstract
The analysis of land–atmosphere feedbacks requires detailed representation of land processes in atmospheric models. The focus here is on runoff–infiltration partitioning and resolved overland flow. In the standard version of WRF, runoff–infiltration partitioning is described as a purely vertical process. In WRF-Hydro, runoff is enhanced with lateral water flows. The study region is the Sissili catchment (12 800 km2) in West Africa, and the study period is from March 2003 to February 2004. The WRF setup here includes an outer and inner domain at 10- and 2-km resolution covering the West Africa and Sissili regions, respectively. In this WRF-Hydro setup, the inner domain is coupled with a subgrid at 500-m resolution to compute overland and river flow. Model results are compared with TRMM precipitation, model tree ensemble (MTE) evapotranspiration, Climate Change Initiative (CCI) soil moisture, CRU temperature, and streamflow observation. The role of runoff–infiltration partitioning and resolved overland flow on land–atmosphere feedbacks is addressed with a sensitivity analysis of WRF results to the runoff–infiltration partitioning parameter and a comparison between WRF and WRF-Hydro results, respectively. In the outer domain, precipitation is sensitive to runoff–infiltration partitioning at the scale of the Sissili area (~100 × 100 km2), but not of area A (500 × 2500 km2). In the inner domain, where precipitation patterns are mainly prescribed by lateral boundary conditions, sensitivity is small, but additionally resolved overland flow here clearly increases infiltration and evapotranspiration at the beginning of the wet season when soils are still dry. The WRF-Hydro setup presented here shows potential for joint atmospheric and terrestrial water balance studies and reproduces observed daily discharge with a Nash–Sutcliffe model efficiency coefficient of 0.43.
Abstract
The analysis of land–atmosphere feedbacks requires detailed representation of land processes in atmospheric models. The focus here is on runoff–infiltration partitioning and resolved overland flow. In the standard version of WRF, runoff–infiltration partitioning is described as a purely vertical process. In WRF-Hydro, runoff is enhanced with lateral water flows. The study region is the Sissili catchment (12 800 km2) in West Africa, and the study period is from March 2003 to February 2004. The WRF setup here includes an outer and inner domain at 10- and 2-km resolution covering the West Africa and Sissili regions, respectively. In this WRF-Hydro setup, the inner domain is coupled with a subgrid at 500-m resolution to compute overland and river flow. Model results are compared with TRMM precipitation, model tree ensemble (MTE) evapotranspiration, Climate Change Initiative (CCI) soil moisture, CRU temperature, and streamflow observation. The role of runoff–infiltration partitioning and resolved overland flow on land–atmosphere feedbacks is addressed with a sensitivity analysis of WRF results to the runoff–infiltration partitioning parameter and a comparison between WRF and WRF-Hydro results, respectively. In the outer domain, precipitation is sensitive to runoff–infiltration partitioning at the scale of the Sissili area (~100 × 100 km2), but not of area A (500 × 2500 km2). In the inner domain, where precipitation patterns are mainly prescribed by lateral boundary conditions, sensitivity is small, but additionally resolved overland flow here clearly increases infiltration and evapotranspiration at the beginning of the wet season when soils are still dry. The WRF-Hydro setup presented here shows potential for joint atmospheric and terrestrial water balance studies and reproduces observed daily discharge with a Nash–Sutcliffe model efficiency coefficient of 0.43.
Abstract
Precipitation change is critical for the Three-River Headwaters (TRH) region, which serves downstream communities in East Asia. The spring (March–May) precipitation over the TRH region shows an increasing trend from 1979 to 2018, as revealed by a Chinese gridded precipitation product (CN05.1). However, the physical processes responsible for this precipitation change are still unclear. This study investigated the characteristics of spring precipitation and the water budget over the TRH region using the ERA5 global reanalysis and the Weather Research and Forecast (WRF) Model. The WRF version employed in this study includes online calculations of the atmospheric water budget and an evapotranspiration (ET) tagging procedure to trace evapotranspired water in the atmosphere. Both ERA5 and WRF reproduce the spring precipitation increase. Moreover, WRFD02 (with a 3-km domain) reduces the wet bias by around 60% and 77% compared to WRFD01 (9 km) and ERA5 (30 km). Both ERA5 and WRF demonstrate that the increase of spring precipitation is dominated by moisture convergence, especially the atmospheric water fluxes from the southern boundary. The enhanced moisture inflow is sustained by enhanced mass flux while the enhanced moisture outflow is sustained by increased moisture. The ET-tagging results further demonstrate the weakened precipitation recycling process because of the significant increase of precipitation produced by external moisture. Compared to ERA5, the reduced wet bias with WRF is attributed to a better spatial resolution of orographic barrier effects, which reduce the southerly water fluxes. The results highlight the potential of regional climate downscaling to better represent the atmospheric water budget in complex terrain.
Abstract
Precipitation change is critical for the Three-River Headwaters (TRH) region, which serves downstream communities in East Asia. The spring (March–May) precipitation over the TRH region shows an increasing trend from 1979 to 2018, as revealed by a Chinese gridded precipitation product (CN05.1). However, the physical processes responsible for this precipitation change are still unclear. This study investigated the characteristics of spring precipitation and the water budget over the TRH region using the ERA5 global reanalysis and the Weather Research and Forecast (WRF) Model. The WRF version employed in this study includes online calculations of the atmospheric water budget and an evapotranspiration (ET) tagging procedure to trace evapotranspired water in the atmosphere. Both ERA5 and WRF reproduce the spring precipitation increase. Moreover, WRFD02 (with a 3-km domain) reduces the wet bias by around 60% and 77% compared to WRFD01 (9 km) and ERA5 (30 km). Both ERA5 and WRF demonstrate that the increase of spring precipitation is dominated by moisture convergence, especially the atmospheric water fluxes from the southern boundary. The enhanced moisture inflow is sustained by enhanced mass flux while the enhanced moisture outflow is sustained by increased moisture. The ET-tagging results further demonstrate the weakened precipitation recycling process because of the significant increase of precipitation produced by external moisture. Compared to ERA5, the reduced wet bias with WRF is attributed to a better spatial resolution of orographic barrier effects, which reduce the southerly water fluxes. The results highlight the potential of regional climate downscaling to better represent the atmospheric water budget in complex terrain.
Abstract
Precipitation is affected by soil moisture spatial variability. However, this variability is not well represented in atmospheric models that do not consider soil moisture transport as a three-dimensional process. This study investigates the sensitivity of precipitation to the uncertainty in the representation of terrestrial water flow. The tools used for this investigation are the Weather Research and Forecasting (WRF) Model and its hydrologically enhanced version, WRF-Hydro, applied over central Europe during April–October 2008. The model grid is convection permitting, with a horizontal spacing of 2.8 km. The WRF-Hydro subgrid employs a 280-m resolution to resolve lateral terrestrial water flow. A WRF/WRF-Hydro ensemble is constructed by modifying the parameter controlling the partitioning between surface runoff and infiltration and by varying the planetary boundary layer (PBL) scheme. This ensemble represents terrestrial water flow uncertainty originating from the consideration of resolved lateral flow, terrestrial water flow uncertainty in the vertical direction, and turbulence parameterization uncertainty. The uncertainty of terrestrial water flow noticeably increases the normalized ensemble spread of daily precipitation where topography is moderate, surface flux spatial variability is high, and the weather regime is dominated by local processes. The adjusted continuous ranked probability score shows that the PBL uncertainty improves the skill of an ensemble subset in reproducing daily precipitation from the E-OBS observational product by 16%–20%. In comparison to WRF, WRF-Hydro improves this skill by 0.4%–0.7%. The reproduction of observed daily discharge with Nash–Sutcliffe model efficiency coefficients generally above 0.3 demonstrates the potential of WRF-Hydro in hydrological science.
Abstract
Precipitation is affected by soil moisture spatial variability. However, this variability is not well represented in atmospheric models that do not consider soil moisture transport as a three-dimensional process. This study investigates the sensitivity of precipitation to the uncertainty in the representation of terrestrial water flow. The tools used for this investigation are the Weather Research and Forecasting (WRF) Model and its hydrologically enhanced version, WRF-Hydro, applied over central Europe during April–October 2008. The model grid is convection permitting, with a horizontal spacing of 2.8 km. The WRF-Hydro subgrid employs a 280-m resolution to resolve lateral terrestrial water flow. A WRF/WRF-Hydro ensemble is constructed by modifying the parameter controlling the partitioning between surface runoff and infiltration and by varying the planetary boundary layer (PBL) scheme. This ensemble represents terrestrial water flow uncertainty originating from the consideration of resolved lateral flow, terrestrial water flow uncertainty in the vertical direction, and turbulence parameterization uncertainty. The uncertainty of terrestrial water flow noticeably increases the normalized ensemble spread of daily precipitation where topography is moderate, surface flux spatial variability is high, and the weather regime is dominated by local processes. The adjusted continuous ranked probability score shows that the PBL uncertainty improves the skill of an ensemble subset in reproducing daily precipitation from the E-OBS observational product by 16%–20%. In comparison to WRF, WRF-Hydro improves this skill by 0.4%–0.7%. The reproduction of observed daily discharge with Nash–Sutcliffe model efficiency coefficients generally above 0.3 demonstrates the potential of WRF-Hydro in hydrological science.
Abstract
Precipitation in the Three-River Headwater (TRH) region has undergone significant changes as a result of global warming, which can affect water resources in downstream regions of Asia. However, the underlying mechanisms of the precipitation variability during the cold season (October–April) are still not fully understood. In this study, the daily China gridded precipitation product CN05.1 as well as the NCEP–NCAR reanalysis are used to investigate the characteristics of the cold season precipitation variability over the TRH region and associated atmospheric mechanisms. The cold season precipitation shows an increasing trend (5.5 mm decade−1) from 1961 to 2014, with a dry-to-wet shift in around the late 1980s. The results indicate that the increased precipitation is associated with the enhanced easterly anomalies over the Tibetan Plateau (TP) and enhanced southeasterly water vapor transport. The enhanced Walker circulations, caused by the gradients of sea surface temperature between the equatorial central-eastern Pacific and Indo–western Pacific in tropical oceans, resulted in strengthened easterly anomalies over the TP and the westward expansion of the anticyclone in the western North Pacific. Meanwhile, the changed Walker circulation is accompanied by a strengthened local Hadley circulation, which leads to enhanced meridional water vapor transport from tropical oceans and the South China Sea toward the TRH region. Furthermore, the strengthened East Asia subtropical westerly jet may contribute to the enhanced divergence at upper levels and anomalous ascending motion above the TRH region, leading to more precipitation.
Abstract
Precipitation in the Three-River Headwater (TRH) region has undergone significant changes as a result of global warming, which can affect water resources in downstream regions of Asia. However, the underlying mechanisms of the precipitation variability during the cold season (October–April) are still not fully understood. In this study, the daily China gridded precipitation product CN05.1 as well as the NCEP–NCAR reanalysis are used to investigate the characteristics of the cold season precipitation variability over the TRH region and associated atmospheric mechanisms. The cold season precipitation shows an increasing trend (5.5 mm decade−1) from 1961 to 2014, with a dry-to-wet shift in around the late 1980s. The results indicate that the increased precipitation is associated with the enhanced easterly anomalies over the Tibetan Plateau (TP) and enhanced southeasterly water vapor transport. The enhanced Walker circulations, caused by the gradients of sea surface temperature between the equatorial central-eastern Pacific and Indo–western Pacific in tropical oceans, resulted in strengthened easterly anomalies over the TP and the westward expansion of the anticyclone in the western North Pacific. Meanwhile, the changed Walker circulation is accompanied by a strengthened local Hadley circulation, which leads to enhanced meridional water vapor transport from tropical oceans and the South China Sea toward the TRH region. Furthermore, the strengthened East Asia subtropical westerly jet may contribute to the enhanced divergence at upper levels and anomalous ascending motion above the TRH region, leading to more precipitation.
Abstract
Global warming is assumed to accelerate the global water cycle. However, quantification of the acceleration and regional analyses remain open. Accordingly, in this study we address the fundamental hydrological question: Is the water cycle regionally accelerating/decelerating under global warming? For our investigation we have implemented the age-weighted regional water tagging approach into the Weather Research and Forecasting WRF model, namely WRF-age, to follow the atmospheric water pathways and to derive atmospheric water residence times defined as the age of tagged water since its source. We apply a three-dimensional online budget analysis of the total, tagged, and aged atmospheric water into WRF-age to provide a prognostic equation of the atmospheric water residence times and to derive atmospheric water transit times defined as the age of tagged water since its source originating from a particular physical or dynamical process. The newly developed, physics-based WRF-age model is used to regionally downscale the reanalysis of ERA-Interim and the MPI-ESM Representative Concentration Pathway 8.5 scenario exemplarily for an East Asian monsoon region, i.e., the Poyang Lake basin (the tagged water source area), for historical (1980-1989) and future (2040-2049) times. In the warmer (+1.9 °C for temperature and +2% for evaporation) and drier (−21% for precipitation) future, the residence time for the tagged water vapor will regionally decrease by 1.8 hours (from 14.3 hours) due to enhanced local evaporation contributions, but the transit time for the tagged precipitation will increase by 1.8 hours (from 12.9 hours) partly due to slower fallout of precipitating moisture components.
Abstract
Global warming is assumed to accelerate the global water cycle. However, quantification of the acceleration and regional analyses remain open. Accordingly, in this study we address the fundamental hydrological question: Is the water cycle regionally accelerating/decelerating under global warming? For our investigation we have implemented the age-weighted regional water tagging approach into the Weather Research and Forecasting WRF model, namely WRF-age, to follow the atmospheric water pathways and to derive atmospheric water residence times defined as the age of tagged water since its source. We apply a three-dimensional online budget analysis of the total, tagged, and aged atmospheric water into WRF-age to provide a prognostic equation of the atmospheric water residence times and to derive atmospheric water transit times defined as the age of tagged water since its source originating from a particular physical or dynamical process. The newly developed, physics-based WRF-age model is used to regionally downscale the reanalysis of ERA-Interim and the MPI-ESM Representative Concentration Pathway 8.5 scenario exemplarily for an East Asian monsoon region, i.e., the Poyang Lake basin (the tagged water source area), for historical (1980-1989) and future (2040-2049) times. In the warmer (+1.9 °C for temperature and +2% for evaporation) and drier (−21% for precipitation) future, the residence time for the tagged water vapor will regionally decrease by 1.8 hours (from 14.3 hours) due to enhanced local evaporation contributions, but the transit time for the tagged precipitation will increase by 1.8 hours (from 12.9 hours) partly due to slower fallout of precipitating moisture components.
