2023 Vol. 40, No. 4
Display Method:
2023, 40(4): 1-1.
Abstract:
2023, 40(4): 541-548.
doi: 10.1007/s00376-022-2331-8
Abstract:
The Northern Hemisphere (NH) often experiences frequent cold air outbreaks and heavy snowfalls during La Niña winters. In 2022, a third-year La Niña event has exceeded both the oceanic and atmospheric thresholds since spring and is predicted to reach its mature phase in December 2022. Under such a significant global climate signal, whether the Eurasian Continent will experience a tough cold winter should not be assumed, despite the direct influence of mid- to high-latitude, large-scale atmospheric circulations upon frequent Eurasian cold extremes, whose teleconnection physically operates by favoring Arctic air invasions into Eurasia as a consequence of the reduction of the meridional background temperature gradient in the NH. In the 2022/23 winter, as indicated by the seasonal predictions from various climate models and statistical approaches developed at the Institute of Atmospheric Physics, abnormal warming will very likely cover most parts of Europe under the control of the North Atlantic Oscillation and the anomalous anticyclone near the Ural Mountains, despite the cooling effects of La Niña. At the same time, the possibility of frequent cold conditions in mid-latitude Asia is also recognized for this upcoming winter, in accordance with the tendency for cold air invasions to be triggered by the synergistic effect of a warm Arctic and a cold tropical Pacific on the hemispheric scale. However, how the future climate will evolve in the 2022/23 winter is still subject to some uncertainty, mostly in terms of unpredictable internal atmospheric variability. Consequently, the status of the mid- to high-latitude atmospheric circulation should be timely updated by medium-term numerical weather forecasts and sub-seasonal-to-seasonal prediction for the necessary date information and early warnings.
The Northern Hemisphere (NH) often experiences frequent cold air outbreaks and heavy snowfalls during La Niña winters. In 2022, a third-year La Niña event has exceeded both the oceanic and atmospheric thresholds since spring and is predicted to reach its mature phase in December 2022. Under such a significant global climate signal, whether the Eurasian Continent will experience a tough cold winter should not be assumed, despite the direct influence of mid- to high-latitude, large-scale atmospheric circulations upon frequent Eurasian cold extremes, whose teleconnection physically operates by favoring Arctic air invasions into Eurasia as a consequence of the reduction of the meridional background temperature gradient in the NH. In the 2022/23 winter, as indicated by the seasonal predictions from various climate models and statistical approaches developed at the Institute of Atmospheric Physics, abnormal warming will very likely cover most parts of Europe under the control of the North Atlantic Oscillation and the anomalous anticyclone near the Ural Mountains, despite the cooling effects of La Niña. At the same time, the possibility of frequent cold conditions in mid-latitude Asia is also recognized for this upcoming winter, in accordance with the tendency for cold air invasions to be triggered by the synergistic effect of a warm Arctic and a cold tropical Pacific on the hemispheric scale. However, how the future climate will evolve in the 2022/23 winter is still subject to some uncertainty, mostly in terms of unpredictable internal atmospheric variability. Consequently, the status of the mid- to high-latitude atmospheric circulation should be timely updated by medium-term numerical weather forecasts and sub-seasonal-to-seasonal prediction for the necessary date information and early warnings.
2023, 40(4): 549-552.
doi: 10.1007/s00376-022-2305-x
Abstract:
Unanticipated sabotage of two underwater pipelines in the Baltic Sea (Nord Stream 1 and 2) happened on 26 September 2022. Massive quantities of natural gas, primarily methane, were released into the atmosphere, which lasted for about one week. As a more powerful greenhouse gas than CO2, the potential climatic impact of methane is a global concern. Using multiple methods and datasets, a recent study reported a relatively accurate magnitude of the leaked methane at 0.22 ± 0.03 million tons (Mt), which was lower than the initial estimate in the immediate aftermath of the event. Under an energy conservation framework used in IPCC AR6, we derived a negligible increase in global surface air temperature of 1.8 × 10−5 °C in a 20-year time horizon caused by the methane leaks with an upper limit of 0.25 Mt. Although the resultant warming from this methane leak incident was minor, future carbon release from additional Earth system feedbacks, such as thawing permafrost, and its impact on the methane mitigation pathways of the Paris Agreement, warrants investigation.
Unanticipated sabotage of two underwater pipelines in the Baltic Sea (Nord Stream 1 and 2) happened on 26 September 2022. Massive quantities of natural gas, primarily methane, were released into the atmosphere, which lasted for about one week. As a more powerful greenhouse gas than CO2, the potential climatic impact of methane is a global concern. Using multiple methods and datasets, a recent study reported a relatively accurate magnitude of the leaked methane at 0.22 ± 0.03 million tons (Mt), which was lower than the initial estimate in the immediate aftermath of the event. Under an energy conservation framework used in IPCC AR6, we derived a negligible increase in global surface air temperature of 1.8 × 10−5 °C in a 20-year time horizon caused by the methane leaks with an upper limit of 0.25 Mt. Although the resultant warming from this methane leak incident was minor, future carbon release from additional Earth system feedbacks, such as thawing permafrost, and its impact on the methane mitigation pathways of the Paris Agreement, warrants investigation.
2023, 40(4): 553-569.
doi: 10.1007/s00376-022-2194-z
Abstract:
In this paper, we first review the research advancements in blocking dynamics and highlight the merits and drawbacks of the previous theories of atmospheric blocking. Then, the dynamical mechanisms of atmospheric blocking are presented based on a nonlinear multi-scale interaction (NMI) model. Previous studies suggested that the eddy deformation (e.g., eddy straining, wave breaking, and eddy merging) might lead to the formation and maintenance of atmospheric blocking. However, the results were speculative and problematic because the previous studies, based on the time-mean eddy-mean flow interaction model, cannot identify the causal relationship between the evolution of atmospheric blocking and the eddy deformation. Based on the NMI model, we indicate that the onset, growth, maintenance, and decay of atmospheric blocking is mainly produced by the spatiotemporal evolution of pre-existing upstream synoptic-scale eddies, whereas the eddy deformation is a concomitant phenomenon of the blocking formation. The lifetime of blocking is mainly determined by the meridional background potential vorticity gradient (PVy) because a small PVy favors weak energy dispersion and strong nonlinearity to sustain the blocking. But the zonal movement of atmospheric blocking is associated with the background westerly wind, PVy, and the blocking amplitude. Using this NMI model, a bridge from the climate change to sub-seasonal atmospheric blocking and weather extremes might be established via examining the effect of climate change on PVy. Thus, it is expected that using the NMI model to explore the dynamics of atmospheric blocking and its change is a new direction in the future.
In this paper, we first review the research advancements in blocking dynamics and highlight the merits and drawbacks of the previous theories of atmospheric blocking. Then, the dynamical mechanisms of atmospheric blocking are presented based on a nonlinear multi-scale interaction (NMI) model. Previous studies suggested that the eddy deformation (e.g., eddy straining, wave breaking, and eddy merging) might lead to the formation and maintenance of atmospheric blocking. However, the results were speculative and problematic because the previous studies, based on the time-mean eddy-mean flow interaction model, cannot identify the causal relationship between the evolution of atmospheric blocking and the eddy deformation. Based on the NMI model, we indicate that the onset, growth, maintenance, and decay of atmospheric blocking is mainly produced by the spatiotemporal evolution of pre-existing upstream synoptic-scale eddies, whereas the eddy deformation is a concomitant phenomenon of the blocking formation. The lifetime of blocking is mainly determined by the meridional background potential vorticity gradient (PVy) because a small PVy favors weak energy dispersion and strong nonlinearity to sustain the blocking. But the zonal movement of atmospheric blocking is associated with the background westerly wind, PVy, and the blocking amplitude. Using this NMI model, a bridge from the climate change to sub-seasonal atmospheric blocking and weather extremes might be established via examining the effect of climate change on PVy. Thus, it is expected that using the NMI model to explore the dynamics of atmospheric blocking and its change is a new direction in the future.
2023, 40(4): 570-586.
doi: 10.1007/s00376-022-2161-8
Abstract:
In situ data in West Africa are scarce, and reanalysis datasets could be an alternative source to alleviate the problem of data availability. Nevertheless, because of uncertainties in numerical prediction models and assimilation methods, among other things, existing reanalysis datasets can perform with various degrees of quality and accuracy. Therefore, a proper assessment of their shortcomings and strengths should be performed prior to their usage. In this study, we examine the performance of ERA5 and ERA-interim (ERAI) products in representing the mean and extreme climates over West Africa for the period 1981–2018 using observations from CRU and CHIRPS. The major conclusion is that ERA5 showed a considerable decrease in precipitation and temperature biases and an improved representation of inter-annual variability in much of western Africa. Also, the annual cycle is better captured by ERA5 in three of the region’s climatic zones; specifically, precipitation is well-reproduced in the Savannah and Guinea Coast, and temperature in the Sahel. In terms of extremes, the ERA5 performance is superior. Still, both reanalyses underestimate the intensity and frequency of heavy precipitations and overestimate the number of wet days, as the numerical models used in reanalyses tend to produce drizzle more often. While ERA5 performs better than ERAI, both datasets are less successful in capturing the observed long-term trends. Although ERA5 has achieved considerable progress compared to its predecessor, improved datasets with better resolution and accuracy continue to be needed in sectors like agriculture and water resources to enable climate impact assessment.
In situ data in West Africa are scarce, and reanalysis datasets could be an alternative source to alleviate the problem of data availability. Nevertheless, because of uncertainties in numerical prediction models and assimilation methods, among other things, existing reanalysis datasets can perform with various degrees of quality and accuracy. Therefore, a proper assessment of their shortcomings and strengths should be performed prior to their usage. In this study, we examine the performance of ERA5 and ERA-interim (ERAI) products in representing the mean and extreme climates over West Africa for the period 1981–2018 using observations from CRU and CHIRPS. The major conclusion is that ERA5 showed a considerable decrease in precipitation and temperature biases and an improved representation of inter-annual variability in much of western Africa. Also, the annual cycle is better captured by ERA5 in three of the region’s climatic zones; specifically, precipitation is well-reproduced in the Savannah and Guinea Coast, and temperature in the Sahel. In terms of extremes, the ERA5 performance is superior. Still, both reanalyses underestimate the intensity and frequency of heavy precipitations and overestimate the number of wet days, as the numerical models used in reanalyses tend to produce drizzle more often. While ERA5 performs better than ERAI, both datasets are less successful in capturing the observed long-term trends. Although ERA5 has achieved considerable progress compared to its predecessor, improved datasets with better resolution and accuracy continue to be needed in sectors like agriculture and water resources to enable climate impact assessment.
2023, 40(4): 587-600.
doi: 10.1007/s00376-022-2092-4
Abstract:
Based on 20 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), this article explored possible reasons for differences in simulation biases and projected changes in precipitation in northern China among the all-model ensemble (AMME), “highest-ranked” model ensemble (BMME), and “lowest-ranked” model ensemble (WMME), from the perspective of atmospheric circulations and moisture budgets. The results show that the BMME and AMME reproduce the East Asian winter circulations better than the WMME. Compared with the AMME and WMME, the BMME reduces the overestimation of evaporation, thereby improving the simulation of winter precipitation. The three ensemble simulated biases for the East Asian summer circulations are generally similar, characterized by a stronger zonal pressure gradient between the mid-latitudes of the North Pacific and East Asia and a northward displacement of the East Asian westerly jet. However, the simulated vertical moisture advection is improved in the BMME, contributing to the slightly higher performance of the BMME than the AMME and WMME on summer precipitation in North and Northeast China. Compared to the AMME and WMME, the BMME projects larger increases in precipitation in northern China during both seasons by the end of the 21st century under the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). One of the reasons is that the increase in evaporation projected by the BMME is larger. The projection of a greater dynamic contribution by the BMME also plays a role. In addition, larger changes in the nonlinear components in the BMME projection contribute to a larger increase in winter precipitation in northern China.
Based on 20 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), this article explored possible reasons for differences in simulation biases and projected changes in precipitation in northern China among the all-model ensemble (AMME), “highest-ranked” model ensemble (BMME), and “lowest-ranked” model ensemble (WMME), from the perspective of atmospheric circulations and moisture budgets. The results show that the BMME and AMME reproduce the East Asian winter circulations better than the WMME. Compared with the AMME and WMME, the BMME reduces the overestimation of evaporation, thereby improving the simulation of winter precipitation. The three ensemble simulated biases for the East Asian summer circulations are generally similar, characterized by a stronger zonal pressure gradient between the mid-latitudes of the North Pacific and East Asia and a northward displacement of the East Asian westerly jet. However, the simulated vertical moisture advection is improved in the BMME, contributing to the slightly higher performance of the BMME than the AMME and WMME on summer precipitation in North and Northeast China. Compared to the AMME and WMME, the BMME projects larger increases in precipitation in northern China during both seasons by the end of the 21st century under the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). One of the reasons is that the increase in evaporation projected by the BMME is larger. The projection of a greater dynamic contribution by the BMME also plays a role. In addition, larger changes in the nonlinear components in the BMME projection contribute to a larger increase in winter precipitation in northern China.
2023, 40(4): 601-618.
doi: 10.1007/s00376-022-2039-9
Abstract:
In recent years, China has implemented several measures to improve air quality. The Beijing-Tianjin-Hebei (BTH) region is one area that has suffered from the most serious air pollution in China and has undergone huge changes in air quality in the past few years. How to scientifically assess these change processes remain the key issue in further improving the air quality over this region in the future. To evaluate the changes in major air pollutant emissions over this region, this paper employs ensemble Kalman filtering (EnKF) for integrating the national ground monitoring pollutant observation data and the Nested Air Quality Prediction Modeling System (NAQPMS) simulation data to inversely estimate the emission rates of SO2, NOX, CO, and primary PM2.5 over BTH region in February from 2014 to 2019. The results show that SO2, NOX, CO, and primary PM2.5 emissions in the BTH region decreased in February from 2014 to 2019 by 83%, 37%, 41%, and 42%, while decreases in Beijing during this period were 86%, 67%, 59%, and 65%, respectively. Compared with the prior emission inventory, the inversion emission inventory reduces the uncertainty of multi-pollutant simulation in the BTH region, with simulated root mean square errors of the monthly average concentrations of SO2, NOX, PM2.5, and CO reduced by 41%, 30%, 31%, and 22%, respectively. The average uncertainties of SO2, NOX, PM2.5, and CO inversion emissions in 2014–19 are ±14.03% yr–1, ±28.91% yr–1, ±126.15% yr–1, and ±43.58% yr–1. Compared with the uncertainty of MEIC emission, the uncertainties of all species changed by +2% yr–1, –2% yr–1, –26% yr–1, and –4% yr–1, respectively. The spatial distribution results illustrate that air pollutant emissions are mainly distributed over the eastern and southern BTH regions. The spatial gap between the inversion emissions and MEIC emissions was further closed in 2019 compared to 2014. The results of this paper can provide a new reference for assessing changes in air pollution emissions over the BTH region in recent years and validating a bottom-up emission inventory.
In recent years, China has implemented several measures to improve air quality. The Beijing-Tianjin-Hebei (BTH) region is one area that has suffered from the most serious air pollution in China and has undergone huge changes in air quality in the past few years. How to scientifically assess these change processes remain the key issue in further improving the air quality over this region in the future. To evaluate the changes in major air pollutant emissions over this region, this paper employs ensemble Kalman filtering (EnKF) for integrating the national ground monitoring pollutant observation data and the Nested Air Quality Prediction Modeling System (NAQPMS) simulation data to inversely estimate the emission rates of SO2, NOX, CO, and primary PM2.5 over BTH region in February from 2014 to 2019. The results show that SO2, NOX, CO, and primary PM2.5 emissions in the BTH region decreased in February from 2014 to 2019 by 83%, 37%, 41%, and 42%, while decreases in Beijing during this period were 86%, 67%, 59%, and 65%, respectively. Compared with the prior emission inventory, the inversion emission inventory reduces the uncertainty of multi-pollutant simulation in the BTH region, with simulated root mean square errors of the monthly average concentrations of SO2, NOX, PM2.5, and CO reduced by 41%, 30%, 31%, and 22%, respectively. The average uncertainties of SO2, NOX, PM2.5, and CO inversion emissions in 2014–19 are ±14.03% yr–1, ±28.91% yr–1, ±126.15% yr–1, and ±43.58% yr–1. Compared with the uncertainty of MEIC emission, the uncertainties of all species changed by +2% yr–1, –2% yr–1, –26% yr–1, and –4% yr–1, respectively. The spatial distribution results illustrate that air pollutant emissions are mainly distributed over the eastern and southern BTH regions. The spatial gap between the inversion emissions and MEIC emissions was further closed in 2019 compared to 2014. The results of this paper can provide a new reference for assessing changes in air pollution emissions over the BTH region in recent years and validating a bottom-up emission inventory.
2023, 40(4): 619-633.
doi: 10.1007/s00376-022-2047-9
Abstract:
Temperature trends in the upper stratosphere are investigated using satellite measurements from Stratospheric Sounding Unit (SSU) outputs and simulations from chemistry–climate models (CCMs) and the Coupled Model Intercomparison Project Phase 6 (CMIP6). Observational evidence shows a lack of cooling in the Antarctic, in contrast to strong cooling at other latitudes, during austral winter over 1979–97. Analysis of CCM simulations for a longer period of 1961–97 also shows a significant contrast in the upper stratospheric temperature trends between the Antarctic and lower latitudes. Results from two sets of model integrations with fixed ozone-depleting substances (ODSs) and fixed greenhouse gases (GHGs) at their 1960 levels suggest that the ODSs have made a major contribution to the lack of cooling in the Antarctic upper stratosphere. Results from CMIP6 simulations with prescribed GHGs and ozone confirm that changes in the dynamical processes associated with observed ozone depletion are largely responsible for the lack of cooling in the Antarctic upper stratosphere. The lack of cooling is found to be dynamically induced through increased upward wave activity into the upper stratosphere, which is attributed mainly to ODSs forcing. Specifically, the radiative cooling caused by the ozone depletion results in a stronger meridional temperature gradient between middle and high latitudes in the upper stratosphere, allowing more planetary waves propagating upward to warm the Antarctic upper stratosphere. These findings improve our understanding of the chemistry–climate coupling in the southern upper stratosphere.
Temperature trends in the upper stratosphere are investigated using satellite measurements from Stratospheric Sounding Unit (SSU) outputs and simulations from chemistry–climate models (CCMs) and the Coupled Model Intercomparison Project Phase 6 (CMIP6). Observational evidence shows a lack of cooling in the Antarctic, in contrast to strong cooling at other latitudes, during austral winter over 1979–97. Analysis of CCM simulations for a longer period of 1961–97 also shows a significant contrast in the upper stratospheric temperature trends between the Antarctic and lower latitudes. Results from two sets of model integrations with fixed ozone-depleting substances (ODSs) and fixed greenhouse gases (GHGs) at their 1960 levels suggest that the ODSs have made a major contribution to the lack of cooling in the Antarctic upper stratosphere. Results from CMIP6 simulations with prescribed GHGs and ozone confirm that changes in the dynamical processes associated with observed ozone depletion are largely responsible for the lack of cooling in the Antarctic upper stratosphere. The lack of cooling is found to be dynamically induced through increased upward wave activity into the upper stratosphere, which is attributed mainly to ODSs forcing. Specifically, the radiative cooling caused by the ozone depletion results in a stronger meridional temperature gradient between middle and high latitudes in the upper stratosphere, allowing more planetary waves propagating upward to warm the Antarctic upper stratosphere. These findings improve our understanding of the chemistry–climate coupling in the southern upper stratosphere.
2023, 40(4): 634-652.
doi: 10.1007/s00376-022-2159-2
Abstract:
This study compares the atmosphere-only HighResMIP simulations from FGOALS-f3-H (FGOALS) and MRI-AGCM3-2-S (MRI) with respect to tropical cyclone (TC) characteristics over the Western North Pacific (WNP) for the July–October months of 1985–2014. The focus is on investigating the role of the tropical easterly jet over the Western Pacific (WP_TEJ) in modulating the simulation biases in terms of their climatological distribution and interannual variability of WNP TC genesis frequency (TCGF) based on the analysis of the genesis potential index (GPI). Results show that the two models reasonably capture the main TC genesis location, the maximum center of frequency, and track density; however, their biases mainly lie in simulating the intense TCs and TCGF distributions. The MRI better simulates the wind-pressure relationship (WPR) but overestimates the proportion of super typhoons (SSTYs). At the same time, FGOALS underestimates the WPR and the proportion of SSTYs but better simulates the total WNP TC precipitation. In particular, FGOALS overestimates the TCGF in the northeastern WNP, which is strongly tied to an overestimated WP_TEJ and the enhanced vertical circulation to the north of its entrance region. In contrast, the MRI simulates a weaker WP_TEJ and vertical circulation, leading to a negative TCGF bias in most of the WNP. Both models exhibit comparable capability in simulating the interannual variability of WP_TEJ intensity, but the composite difference of large-scale atmospheric factors between strong and weak WP_TEJ years is overestimated, resulting in larger interannual anomalies of WNP TCGF, especially for FGOALS. Therefore, accurate simulations of the WP_TEJ and the associated oceanic and atmospheric factors are crucial to further improving WNP TC simulations for both models.
This study compares the atmosphere-only HighResMIP simulations from FGOALS-f3-H (FGOALS) and MRI-AGCM3-2-S (MRI) with respect to tropical cyclone (TC) characteristics over the Western North Pacific (WNP) for the July–October months of 1985–2014. The focus is on investigating the role of the tropical easterly jet over the Western Pacific (WP_TEJ) in modulating the simulation biases in terms of their climatological distribution and interannual variability of WNP TC genesis frequency (TCGF) based on the analysis of the genesis potential index (GPI). Results show that the two models reasonably capture the main TC genesis location, the maximum center of frequency, and track density; however, their biases mainly lie in simulating the intense TCs and TCGF distributions. The MRI better simulates the wind-pressure relationship (WPR) but overestimates the proportion of super typhoons (SSTYs). At the same time, FGOALS underestimates the WPR and the proportion of SSTYs but better simulates the total WNP TC precipitation. In particular, FGOALS overestimates the TCGF in the northeastern WNP, which is strongly tied to an overestimated WP_TEJ and the enhanced vertical circulation to the north of its entrance region. In contrast, the MRI simulates a weaker WP_TEJ and vertical circulation, leading to a negative TCGF bias in most of the WNP. Both models exhibit comparable capability in simulating the interannual variability of WP_TEJ intensity, but the composite difference of large-scale atmospheric factors between strong and weak WP_TEJ years is overestimated, resulting in larger interannual anomalies of WNP TCGF, especially for FGOALS. Therefore, accurate simulations of the WP_TEJ and the associated oceanic and atmospheric factors are crucial to further improving WNP TC simulations for both models.
2023, 40(4): 653-662.
doi: 10.1007/s00376-022-2187-y
Abstract:
Extremely heavy rainfall occurred over both Northwest India and North China in September 2021. The precipitation anomalies were 4.1 and 6.2 times interannual standard deviation over the two regions, respectively, and broke the record since the observational data were available, i.e., 1901 for India and 1951 for China. In this month, the Asian upper-tropospheric westerly jet was greatly displaced poleward over West Asia, and correspondingly, an anomalous cyclone appeared over India. The anomalous cyclone transported abundant water vapor into Northwest India, leading to the heavy rainfall there. In addition, the Silk Road pattern, a teleconnection pattern of upper-level meridional wind over the Eurasian continent and fueled by the heavy rainfall in Northwest India, contributed to the heavy rainfall in North China. Our study emphasizes the roles of atmospheric teleconnection patterns in concurrent rainfall extremes in the two regions far away from each other, and the occurrence of rainfall extremes during the post- or pre-monsoon period in the northern margins of monsoon regions.
Extremely heavy rainfall occurred over both Northwest India and North China in September 2021. The precipitation anomalies were 4.1 and 6.2 times interannual standard deviation over the two regions, respectively, and broke the record since the observational data were available, i.e., 1901 for India and 1951 for China. In this month, the Asian upper-tropospheric westerly jet was greatly displaced poleward over West Asia, and correspondingly, an anomalous cyclone appeared over India. The anomalous cyclone transported abundant water vapor into Northwest India, leading to the heavy rainfall there. In addition, the Silk Road pattern, a teleconnection pattern of upper-level meridional wind over the Eurasian continent and fueled by the heavy rainfall in Northwest India, contributed to the heavy rainfall in North China. Our study emphasizes the roles of atmospheric teleconnection patterns in concurrent rainfall extremes in the two regions far away from each other, and the occurrence of rainfall extremes during the post- or pre-monsoon period in the northern margins of monsoon regions.
2023, 40(4): 663-681.
doi: 10.1007/s00376-022-2050-1
Abstract:
During June–July 2020, the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly (biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtropical high (WPSH) is particularly evident. As for the extratropics, geopotential height anomalies near Lake Baikal are active. The former is attributed to the intensified biweekly activity of the southwest–northeast oriented East-Asian Pacific wave train (EAP) originating from the tropical western Pacific, while the latter is associated with the biweekly activities of the eastward propagating Eurasia mid-high latitudinal wave train and the westward propagating North Pacific wave train. Why the biweekly activities of these wave trains intensified is further diagnosed from the perspective of thermodynamical forcing and also from the modulation of interannual background on intraseasonal variability. It is found that the strongest recorded convection anchoring over the tropical western Indian Ocean (IO) triggers anomalous descent over the tropical western Pacific, which modulates the biweekly activity of the EAP. Meanwhile, the anomalous diabatic heating over the IO causes changes of the meridional thermodynamic contrast across the IO to the high latitudes, which modulates the extratropical wave trains. A further diagnosis of barotropic kinetic energy conversion suggests that the active occurrence of two extratropical biweekly wave trains is attributed to the increased efficiency of energy conversion from basic flow. The westward propagation of the extratropical North Pacific wave train is attributed to the weakened and north-shifted upper-level westerly, which is caused by the SST warmth near the Kuroshio extension.
During June–July 2020, the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly (biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtropical high (WPSH) is particularly evident. As for the extratropics, geopotential height anomalies near Lake Baikal are active. The former is attributed to the intensified biweekly activity of the southwest–northeast oriented East-Asian Pacific wave train (EAP) originating from the tropical western Pacific, while the latter is associated with the biweekly activities of the eastward propagating Eurasia mid-high latitudinal wave train and the westward propagating North Pacific wave train. Why the biweekly activities of these wave trains intensified is further diagnosed from the perspective of thermodynamical forcing and also from the modulation of interannual background on intraseasonal variability. It is found that the strongest recorded convection anchoring over the tropical western Indian Ocean (IO) triggers anomalous descent over the tropical western Pacific, which modulates the biweekly activity of the EAP. Meanwhile, the anomalous diabatic heating over the IO causes changes of the meridional thermodynamic contrast across the IO to the high latitudes, which modulates the extratropical wave trains. A further diagnosis of barotropic kinetic energy conversion suggests that the active occurrence of two extratropical biweekly wave trains is attributed to the increased efficiency of energy conversion from basic flow. The westward propagation of the extratropical North Pacific wave train is attributed to the weakened and north-shifted upper-level westerly, which is caused by the SST warmth near the Kuroshio extension.
2023, 40(4): 682-696.
doi: 10.1007/s00376-022-2180-5
Abstract:
The WRF-lake vertically one-dimensional (1D) water temperature model, as a submodule of the Weather Research and Forecasting (WRF) system, is being widely used to investigate water–atmosphere interactions. But previous applications revealed that it cannot accurately simulate the water temperature in a deep riverine reservoir during a large flow rate period, and whether it can produce sufficiently accurate heat flux through the water surface of deep riverine reservoirs remains uncertain. In this study, the WRF-lake model was improved for applications in large, deep riverine reservoirs by parametric scheme optimization, and the accuracy of heat flux calculation was evaluated compared with the results of a better physically based model, the Delft3D-Flow, which was previously applied to different kinds of reservoirs successfully. The results show: (1) The latest version of WRF-lake can describe the surface water temperature to some extent but performs poorly in the large flow period. We revised WRF-lake by modifying the vertical thermal diffusivity, and then, the water temperature simulation in the large flow period was improved significantly. (2) The latest version of WRF-lake overestimates the reservoir–atmosphere heat exchange throughout the year, mainly because of underestimating the downward energy transfer in the reservoir, resulting in more heat remaining at the surface and returning to the atmosphere. The modification of vertical thermal diffusivity can improve the surface heat flux calculation significantly. (3) The longitudinal temperature variation and the temperature difference between inflow and outflow, which cannot be considered in the 1D WRF-lake, can also affect the water surface heat flux.
The WRF-lake vertically one-dimensional (1D) water temperature model, as a submodule of the Weather Research and Forecasting (WRF) system, is being widely used to investigate water–atmosphere interactions. But previous applications revealed that it cannot accurately simulate the water temperature in a deep riverine reservoir during a large flow rate period, and whether it can produce sufficiently accurate heat flux through the water surface of deep riverine reservoirs remains uncertain. In this study, the WRF-lake model was improved for applications in large, deep riverine reservoirs by parametric scheme optimization, and the accuracy of heat flux calculation was evaluated compared with the results of a better physically based model, the Delft3D-Flow, which was previously applied to different kinds of reservoirs successfully. The results show: (1) The latest version of WRF-lake can describe the surface water temperature to some extent but performs poorly in the large flow period. We revised WRF-lake by modifying the vertical thermal diffusivity, and then, the water temperature simulation in the large flow period was improved significantly. (2) The latest version of WRF-lake overestimates the reservoir–atmosphere heat exchange throughout the year, mainly because of underestimating the downward energy transfer in the reservoir, resulting in more heat remaining at the surface and returning to the atmosphere. The modification of vertical thermal diffusivity can improve the surface heat flux calculation significantly. (3) The longitudinal temperature variation and the temperature difference between inflow and outflow, which cannot be considered in the 1D WRF-lake, can also affect the water surface heat flux.
2023, 40(4): 697-710.
doi: 10.1007/s00376-022-2114-2
Abstract:
The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR's dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear, and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamic and thermodynamic responses reveal that exhausted spring El Niño events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.
The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR's dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear, and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamic and thermodynamic responses reveal that exhausted spring El Niño events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.
2023, 40(4): 711-724.
doi: 10.1007/s00376-022-2107-1
Abstract:
Here, we analyze the characteristics and the formation mechanisms of low-level jets (LLJs) in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts (ERA5) reanalysis dataset. Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets (BLJs) concentrated at heights of 900–1200 m. The BLJs occur most frequently at 2300 LST (LST=UTC+ 8 hours) but are strongest at 0200 LST, with composite wind velocities >14 m s–1. Synoptic-system-related LLJs (SLLJs) occur most frequently at 0800 LST but are strongest at 1100 LST, with composite wind velocities >12 m s−1. Both BLJs and SLLJs are characterized by a southwesterly wind direction, although the wind direction of SLLJs is more westerly, and northeasterly SLLJs occur more frequently than northeasterly BLJs. When Wuhan is south of the mei-yu front, the westward extension of the northwest Pacific subtropical high intensifies, and the low-pressure system in the eastern Tibetan Plateau strengthens, favoring the formation of LLJs, which are closely related to precipitation. The wind speeds on rainstorm days are greater than those on LLJ days. Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation. BLJs were shown to develop earlier than SLLJs.
Here, we analyze the characteristics and the formation mechanisms of low-level jets (LLJs) in the middle reaches of the Yangtze River during the 2010 mei-yu season using Wuhan station radiosonde data and the fifth generation of the European Centre for Medium-Range Weather Forecasts (ERA5) reanalysis dataset. Our results show that the vertical structure of LLJs is characterized by a predominance of boundary layer jets (BLJs) concentrated at heights of 900–1200 m. The BLJs occur most frequently at 2300 LST (LST=UTC+ 8 hours) but are strongest at 0200 LST, with composite wind velocities >14 m s–1. Synoptic-system-related LLJs (SLLJs) occur most frequently at 0800 LST but are strongest at 1100 LST, with composite wind velocities >12 m s−1. Both BLJs and SLLJs are characterized by a southwesterly wind direction, although the wind direction of SLLJs is more westerly, and northeasterly SLLJs occur more frequently than northeasterly BLJs. When Wuhan is south of the mei-yu front, the westward extension of the northwest Pacific subtropical high intensifies, and the low-pressure system in the eastern Tibetan Plateau strengthens, favoring the formation of LLJs, which are closely related to precipitation. The wind speeds on rainstorm days are greater than those on LLJ days. Our analysis of four typical heavy precipitation events shows the presence of LLJs at the center of the precipitation and on its southern side before the onset of heavy precipitation. BLJs were shown to develop earlier than SLLJs.
2023, 40(4): 725-742.
doi: 10.1007/s00376-022-2131-1
Abstract:
The characteristics of the raindrop size distribution (DSD) during regional freezing rain (FR) events that occur throughout the phase change (from liquid to solid) are poorly understood due to limited observations. We investigate the evolution of microphysical parameters and the key formation mechanisms of regional FR using the DSDs from five disdrometer sites in January 2018 in the Jianghan Plain (JHP) of Central China. FR is identified via the size and velocity distribution measured from a disdrometer, the discrete Fréchet distancemethod, surface temperature, human observations, and sounding data. With the persistence of precipitation, the emergence of graupel or snowflakes significantly reduces the proportion of FR. The enhancement of this regional FR event is mainly dominated by the increase in the number concentration of raindrops but weakly affected by the diameters. To improve the accuracy of quantitative precipitation estimation for the FR event, a modified second-degree polynomial relation between the shape μ and slope Λ of gamma DSDs is derived, and a new Z-R (radar reflectivity to rain rate) relationship is developed. The mean values of mass-weighted mean diameters (Dm) and generalized intercepts (lgNw) in FR are close to the stratiform results in the northern region of China. Both the melting of tiny-rimed graupels and large-dry snowflakes are a response to the formation of this regional FR process in the JHP, dominated by the joint influence of the physical mechanism of warm rain, vapor deposition, and aggregation/riming coupled with the effect of weak convective motion in some periods.
The characteristics of the raindrop size distribution (DSD) during regional freezing rain (FR) events that occur throughout the phase change (from liquid to solid) are poorly understood due to limited observations. We investigate the evolution of microphysical parameters and the key formation mechanisms of regional FR using the DSDs from five disdrometer sites in January 2018 in the Jianghan Plain (JHP) of Central China. FR is identified via the size and velocity distribution measured from a disdrometer, the discrete Fréchet distancemethod, surface temperature, human observations, and sounding data. With the persistence of precipitation, the emergence of graupel or snowflakes significantly reduces the proportion of FR. The enhancement of this regional FR event is mainly dominated by the increase in the number concentration of raindrops but weakly affected by the diameters. To improve the accuracy of quantitative precipitation estimation for the FR event, a modified second-degree polynomial relation between the shape μ and slope Λ of gamma DSDs is derived, and a new Z-R (radar reflectivity to rain rate) relationship is developed. The mean values of mass-weighted mean diameters (Dm) and generalized intercepts (lgNw) in FR are close to the stratiform results in the northern region of China. Both the melting of tiny-rimed graupels and large-dry snowflakes are a response to the formation of this regional FR process in the JHP, dominated by the joint influence of the physical mechanism of warm rain, vapor deposition, and aggregation/riming coupled with the effect of weak convective motion in some periods.
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