2020 Vol. 37, No. 9
Display Method:
2020, 37(9): 939-950.
doi: 10.1007/s00376-020-9220-9
Abstract:
Predicting tropical cyclone (TC) genesis is of great societal importance but scientifically challenging. It requires fine-resolution coupled models that properly represent air−sea interactions in the atmospheric responses to local warm sea surface temperatures and feedbacks, with aid from coherent coupled initialization. This study uses three sets of high-resolution regional coupled models (RCMs) covering the Asia−Pacific (AP) region initialized with local observations and dynamically downscaled coupled data assimilation to evaluate the predictability of TC genesis in the West Pacific. The AP-RCMs consist of three sets of high-resolution configurations of the Weather Research and Forecasting−Regional Ocean Model System (WRF-ROMS): 27-km WRF with 9-km ROMS, and 9-km WRF with 3-km ROMS. In this study, a 9-km WRF with 9-km ROMS coupled model system is also used in a case test for the predictability of TC genesis. Since the local sea surface temperatures and wind shear conditions that favor TC formation are better resolved, the enhanced-resolution coupled model tends to improve the predictability of TC genesis, which could be further improved by improving planetary boundary layer physics, thus resolving better air−sea and air−land interactions.
Predicting tropical cyclone (TC) genesis is of great societal importance but scientifically challenging. It requires fine-resolution coupled models that properly represent air−sea interactions in the atmospheric responses to local warm sea surface temperatures and feedbacks, with aid from coherent coupled initialization. This study uses three sets of high-resolution regional coupled models (RCMs) covering the Asia−Pacific (AP) region initialized with local observations and dynamically downscaled coupled data assimilation to evaluate the predictability of TC genesis in the West Pacific. The AP-RCMs consist of three sets of high-resolution configurations of the Weather Research and Forecasting−Regional Ocean Model System (WRF-ROMS): 27-km WRF with 9-km ROMS, and 9-km WRF with 3-km ROMS. In this study, a 9-km WRF with 9-km ROMS coupled model system is also used in a case test for the predictability of TC genesis. Since the local sea surface temperatures and wind shear conditions that favor TC formation are better resolved, the enhanced-resolution coupled model tends to improve the predictability of TC genesis, which could be further improved by improving planetary boundary layer physics, thus resolving better air−sea and air−land interactions.
2020, 37(9): 951-958.
doi: 10.1007/s00376-020-2100-5
Abstract:
The backward nonlinear local Lyapunov exponent method (BNLLE) is applied to quantify the predictability of warm and cold events in the Lorenz model. Results show that the maximum prediction lead times of warm and cold events present obvious layered structures in phase space. The maximum prediction lead times of each warm (cold) event on individual circles concentric with the distribution of warm (cold) regime events are roughly the same, whereas the maximum prediction lead time of events on other circles are different. Statistical results show that warm events are more predictable than cold events.
The backward nonlinear local Lyapunov exponent method (BNLLE) is applied to quantify the predictability of warm and cold events in the Lorenz model. Results show that the maximum prediction lead times of warm and cold events present obvious layered structures in phase space. The maximum prediction lead times of each warm (cold) event on individual circles concentric with the distribution of warm (cold) regime events are roughly the same, whereas the maximum prediction lead time of events on other circles are different. Statistical results show that warm events are more predictable than cold events.
2020, 37(9): 959-974.
doi: 10.1007/s00376-020-2105-0
Abstract:
Potential evapotranspiration (EPET) is usually calculated by empirical methods from surface meteorological variables, such as temperature, radiation and wind speed. The in-situ measured pan evaporation (ETpan) can also be used as a proxy for EPET. In this study, EPET values computed from ten models are compared with observed ETpan data in ten Chinese river basins for the period 1961−2013. The daily observed meteorological variables at 2267 stations are used as the input to those models, and a ranking scheme is applied to rank the statistical quantities (ratio of standard deviations, correlation coefficient, and ratio of trends) between ETpan and modeled EPET in different river basins. There are large deviations between the modeled EPET and the ETpan in both the magnitude and the annual trend at most stations. In eight of the basins (except for Southeast and Southwest China), ETpan shows decreasing trends with magnitudes ranging between −0.01 mm d−1 yr−1 and −0.03 mm d−1 yr−1, while the decreasing trends in modeled EPET are less than −0.01 mm d−1 yr−1. Inter comparisons among different models in different river basins suggest that PETHam1 is the best model in the Pearl River basin, PETHam2 outperforms other models in the Huaihe River, Yangtze River and Yellow River basins, and PETFAO is the best model for the remaining basins. Sensitivity analyses reveal that wind speed and sunshine duration are two important factors for decreasing EPET in most basins except in Southeast and Southwest China. The increasing EPET trend in Southeast China is mainly attributed to the reduced relative humidity.
Potential evapotranspiration (EPET) is usually calculated by empirical methods from surface meteorological variables, such as temperature, radiation and wind speed. The in-situ measured pan evaporation (ETpan) can also be used as a proxy for EPET. In this study, EPET values computed from ten models are compared with observed ETpan data in ten Chinese river basins for the period 1961−2013. The daily observed meteorological variables at 2267 stations are used as the input to those models, and a ranking scheme is applied to rank the statistical quantities (ratio of standard deviations, correlation coefficient, and ratio of trends) between ETpan and modeled EPET in different river basins. There are large deviations between the modeled EPET and the ETpan in both the magnitude and the annual trend at most stations. In eight of the basins (except for Southeast and Southwest China), ETpan shows decreasing trends with magnitudes ranging between −0.01 mm d−1 yr−1 and −0.03 mm d−1 yr−1, while the decreasing trends in modeled EPET are less than −0.01 mm d−1 yr−1. Inter comparisons among different models in different river basins suggest that PETHam1 is the best model in the Pearl River basin, PETHam2 outperforms other models in the Huaihe River, Yangtze River and Yellow River basins, and PETFAO is the best model for the remaining basins. Sensitivity analyses reveal that wind speed and sunshine duration are two important factors for decreasing EPET in most basins except in Southeast and Southwest China. The increasing EPET trend in Southeast China is mainly attributed to the reduced relative humidity.
2020, 37(9): 975-986.
doi: 10.1007/s00376-020-0079-6
Abstract:
Most large-scale evapotranspiration (ET) estimation methods require detailed information of land use, land cover, and/or soil type on top of various atmospheric measurements. The complementary relationship of evaporation (CR) takes advantage of the inherent dynamic feedback mechanisms found in the soil−vegetation−atmosphere interface for its estimation of ET rates without the need of such biogeophysical data. ET estimates over the conterminous United States by a new, globally calibrated, static scaling (GCR-stat) of the generalized complementary relationship (GCR) of evaporation were compared to similar estimates of an existing, calibration-free version (GCR-dyn) of the GCR that employs a temporally varying dynamic scaling. Simplified annual water balances of 327 medium and 18 large watersheds served as ground-truth ET values. With long-term monthly mean forcing, GCR-stat (also utilizing precipitation measurements) outperforms GCR-dyn as the latter cannot fully take advantage of its dynamic scaling with such data of reduced temporal variability. However, in a continuous monthly simulation, GCR-dyn is on a par with GCR-stat, and especially excels in reproducing long-term tendencies in annual catchment ET rates even though it does not require precipitation information. The same GCR-dyn estimates were also compared to similar estimates of eight other popular ET products and they generally outperform all of them. For this reason, a dynamic scaling of the GCR is recommended over a static one for modeling long-term behavior of terrestrial ET.
Most large-scale evapotranspiration (ET) estimation methods require detailed information of land use, land cover, and/or soil type on top of various atmospheric measurements. The complementary relationship of evaporation (CR) takes advantage of the inherent dynamic feedback mechanisms found in the soil−vegetation−atmosphere interface for its estimation of ET rates without the need of such biogeophysical data. ET estimates over the conterminous United States by a new, globally calibrated, static scaling (GCR-stat) of the generalized complementary relationship (GCR) of evaporation were compared to similar estimates of an existing, calibration-free version (GCR-dyn) of the GCR that employs a temporally varying dynamic scaling. Simplified annual water balances of 327 medium and 18 large watersheds served as ground-truth ET values. With long-term monthly mean forcing, GCR-stat (also utilizing precipitation measurements) outperforms GCR-dyn as the latter cannot fully take advantage of its dynamic scaling with such data of reduced temporal variability. However, in a continuous monthly simulation, GCR-dyn is on a par with GCR-stat, and especially excels in reproducing long-term tendencies in annual catchment ET rates even though it does not require precipitation information. The same GCR-dyn estimates were also compared to similar estimates of eight other popular ET products and they generally outperform all of them. For this reason, a dynamic scaling of the GCR is recommended over a static one for modeling long-term behavior of terrestrial ET.
2020, 37(9): 987-1006.
doi: 10.1007/s00376-020-2038-7
Abstract:
Two different initialization schemes for tropical cyclone (TC) prediction in numerical models are evaluated based on a case study of Typhoon Lekima (2019). The first is a dynamical initialization (DI) scheme where the axisymmetric TC vortex in the initial conditions is spun up through the 6-h cycle runs before the initial forecast time. The second scheme is a bogussing scheme where the analysis TC vortex is replaced by a synthetic Rankine vortex. Results show that although both initialization schemes can help improve the simulated rapid intensification (RI) of Lekima, the simulation employing the DI scheme (DIS) reproduces better the RI onset and intensification rate than that employing the bogussing scheme (BOG). Further analyses show the cycle runs of DI help establish a realistic TC structure with stronger secondary circulation than those in the control run and BOG, leading to fast vortex spinup and contraction of the radius of maximum wind (RMW). The resultant strong inner-core primary circulation favors precession of the midlevel vortex under the moderate vertical wind shear (VWS) and thus helps vortex alignment, contributing to an earlier RI onset. Afterwards, the decreased vertical shear and the stronger convection inside the RMW support the persistent RI of Lekima in DIS. In contrast, the reduced VWS is not well captured and the inner-core convection is weaker and resides farther away from the TC center in BOG, leading to slower intensification. The results imply that the DI effectively improves the prediction of the inner-core process, which is crucial to the RI forecast.
Two different initialization schemes for tropical cyclone (TC) prediction in numerical models are evaluated based on a case study of Typhoon Lekima (2019). The first is a dynamical initialization (DI) scheme where the axisymmetric TC vortex in the initial conditions is spun up through the 6-h cycle runs before the initial forecast time. The second scheme is a bogussing scheme where the analysis TC vortex is replaced by a synthetic Rankine vortex. Results show that although both initialization schemes can help improve the simulated rapid intensification (RI) of Lekima, the simulation employing the DI scheme (DIS) reproduces better the RI onset and intensification rate than that employing the bogussing scheme (BOG). Further analyses show the cycle runs of DI help establish a realistic TC structure with stronger secondary circulation than those in the control run and BOG, leading to fast vortex spinup and contraction of the radius of maximum wind (RMW). The resultant strong inner-core primary circulation favors precession of the midlevel vortex under the moderate vertical wind shear (VWS) and thus helps vortex alignment, contributing to an earlier RI onset. Afterwards, the decreased vertical shear and the stronger convection inside the RMW support the persistent RI of Lekima in DIS. In contrast, the reduced VWS is not well captured and the inner-core convection is weaker and resides farther away from the TC center in BOG, leading to slower intensification. The results imply that the DI effectively improves the prediction of the inner-core process, which is crucial to the RI forecast.
2020, 37(9): 1007-1018.
doi: 10.1007/s00376-020-9281-9
Abstract:
Cloud radiative kernels were built by BCC_RAD (Beijing Climate Center radiative transfer model) radiative transfer code. Then, short-term cloud feedback and its mechanisms in East Asia (0.5°S−60.5°N, 69.5°−150.5°E) were analyzed quantitatively using the kernels combined with MODIS satellite data from July 2002 to June 2018. According to the surface and monsoon types, four subregions in East Asia—the Tibetan Plateau, northwest, temperate monsoon (TM), and subtropical monsoon (SM)—were selected. The average longwave, shortwave, and net cloud feedbacks in East Asia are −0.68 ± 1.20, 1.34 ± 1.08, and 0.66 ± 0.40 W m−2 K−1 (±2σ), respectively, among which the net feedback is dominated by the positive shortwave feedback. Positive feedback in SM is the strongest of all subregions, mainly due to the contributions of nimbostratus and stratus. In East Asia, short-term feedback in spring is primarily caused by marine stratus in SM, in summer is primarily driven by deep convective cloud in TM, in autumn is mainly caused by land nimbostratus in SM, and in winter is mainly driven by land stratus in SM. Cloud feedback in East Asia is chiefly driven by decreases in mid-level and low cloud fraction owing to the changes in relative humidity, and a decrease in low cloud optical thickness due to the changes in cloud water content.
Cloud radiative kernels were built by BCC_RAD (Beijing Climate Center radiative transfer model) radiative transfer code. Then, short-term cloud feedback and its mechanisms in East Asia (0.5°S−60.5°N, 69.5°−150.5°E) were analyzed quantitatively using the kernels combined with MODIS satellite data from July 2002 to June 2018. According to the surface and monsoon types, four subregions in East Asia—the Tibetan Plateau, northwest, temperate monsoon (TM), and subtropical monsoon (SM)—were selected. The average longwave, shortwave, and net cloud feedbacks in East Asia are −0.68 ± 1.20, 1.34 ± 1.08, and 0.66 ± 0.40 W m−2 K−1 (±2σ), respectively, among which the net feedback is dominated by the positive shortwave feedback. Positive feedback in SM is the strongest of all subregions, mainly due to the contributions of nimbostratus and stratus. In East Asia, short-term feedback in spring is primarily caused by marine stratus in SM, in summer is primarily driven by deep convective cloud in TM, in autumn is mainly caused by land nimbostratus in SM, and in winter is mainly driven by land stratus in SM. Cloud feedback in East Asia is chiefly driven by decreases in mid-level and low cloud fraction owing to the changes in relative humidity, and a decrease in low cloud optical thickness due to the changes in cloud water content.
2020, 37(9): 1019-1031.
doi: 10.1007/s00376-020-9271-y
Abstract:
By employing NCEP−NCAR 1°×1° reanalysis datasets, the mechanism of the easterlies vortex (EV) affecting the short-term movement of the subtropical anticyclone over the western Pacific (WPSA) in the mei-yu period is examined using potential vorticity(PV) theory. The results show that when the EV and the westerlies vortex (WV) travel west/east to the same longitude of 120°E, the WPSA suddenly retreats. The EV and WV manifest as the downward transport of PV in the upper troposphere, and the variation of the corresponding high-value regions of PV significantly reflects the intensity changes of the EV and WV. The meridional propagation of PV causes the intensity change of the EV. The vertical movement on both sides of the EV is related to the position of the EV relative to the WPSA and the South Asian high (SAH). When the high PV in the easterlies and westerlies arrive at the same longitude in the meridional direction, the special circulation pattern will lower the position of PV isolines at the ridge line of the WPSA. Thus, the cyclonic circulation at the lower level will be strengthened, causing the abnormally eastward retreat of the WPSA. Analysis of the PV equation at the isentropic surface indicates that when the positive PV variation west of the EV intensifies, it connects with the positive PV variation east of the WV, forming a positive PV band and making the WPSA retreat abnormally. The horizontal advection of the PV has the greatest effect. The contribution of the vertical advection of PV and the vertical differential of heating is also positive, but the values are relatively small. The contribution of the residual was negative and it becomes smaller before and after the WPSA retreats.
By employing NCEP−NCAR 1°×1° reanalysis datasets, the mechanism of the easterlies vortex (EV) affecting the short-term movement of the subtropical anticyclone over the western Pacific (WPSA) in the mei-yu period is examined using potential vorticity(PV) theory. The results show that when the EV and the westerlies vortex (WV) travel west/east to the same longitude of 120°E, the WPSA suddenly retreats. The EV and WV manifest as the downward transport of PV in the upper troposphere, and the variation of the corresponding high-value regions of PV significantly reflects the intensity changes of the EV and WV. The meridional propagation of PV causes the intensity change of the EV. The vertical movement on both sides of the EV is related to the position of the EV relative to the WPSA and the South Asian high (SAH). When the high PV in the easterlies and westerlies arrive at the same longitude in the meridional direction, the special circulation pattern will lower the position of PV isolines at the ridge line of the WPSA. Thus, the cyclonic circulation at the lower level will be strengthened, causing the abnormally eastward retreat of the WPSA. Analysis of the PV equation at the isentropic surface indicates that when the positive PV variation west of the EV intensifies, it connects with the positive PV variation east of the WV, forming a positive PV band and making the WPSA retreat abnormally. The horizontal advection of the PV has the greatest effect. The contribution of the vertical advection of PV and the vertical differential of heating is also positive, but the values are relatively small. The contribution of the residual was negative and it becomes smaller before and after the WPSA retreats.
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