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Abstract
The variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I), is applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. The dual-Doppler-analyzed VF field reveals the following features: The axisymmetric part of the VF is a well-defined slantwise two-cell vortex in which the maximum tangential velocity is nearly 40 m s−1 at the edge of the vortex core (0.6 km from the vortex center), the central downdraft velocity reaches −35 m s−1 at 3-km height, and the surrounding-updraft velocity reaches 26 m s−1 at 5-km height. The total VF field is a loosely defined slantwise two-cell vortex consisting of a nearly axisymmetric vortex core (in which the ground-relative surface wind speed reaches 50 m s−1 on the southeast edge), a strong nonaxisymmetric slantwise downdraft in the vortex core, and a main updraft in a banana-shaped area southeast of the vortex core, which extends slantwise upward and spirals cyclonically around the vortex core. The single-Doppler analysis with observations from the KTLX radar only exhibits roughly the same features as the dual-Doppler analysis but contains spurious vertical-motion structures in and around the vortex core. Analysis errors are assessed by leveraging the findings from Parts II and III, which indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF but the single-Doppler-analyzed VF is not (especially for nonaxisymmetric vertical motions in and around the vortex core), so the dual-Doppler-analyzed VF should be useful for initializing/verifying high-resolution tornado simulations.
Significance Statement
After the variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I of this paper series), was tested successfully with simulated radar observations in Part II and its sensitivity to vortex center location error was examined in Part III, the method is now applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. Analysis errors are assessed by leveraging the findings from Parts II and III. The results indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF (although the single-Doppler-analyzed VF is not especially for nonaxisymmetric vertical motions in and around the vortex core) and thus should be useful for initializing/verifying high-resolution tornado simulations.
Abstract
The variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I), is applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. The dual-Doppler-analyzed VF field reveals the following features: The axisymmetric part of the VF is a well-defined slantwise two-cell vortex in which the maximum tangential velocity is nearly 40 m s−1 at the edge of the vortex core (0.6 km from the vortex center), the central downdraft velocity reaches −35 m s−1 at 3-km height, and the surrounding-updraft velocity reaches 26 m s−1 at 5-km height. The total VF field is a loosely defined slantwise two-cell vortex consisting of a nearly axisymmetric vortex core (in which the ground-relative surface wind speed reaches 50 m s−1 on the southeast edge), a strong nonaxisymmetric slantwise downdraft in the vortex core, and a main updraft in a banana-shaped area southeast of the vortex core, which extends slantwise upward and spirals cyclonically around the vortex core. The single-Doppler analysis with observations from the KTLX radar only exhibits roughly the same features as the dual-Doppler analysis but contains spurious vertical-motion structures in and around the vortex core. Analysis errors are assessed by leveraging the findings from Parts II and III, which indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF but the single-Doppler-analyzed VF is not (especially for nonaxisymmetric vertical motions in and around the vortex core), so the dual-Doppler-analyzed VF should be useful for initializing/verifying high-resolution tornado simulations.
Significance Statement
After the variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I of this paper series), was tested successfully with simulated radar observations in Part II and its sensitivity to vortex center location error was examined in Part III, the method is now applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. Analysis errors are assessed by leveraging the findings from Parts II and III. The results indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF (although the single-Doppler-analyzed VF is not especially for nonaxisymmetric vertical motions in and around the vortex core) and thus should be useful for initializing/verifying high-resolution tornado simulations.
Abstract
The accurate prediction of surface soil moisture (SM) is crucial for understanding hydrological processes. Deep learning (DL) models such as the long short-term memory model (LSTM) provide a powerful method and have been widely used in SM prediction. However, few studies have notably high success rates due to lacking prior knowledge in forms such as causality. Here we present a new causality-structure-based LSTM model (CLSTM), which could learn time interdependency and causality information for hydrometeorological applications. We applied and compared LSTM and CLSTM methods for forecasting SM across 64 FLUXNET sites globally. The results showed that CLSTM dramatically increased the predictive performance compared with LSTM. The Nash–Sutcliffe efficiency (NSE) suggested that more than 67% of sites witnessed an improvement of SM simulation larger than 10%. It is highlighted that CLSTM had a much better generalization ability that can adapt to extreme soil conditions, such as SM response to drought and precipitation events. By incorporating causal relations, CLSTM increased predictive ability across different lead times compared to LSTM. We also highlighted the critical role of physical information in the form of causality structure to improve drought prediction. At the same time, CLSTM has the potential to improve predictions of other hydrometeorological variables.
Abstract
The accurate prediction of surface soil moisture (SM) is crucial for understanding hydrological processes. Deep learning (DL) models such as the long short-term memory model (LSTM) provide a powerful method and have been widely used in SM prediction. However, few studies have notably high success rates due to lacking prior knowledge in forms such as causality. Here we present a new causality-structure-based LSTM model (CLSTM), which could learn time interdependency and causality information for hydrometeorological applications. We applied and compared LSTM and CLSTM methods for forecasting SM across 64 FLUXNET sites globally. The results showed that CLSTM dramatically increased the predictive performance compared with LSTM. The Nash–Sutcliffe efficiency (NSE) suggested that more than 67% of sites witnessed an improvement of SM simulation larger than 10%. It is highlighted that CLSTM had a much better generalization ability that can adapt to extreme soil conditions, such as SM response to drought and precipitation events. By incorporating causal relations, CLSTM increased predictive ability across different lead times compared to LSTM. We also highlighted the critical role of physical information in the form of causality structure to improve drought prediction. At the same time, CLSTM has the potential to improve predictions of other hydrometeorological variables.
Abstract
A Pacific–Japan (PJ) pattern index is defined based on the singular value decomposition (SVD) analysis of summertime 500-hPa height in East Asia and precipitation in the tropical western North Pacific (WNP). The time series of this PJ index shows clearly the interannual and interdecadal variations since 1948. Idealized atmospheric general circulation model (AGCM) experiments were carried out to understand the remote and local SST forcing in causing the interannual variations of the PJ pattern and interdecadal variations of the PJ-like pattern. It is found that the PJ interannual variation is closely related to El Niño–Southern Oscillation (ENSO). A basinwide warming occurs in the tropical Indian Ocean (TIO) during El Niño mature winter. The TIO warming persists from the El Niño peak winter to the succeeding summer. Meanwhile, a cold SST anomaly (SSTA) appears in the eastern WNP and persists from the El Niño mature winter to the succeeding summer. Idealized AGCM experiments that separate the TIO and WNP SSTA forcing effects show that both the remote eastern TIO forcing and local WNP SSTA forcing are important in affecting atmospheric heating anomaly in the WNP monsoon region, which further impacts the PJ interannual teleconnection pattern over East Asia. In contrast to the interannual variation, the interdecadal change of the PJ-like pattern is primarily affected by the interdecadal change of SST in the TIO rather than by the local SSTA in the WNP.
Abstract
A Pacific–Japan (PJ) pattern index is defined based on the singular value decomposition (SVD) analysis of summertime 500-hPa height in East Asia and precipitation in the tropical western North Pacific (WNP). The time series of this PJ index shows clearly the interannual and interdecadal variations since 1948. Idealized atmospheric general circulation model (AGCM) experiments were carried out to understand the remote and local SST forcing in causing the interannual variations of the PJ pattern and interdecadal variations of the PJ-like pattern. It is found that the PJ interannual variation is closely related to El Niño–Southern Oscillation (ENSO). A basinwide warming occurs in the tropical Indian Ocean (TIO) during El Niño mature winter. The TIO warming persists from the El Niño peak winter to the succeeding summer. Meanwhile, a cold SST anomaly (SSTA) appears in the eastern WNP and persists from the El Niño mature winter to the succeeding summer. Idealized AGCM experiments that separate the TIO and WNP SSTA forcing effects show that both the remote eastern TIO forcing and local WNP SSTA forcing are important in affecting atmospheric heating anomaly in the WNP monsoon region, which further impacts the PJ interannual teleconnection pattern over East Asia. In contrast to the interannual variation, the interdecadal change of the PJ-like pattern is primarily affected by the interdecadal change of SST in the TIO rather than by the local SSTA in the WNP.
Abstract
The predictability of temperatures in North America is of great importance for local agriculture and human health. In autumn (SON), the temperature in North America is correlated with ENSO, but its predictive skill is limited. Here, we show that spring (MAM) sea surface temperature (SST) anomalies in the southwest Pacific (SWP) exhibit a higher correlation with autumn temperature in North America (r = 0.73) than ENSO. This cross-seasonal and cross-hemispheric relationship is established via the western tropical Pacific (WTP) region. The spring SWP SST anomalies show a cross-hemispheric propagation embedded in the southerly monsoonal flow through the wind–evaporation–SST feedback, which sustains the progression of SST anomalies toward the WTP in autumn. The AGCM simulations from multimodels show that active convection in the WTP caused by SST warming stimulate a Rossby wave train propagating downward to North America, where it is governed by an anomalous high with increased atmospheric thickness and tropospheric temperature. Consequently, the longwave radiative heating over North America is enhanced, raising the surface air temperature. These results indicate that the SWP SST is a useful predictor of North American temperature 6 months in advance through the cross-hemispheric influence. A model based on the spring SWP SST and preceding winter ENSO (Niño-3.4) shows a high predictive skill for the autumn temperature anomalies in North America.
Significance Statement
Seasonal predictions of surface temperature are important for agricultural decision-making, disaster prevention, and mitigation. Over the western Pacific, associated with the seasonal evolution of ITCZ, there is a strong northward progression of SST anomaly signals generated in the southwest Pacific. This study finds that this cross-hemispheric SST propagation can significantly impact the autumn surface temperature over central North America through the atmospheric bridge. A statistical model incorporating southwest Pacific SST for spring is constructed to predict the North American autumn surface temperature and exhibits high consistency with observations. This improves our understanding of the interhemispheric interactions at seasonal time scales and the seasonal predictability of the North American climate.
Abstract
The predictability of temperatures in North America is of great importance for local agriculture and human health. In autumn (SON), the temperature in North America is correlated with ENSO, but its predictive skill is limited. Here, we show that spring (MAM) sea surface temperature (SST) anomalies in the southwest Pacific (SWP) exhibit a higher correlation with autumn temperature in North America (r = 0.73) than ENSO. This cross-seasonal and cross-hemispheric relationship is established via the western tropical Pacific (WTP) region. The spring SWP SST anomalies show a cross-hemispheric propagation embedded in the southerly monsoonal flow through the wind–evaporation–SST feedback, which sustains the progression of SST anomalies toward the WTP in autumn. The AGCM simulations from multimodels show that active convection in the WTP caused by SST warming stimulate a Rossby wave train propagating downward to North America, where it is governed by an anomalous high with increased atmospheric thickness and tropospheric temperature. Consequently, the longwave radiative heating over North America is enhanced, raising the surface air temperature. These results indicate that the SWP SST is a useful predictor of North American temperature 6 months in advance through the cross-hemispheric influence. A model based on the spring SWP SST and preceding winter ENSO (Niño-3.4) shows a high predictive skill for the autumn temperature anomalies in North America.
Significance Statement
Seasonal predictions of surface temperature are important for agricultural decision-making, disaster prevention, and mitigation. Over the western Pacific, associated with the seasonal evolution of ITCZ, there is a strong northward progression of SST anomaly signals generated in the southwest Pacific. This study finds that this cross-hemispheric SST propagation can significantly impact the autumn surface temperature over central North America through the atmospheric bridge. A statistical model incorporating southwest Pacific SST for spring is constructed to predict the North American autumn surface temperature and exhibits high consistency with observations. This improves our understanding of the interhemispheric interactions at seasonal time scales and the seasonal predictability of the North American climate.
Abstract
Turbulent mixing in the northwestern Pacific Ocean is estimated using the Gregg–Henyey–Polzin scaling and Thorpe-scale methods. The data sources are the hydrographic observations during October and November 2005. The results reveal clear spatial patterns of turbulent mixing in the study area. High-level diffusivity on the order of 10−3 m2 s−1 or larger is found within the western boundary region, where the Kuroshio flows northward. The width covered by this prominent diffusivity shows an increase from 12° to 18°N. The horizontal distribution of depth-averaged diffusivity in the top 500 m shows enhanced mixing with diffusivity of 6 × 10−3 m2 s−1 south of 9°N where the Mindanao Eddy remains a quasi-permanent feature. These two distinct patterns of diffusivity distribution suggest that the Kuroshio and the Mindanao Eddy are likely responsible for the elevated turbulent mixing in the study area.
Abstract
Turbulent mixing in the northwestern Pacific Ocean is estimated using the Gregg–Henyey–Polzin scaling and Thorpe-scale methods. The data sources are the hydrographic observations during October and November 2005. The results reveal clear spatial patterns of turbulent mixing in the study area. High-level diffusivity on the order of 10−3 m2 s−1 or larger is found within the western boundary region, where the Kuroshio flows northward. The width covered by this prominent diffusivity shows an increase from 12° to 18°N. The horizontal distribution of depth-averaged diffusivity in the top 500 m shows enhanced mixing with diffusivity of 6 × 10−3 m2 s−1 south of 9°N where the Mindanao Eddy remains a quasi-permanent feature. These two distinct patterns of diffusivity distribution suggest that the Kuroshio and the Mindanao Eddy are likely responsible for the elevated turbulent mixing in the study area.
Abstract
On the basis of the similarity theory of the atmospheric surface layer and the mass conservation principle, a new scheme using a variational method is developed to estimate the surface momentum and sensible and latent heat fluxes. In this scheme, the mass conservation is introduced into the cost function as a weak physical constraint, which leads to an overdetermined system. For the variational method with mass conservation constraint, only the conventional meteorological observational data are taken into account. Data collected in the Yellow River Source Region Climate and Environment Observation and Research Station at Maqu, China, during 11–25 August 2010 are used to test this new scheme. Results indicate that this scheme is more reliable and accurate than both the flux-profile method and the variational method without mass conservation constraint. In addition, the effect of the weights in the cost function is examined. Sensitivity tests show that the fluxes estimated by the proposed scheme are insensitive to the stability functions explored in the cost function and measurement errors.
Abstract
On the basis of the similarity theory of the atmospheric surface layer and the mass conservation principle, a new scheme using a variational method is developed to estimate the surface momentum and sensible and latent heat fluxes. In this scheme, the mass conservation is introduced into the cost function as a weak physical constraint, which leads to an overdetermined system. For the variational method with mass conservation constraint, only the conventional meteorological observational data are taken into account. Data collected in the Yellow River Source Region Climate and Environment Observation and Research Station at Maqu, China, during 11–25 August 2010 are used to test this new scheme. Results indicate that this scheme is more reliable and accurate than both the flux-profile method and the variational method without mass conservation constraint. In addition, the effect of the weights in the cost function is examined. Sensitivity tests show that the fluxes estimated by the proposed scheme are insensitive to the stability functions explored in the cost function and measurement errors.
Abstract
This study examines the validity and limitations associated with retrieval of cloud optical depth τ and effective droplet size r e in the Arctic from Advanced Very High Resolution Radiometer (AVHRR) channels 2 (0.725–1.10 μm), 3 (3.55–3.93 μm), and 4 (10.3–11.3 μm). The error in r e is found to be normally less than 10%, but the uncertainty in τ can be more than 50% for a 10% uncertainty in the satellite-measured radiance. Model simulations show that the satellite-retrieved cloud optical depth τ sat is overestimated by up to 20% if the vertical cloud inhomogeneity is ignored and is underestimated by more than 50% if overlap of cirrus and liquid water clouds is ignored. Under partially cloudy conditions, τ sat is larger than that derived from surface-measured downward solar irradiance (τ surf) by 40%–130%, depending on cloud-cover fraction. Here, τ sat derived from NOAA-14 AVHRR data agrees well with τ surf derived from surface measurements of solar irradiance at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp in summer, but τ sat is about 2.3 times τ surf before the onset of snowmelt. This overestimate of τ sat is mainly due to the high reflectivity in AVHRR channel 2 over snow/ice surfaces, the presence of partial cloud cover, and inaccurate representation of the scattering phase function for mixed-phase clouds.
Abstract
This study examines the validity and limitations associated with retrieval of cloud optical depth τ and effective droplet size r e in the Arctic from Advanced Very High Resolution Radiometer (AVHRR) channels 2 (0.725–1.10 μm), 3 (3.55–3.93 μm), and 4 (10.3–11.3 μm). The error in r e is found to be normally less than 10%, but the uncertainty in τ can be more than 50% for a 10% uncertainty in the satellite-measured radiance. Model simulations show that the satellite-retrieved cloud optical depth τ sat is overestimated by up to 20% if the vertical cloud inhomogeneity is ignored and is underestimated by more than 50% if overlap of cirrus and liquid water clouds is ignored. Under partially cloudy conditions, τ sat is larger than that derived from surface-measured downward solar irradiance (τ surf) by 40%–130%, depending on cloud-cover fraction. Here, τ sat derived from NOAA-14 AVHRR data agrees well with τ surf derived from surface measurements of solar irradiance at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp in summer, but τ sat is about 2.3 times τ surf before the onset of snowmelt. This overestimate of τ sat is mainly due to the high reflectivity in AVHRR channel 2 over snow/ice surfaces, the presence of partial cloud cover, and inaccurate representation of the scattering phase function for mixed-phase clouds.
Abstract
This study investigates the precursors and formation mechanisms of spring (April–May) event-based extreme precipitation (EEP) during 1961–2014 in central-eastern China. The EEP events during springtime are primarily characterized by extreme precipitation that occurs at the first half of an event. During early stages of spring EEP events, a Rossby wave grows over western Europe and the North Atlantic Ocean. The wave propagates eastward toward East Asia, exhibiting a circumglobal teleconnection (CGT) pattern. A strong anticyclone related to the CGT pattern is formed over the islands of Japan in the upper troposphere, enhancing the divergence anomalies and bringing more water vapor anomalies from the Sea of Japan into central-eastern China. Meanwhile, the westerly jet jumps northward and anomalous southwesterly water vapor flux is significantly prevalent, both associated with the onset of the Bay of Bengal summer monsoon (BOBSM). When the anomalous southwesterly and northeasterly moisture fluxes into central-eastern China combine, strong convergence is formed, providing abundant water vapor for extreme precipitation. The moisture budget analysis further suggests that the dynamic processes associated with horizontal wind anomalies play a crucial role in the moisture convergence for the spring EEP events. The advection of zonal and meridional moisture is strongly related to the anomalous winds of the CGT waves and BOBSM, respectively; the horizontal thermodynamic processes related to specific humidity and vertical advection contribute much less. The results indicate the preceding signals in the midlatitudes and subtropics for the spring EEP events, enabling extreme precipitation forecasting and hydrological prediction.
Abstract
This study investigates the precursors and formation mechanisms of spring (April–May) event-based extreme precipitation (EEP) during 1961–2014 in central-eastern China. The EEP events during springtime are primarily characterized by extreme precipitation that occurs at the first half of an event. During early stages of spring EEP events, a Rossby wave grows over western Europe and the North Atlantic Ocean. The wave propagates eastward toward East Asia, exhibiting a circumglobal teleconnection (CGT) pattern. A strong anticyclone related to the CGT pattern is formed over the islands of Japan in the upper troposphere, enhancing the divergence anomalies and bringing more water vapor anomalies from the Sea of Japan into central-eastern China. Meanwhile, the westerly jet jumps northward and anomalous southwesterly water vapor flux is significantly prevalent, both associated with the onset of the Bay of Bengal summer monsoon (BOBSM). When the anomalous southwesterly and northeasterly moisture fluxes into central-eastern China combine, strong convergence is formed, providing abundant water vapor for extreme precipitation. The moisture budget analysis further suggests that the dynamic processes associated with horizontal wind anomalies play a crucial role in the moisture convergence for the spring EEP events. The advection of zonal and meridional moisture is strongly related to the anomalous winds of the CGT waves and BOBSM, respectively; the horizontal thermodynamic processes related to specific humidity and vertical advection contribute much less. The results indicate the preceding signals in the midlatitudes and subtropics for the spring EEP events, enabling extreme precipitation forecasting and hydrological prediction.
Abstract
This study quantifies the contributions of tropical sea surface temperature (SST) variations during the boreal warm season to the interannual-to-decadal variability in tropical cyclone genesis frequency (TCGF) over the Northern Hemisphere ocean basins. The first seven leading modes of tropical SST variability are found to affect basinwide TCGF in one or more basins, and are related to canonical El Niño–Southern Oscillation (ENSO), global warming (GW), the Pacific meridional mode (PMM), Atlantic multidecadal oscillation (AMO), Pacific decadal oscillation (PDO), and the Atlantic meridional mode (AMM). These modes account for approximately 58%, 50%, and 56% of the variance in basinwide TCGF during 1969–2018 over the North Atlantic (NA), northeast Pacific (NEP), and northwest Pacific (NWP) Oceans, respectively. The SST effect is weak on TCGF variability in the north Indian Ocean. The SST modes dominating TCGF variability differ among the basins: ENSO, the AMO, AMM, and GW are dominant for the NA; ENSO and the AMO for the NEP; and the PMM, interannual AMO, and GW for the NWP. A specific mode may have opposite effects on TCGF in different basins, particularly between the NA and NEP. Sliding-window multiple linear regression analyses show that the SST effects on basinwide TCGF are stable in time in the NA and NWP, but have strengthened since the 1990s in the NEP. The SST effects on local TC genesis and occurrence frequency are also explored, and the underlying physical mechanisms are examined by diagnosing a genesis potential index and its components.
Abstract
This study quantifies the contributions of tropical sea surface temperature (SST) variations during the boreal warm season to the interannual-to-decadal variability in tropical cyclone genesis frequency (TCGF) over the Northern Hemisphere ocean basins. The first seven leading modes of tropical SST variability are found to affect basinwide TCGF in one or more basins, and are related to canonical El Niño–Southern Oscillation (ENSO), global warming (GW), the Pacific meridional mode (PMM), Atlantic multidecadal oscillation (AMO), Pacific decadal oscillation (PDO), and the Atlantic meridional mode (AMM). These modes account for approximately 58%, 50%, and 56% of the variance in basinwide TCGF during 1969–2018 over the North Atlantic (NA), northeast Pacific (NEP), and northwest Pacific (NWP) Oceans, respectively. The SST effect is weak on TCGF variability in the north Indian Ocean. The SST modes dominating TCGF variability differ among the basins: ENSO, the AMO, AMM, and GW are dominant for the NA; ENSO and the AMO for the NEP; and the PMM, interannual AMO, and GW for the NWP. A specific mode may have opposite effects on TCGF in different basins, particularly between the NA and NEP. Sliding-window multiple linear regression analyses show that the SST effects on basinwide TCGF are stable in time in the NA and NWP, but have strengthened since the 1990s in the NEP. The SST effects on local TC genesis and occurrence frequency are also explored, and the underlying physical mechanisms are examined by diagnosing a genesis potential index and its components.
Abstract
Based on daily rainfall observations and Japanese 25-year Reanalysis Project data during ~1981–2010, a three-dimensional circulation structure that formed before heavy summer rainfall in central north China (CNC) is revealed in this study. Composite analyses of circulation in advance of 225 heavy rain days show that the circulation structure is characterized by a remarkable upper-tropospheric warm anomaly (UTWA), which covers most of northern China with a center at ~300 hPa. Under hydrostatic and geostrophic equilibriums, the UTWA contributes to the generation of an anticyclonic (cyclonic) anomaly above (below). The anticyclonic anomaly strengthens (weakens) westerly winds to the north (south) of the warm center and pushes the high-level westerly jet to the north. The cyclonic anomaly deepens the trough upstream of CNC and intensifies lower southwesterly winds to the mideast of the warm center. As a result, the northerly stretched high-level jet produces upper divergence in its right-front side and the intensified southwesterly winds induce lower moisture convergence in its left-front side, causing heavy rainfall in CNC. Correlation analyses further confirm the close connections between UTWA and circulation in the upper and lower troposphere. The correlation coefficients between UTWA and the upper geopotential height, upper westerly jet, and lower southerly flow reach 0.95, 0.70, and 0.39, implying that the two critical factors leading to intense rainfall in CNC, the high-level jet and the low-level southerly flow, are closely connected with the UTWA. Consequently, in the future analyses and forecasts of heavy rainfall over northern China, more attention should be paid to the temperature in the upper troposphere.
Abstract
Based on daily rainfall observations and Japanese 25-year Reanalysis Project data during ~1981–2010, a three-dimensional circulation structure that formed before heavy summer rainfall in central north China (CNC) is revealed in this study. Composite analyses of circulation in advance of 225 heavy rain days show that the circulation structure is characterized by a remarkable upper-tropospheric warm anomaly (UTWA), which covers most of northern China with a center at ~300 hPa. Under hydrostatic and geostrophic equilibriums, the UTWA contributes to the generation of an anticyclonic (cyclonic) anomaly above (below). The anticyclonic anomaly strengthens (weakens) westerly winds to the north (south) of the warm center and pushes the high-level westerly jet to the north. The cyclonic anomaly deepens the trough upstream of CNC and intensifies lower southwesterly winds to the mideast of the warm center. As a result, the northerly stretched high-level jet produces upper divergence in its right-front side and the intensified southwesterly winds induce lower moisture convergence in its left-front side, causing heavy rainfall in CNC. Correlation analyses further confirm the close connections between UTWA and circulation in the upper and lower troposphere. The correlation coefficients between UTWA and the upper geopotential height, upper westerly jet, and lower southerly flow reach 0.95, 0.70, and 0.39, implying that the two critical factors leading to intense rainfall in CNC, the high-level jet and the low-level southerly flow, are closely connected with the UTWA. Consequently, in the future analyses and forecasts of heavy rainfall over northern China, more attention should be paid to the temperature in the upper troposphere.
