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- Author or Editor: Jing Wang x
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Abstract
The multiply nested, fully compressible, nonhydrostatic tropical cyclone model version 4 (TCM4) is used to examine and understand the sensitivity of the simulated tropical cyclone (TC) inner-core size to its initial vortex size. The results show that although the simulated TC intensity at the mature stage is weakly dependent on the initial vortex size for the general settings, the simulated TC inner-core size is largely determined by the initial vortex size. The initial vortex size is critical to both the energy input from the ocean and the effectiveness of the inward angular momentum transport by the transverse circulation driven by eyewall convection and diabatic heating in spiral rainbands.
Strong outer winds in a storm with a large initial size lead to large entropy fluxes to a large radial extent outside the eyewall, favoring the development of active spiral rainbands. Latent heat released in spiral rainbands plays a key role in increasing the low-level radial inflow and accelerating tangential winds outside the eyewall, leading to outward expansion of tangential wind fields and thus increasing the inner-core size of the simulated storm. On the contrary, a storm with a small initial size has weaker outer winds and smaller surface entropy fluxes outside the eyewall and is accompanied by less active spiral rainbands and thus a much slower increase in the inner-core size. The effectiveness of the inward transport of absolute angular momentum to increase the tangential winds outside the eyewall is largely determined by the radial extent of the vertical absolute vorticity, which is shown to be higher in a large size vortex.
The relative importance of the initial vortex size and the environmental relative humidity (RH) to the TC inner-core size is also evaluated. It is found that the inner-core size of the simulated storm at the mature stage depends more heavily on the initial vortex size than on the initial RH of the environment.
Abstract
The multiply nested, fully compressible, nonhydrostatic tropical cyclone model version 4 (TCM4) is used to examine and understand the sensitivity of the simulated tropical cyclone (TC) inner-core size to its initial vortex size. The results show that although the simulated TC intensity at the mature stage is weakly dependent on the initial vortex size for the general settings, the simulated TC inner-core size is largely determined by the initial vortex size. The initial vortex size is critical to both the energy input from the ocean and the effectiveness of the inward angular momentum transport by the transverse circulation driven by eyewall convection and diabatic heating in spiral rainbands.
Strong outer winds in a storm with a large initial size lead to large entropy fluxes to a large radial extent outside the eyewall, favoring the development of active spiral rainbands. Latent heat released in spiral rainbands plays a key role in increasing the low-level radial inflow and accelerating tangential winds outside the eyewall, leading to outward expansion of tangential wind fields and thus increasing the inner-core size of the simulated storm. On the contrary, a storm with a small initial size has weaker outer winds and smaller surface entropy fluxes outside the eyewall and is accompanied by less active spiral rainbands and thus a much slower increase in the inner-core size. The effectiveness of the inward transport of absolute angular momentum to increase the tangential winds outside the eyewall is largely determined by the radial extent of the vertical absolute vorticity, which is shown to be higher in a large size vortex.
The relative importance of the initial vortex size and the environmental relative humidity (RH) to the TC inner-core size is also evaluated. It is found that the inner-core size of the simulated storm at the mature stage depends more heavily on the initial vortex size than on the initial RH of the environment.
Abstract
In a recent study by Wang et al. that introduced a dynamical efficiency to the intensification potential of a tropical cyclone (TC) system, a simplified energetically based dynamical system (EBDS) model was shown to be able to capture the intensity dependence of TC potential intensification rate (PIR) in both idealized numerical simulations and observations. Although the EBDS model can capture the intensity dependence of TC intensification as in observations, a detailed evaluation has not yet been done. This study provides an evaluation of the EBDS model in reproducing the intensity-dependent feature of the observed TC PIR based on the best track data for TCs over the North Atlantic and central, eastern, and western North Pacific during 1982–2019. Results show that the theoretical PIR estimated by the EBDS model can capture basic features of the observed PIR reasonably well. The TC PIR in the best track data increases with increasing relative TC intensity [intensity normalized by its corresponding maximum potential intensity (MPI)] and reaches a maximum at an intermediate relative intensity around 0.6, and then decreases with increasing relative intensity to zero as the TC approaches its MPI, as in idealized numerical simulations. Results also show that the PIR for a given relative intensity increases with the increasing MPI and thus increasing sea surface temperature, which is also consistent with the theoretical PIR implied by the EBDS model. In addition, future directions to include environmental effects and make the EBDS model applicable to predict intensity change of real TCs are also discussed.
Abstract
In a recent study by Wang et al. that introduced a dynamical efficiency to the intensification potential of a tropical cyclone (TC) system, a simplified energetically based dynamical system (EBDS) model was shown to be able to capture the intensity dependence of TC potential intensification rate (PIR) in both idealized numerical simulations and observations. Although the EBDS model can capture the intensity dependence of TC intensification as in observations, a detailed evaluation has not yet been done. This study provides an evaluation of the EBDS model in reproducing the intensity-dependent feature of the observed TC PIR based on the best track data for TCs over the North Atlantic and central, eastern, and western North Pacific during 1982–2019. Results show that the theoretical PIR estimated by the EBDS model can capture basic features of the observed PIR reasonably well. The TC PIR in the best track data increases with increasing relative TC intensity [intensity normalized by its corresponding maximum potential intensity (MPI)] and reaches a maximum at an intermediate relative intensity around 0.6, and then decreases with increasing relative intensity to zero as the TC approaches its MPI, as in idealized numerical simulations. Results also show that the PIR for a given relative intensity increases with the increasing MPI and thus increasing sea surface temperature, which is also consistent with the theoretical PIR implied by the EBDS model. In addition, future directions to include environmental effects and make the EBDS model applicable to predict intensity change of real TCs are also discussed.
Abstract
A tropical cyclone (TC) viewed as a heat engine converts heat energy extracted from the ocean into the kinetic energy of the TC, which is eventually dissipated due to surface friction. Since the energy production rate is a linear function while the frictional dissipation rate is a cubic power of surface wind speed, the dissipation rate is generally smaller than the production rate initially but increases faster than the production rate as the storm intensifies. When the dissipation rate eventually reaches the production rate, the TC has no excess energy to intensify. Emanuel hypothesized that a TC achieves its maximum potential intensity (E-MPI) when the surface frictional dissipation rate balances the energy production rate near the radius of maximum wind (RMW). Although the E-MPI agrees well with the maximum intensity of numerically simulated TCs in earlier axisymmetric models, the balance hypothesis near the RMW has not been evaluated. This study shows that the frictional dissipation rate in a numerically simulated mature TC is about 25% larger than the energy production rate near the RMW, while the dissipation rate is lower than the energy production rate outside the eyewall. This finding implies that the excess frictional dissipation under the eyewall should be partially balanced by the energy production outside the eyewall and thus the local balance hypothesis underestimates the TC maximum intensity. Both Lagrangian and control volume equivalent potential temperature (θe ) budget analyses demonstrate that the energy gained by boundary layer inflow air due to surface entropy fluxes outside of and prior to interaction with the eyewall contributes significantly to the energy balance in the eyewall through the lateral inward energy flux. This contribution is further verified using a sensitivity experiment in which the surface entropy fluxes are eliminated outside a radius of 30–45 km, which leads to a 13.5% reduction in the maximum sustained near-surface wind speed and a largely reduced size of the model TC.
Abstract
A tropical cyclone (TC) viewed as a heat engine converts heat energy extracted from the ocean into the kinetic energy of the TC, which is eventually dissipated due to surface friction. Since the energy production rate is a linear function while the frictional dissipation rate is a cubic power of surface wind speed, the dissipation rate is generally smaller than the production rate initially but increases faster than the production rate as the storm intensifies. When the dissipation rate eventually reaches the production rate, the TC has no excess energy to intensify. Emanuel hypothesized that a TC achieves its maximum potential intensity (E-MPI) when the surface frictional dissipation rate balances the energy production rate near the radius of maximum wind (RMW). Although the E-MPI agrees well with the maximum intensity of numerically simulated TCs in earlier axisymmetric models, the balance hypothesis near the RMW has not been evaluated. This study shows that the frictional dissipation rate in a numerically simulated mature TC is about 25% larger than the energy production rate near the RMW, while the dissipation rate is lower than the energy production rate outside the eyewall. This finding implies that the excess frictional dissipation under the eyewall should be partially balanced by the energy production outside the eyewall and thus the local balance hypothesis underestimates the TC maximum intensity. Both Lagrangian and control volume equivalent potential temperature (θe ) budget analyses demonstrate that the energy gained by boundary layer inflow air due to surface entropy fluxes outside of and prior to interaction with the eyewall contributes significantly to the energy balance in the eyewall through the lateral inward energy flux. This contribution is further verified using a sensitivity experiment in which the surface entropy fluxes are eliminated outside a radius of 30–45 km, which leads to a 13.5% reduction in the maximum sustained near-surface wind speed and a largely reduced size of the model TC.
Abstract
The surface energy (entropy) flux is critical to the development and maintenance of a tropical cyclone (TC). However, it is unclear how sensitive the inner-core size and intensity of a TC could be to the radial distribution of the surface entropy flux under the TC. Such a potential sensitivity is examined in this study using the multiply nested, fully compressible, nonhydrostatic TC model TCM4. By artificially eliminating the surface entropy fluxes in different radial extent in different experiments, the effect of the surface entropy flux in the different radial ranges on the inner-core size and intensity of a simulated TC is evaluated. Consistent with recent findings from axisymmetric models, the entropy flux in the eye region of a TC is found to contribute little to the storm intensity, but it plays a role in reducing the radius of maximum wind (RMW). Although surface entropy fluxes under the eyewall contribute greatly to the storm intensity, those outside the eyewall up to a radius of about 2–2.5 times the RMW are also important. Farther outward, the surface entropy fluxes are found to be crucial to the growth of the storm inner-core size but could reduce the storm intensity. The surface entropy flux outside the inner core plays a critical role in maintaining high convective available potential energy (CAPE) outside the eyewall and thus active spiral rainbands. The latent heat release in these rainbands is responsible for the increase in the inner-core size of the simulated TC. A positive feedback is identified to explain changes in the inner-core size of the simulated storms in different experiments. Implications of the results for both observations and numerical prediction of TC structure and intensity changes are briefly discussed.
Abstract
The surface energy (entropy) flux is critical to the development and maintenance of a tropical cyclone (TC). However, it is unclear how sensitive the inner-core size and intensity of a TC could be to the radial distribution of the surface entropy flux under the TC. Such a potential sensitivity is examined in this study using the multiply nested, fully compressible, nonhydrostatic TC model TCM4. By artificially eliminating the surface entropy fluxes in different radial extent in different experiments, the effect of the surface entropy flux in the different radial ranges on the inner-core size and intensity of a simulated TC is evaluated. Consistent with recent findings from axisymmetric models, the entropy flux in the eye region of a TC is found to contribute little to the storm intensity, but it plays a role in reducing the radius of maximum wind (RMW). Although surface entropy fluxes under the eyewall contribute greatly to the storm intensity, those outside the eyewall up to a radius of about 2–2.5 times the RMW are also important. Farther outward, the surface entropy fluxes are found to be crucial to the growth of the storm inner-core size but could reduce the storm intensity. The surface entropy flux outside the inner core plays a critical role in maintaining high convective available potential energy (CAPE) outside the eyewall and thus active spiral rainbands. The latent heat release in these rainbands is responsible for the increase in the inner-core size of the simulated TC. A positive feedback is identified to explain changes in the inner-core size of the simulated storms in different experiments. Implications of the results for both observations and numerical prediction of TC structure and intensity changes are briefly discussed.
Abstract
The dependence of tropical cyclone (TC) intensification rate IR on storm intensity and size was statistically analyzed for North Atlantic TCs during 1988–2012. The results show that IR is positively (negatively) correlated with storm intensity (the maximum sustained near-surface wind speed V max) when V max is below (above) 70–80 knots (kt; 1 kt = 0.51 m s−1), and negatively correlated with storm size in terms of the radius of maximum wind (RMW), the average radius of gale-force wind (AR34), and the outer-core wind skirt parameter DR34 (=AR34 − RMW). The turning point for V max of 70–80 kt is explained as a balance between the potential intensification and the maximum potential intensity (MPI). The highest IR occurs for V max = 80 kt, RMW ≤ 40 km, and AR34 = DR34 = 150 km. The high frequency of occurrence of intensifying TCs occurs for V max ≤ 80 kt and RMW between 20 and 60 km, AR34 ≤ 200 km, and DR34 ≤ 150 km. Rapid intensification (RI) often occurs in a relatively narrow parameter space in storm intensity and both inner- and outer-core sizes. In addition, a theoretical basis for the intensity dependency has also been provided based on a previously constructed simplified dynamical system for TC intensity prediction.
Abstract
The dependence of tropical cyclone (TC) intensification rate IR on storm intensity and size was statistically analyzed for North Atlantic TCs during 1988–2012. The results show that IR is positively (negatively) correlated with storm intensity (the maximum sustained near-surface wind speed V max) when V max is below (above) 70–80 knots (kt; 1 kt = 0.51 m s−1), and negatively correlated with storm size in terms of the radius of maximum wind (RMW), the average radius of gale-force wind (AR34), and the outer-core wind skirt parameter DR34 (=AR34 − RMW). The turning point for V max of 70–80 kt is explained as a balance between the potential intensification and the maximum potential intensity (MPI). The highest IR occurs for V max = 80 kt, RMW ≤ 40 km, and AR34 = DR34 = 150 km. The high frequency of occurrence of intensifying TCs occurs for V max ≤ 80 kt and RMW between 20 and 60 km, AR34 ≤ 200 km, and DR34 ≤ 150 km. Rapid intensification (RI) often occurs in a relatively narrow parameter space in storm intensity and both inner- and outer-core sizes. In addition, a theoretical basis for the intensity dependency has also been provided based on a previously constructed simplified dynamical system for TC intensity prediction.
Abstract
This study extends the statistical analysis on the dependence of tropical cyclone (TC) intensification rate (IR) on sea surface temperature (SST), storm initial intensity (maximum sustained surface wind speed V max), and storm size, in terms of the radius of maximum wind (RMW), the radius of 34-kt (AR34; 1 kt = 0.51 m s−1) wind, and the outer-core wind skirt parameter DR34 (= AR34 − RMW), for North Atlantic TCs to western North Pacific (WNP) TCs during 1982–2015. Results show that the relationship between the TC maximum potential intensification rate (MPIR) and SST also exists in the WNP. TC IR depends strongly on TC intensity and structure, consistent with the findings for North Atlantic TCs. TC IR is positively (negatively) correlated with storm intensity when V max is below (above) 70 kt and negatively correlated with the RMW. Rapid intensification (RI) occurs only in a relatively narrow range of parameter space in storm intensity and both inner- and outer-core sizes, with the highest IR appearing for V max = 70 kt, RMW ≦ 40 km, AR34 = 150 km, and DR34 = 100 km. The highest frequency of occurrence of intensifying TCs occurs for V max ~ 40–60 kt, RMW ~ 20–60 km, AR34 = 200 km, and DR34 = 120 km. Overall, these values are very similar to those for TCs in the North Atlantic. These results suggest the need for the realistic initialization of TC structure in numerical models and the inclusion of size parameters in statistical TC intensity prediction schemes.
Abstract
This study extends the statistical analysis on the dependence of tropical cyclone (TC) intensification rate (IR) on sea surface temperature (SST), storm initial intensity (maximum sustained surface wind speed V max), and storm size, in terms of the radius of maximum wind (RMW), the radius of 34-kt (AR34; 1 kt = 0.51 m s−1) wind, and the outer-core wind skirt parameter DR34 (= AR34 − RMW), for North Atlantic TCs to western North Pacific (WNP) TCs during 1982–2015. Results show that the relationship between the TC maximum potential intensification rate (MPIR) and SST also exists in the WNP. TC IR depends strongly on TC intensity and structure, consistent with the findings for North Atlantic TCs. TC IR is positively (negatively) correlated with storm intensity when V max is below (above) 70 kt and negatively correlated with the RMW. Rapid intensification (RI) occurs only in a relatively narrow range of parameter space in storm intensity and both inner- and outer-core sizes, with the highest IR appearing for V max = 70 kt, RMW ≦ 40 km, AR34 = 150 km, and DR34 = 100 km. The highest frequency of occurrence of intensifying TCs occurs for V max ~ 40–60 kt, RMW ~ 20–60 km, AR34 = 200 km, and DR34 = 120 km. Overall, these values are very similar to those for TCs in the North Atlantic. These results suggest the need for the realistic initialization of TC structure in numerical models and the inclusion of size parameters in statistical TC intensity prediction schemes.
Abstract
The equatorial wave dynamics of sea level variations during negative Indian Ocean dipole (nIOD) events are investigated using the LICOM ocean general circulation model forced with the European Centre for Medium-Range Weather Forecast reanalysis wind stress and heat flux from 1990 to 2001. The work is a continuation of the study by Yuan and Liu, in which the equatorial wave dynamics during positive IOD events are investigated. The model has reproduced the sea level anomalies of satellite altimeter data well. Long equatorial waves extracted from the model output suggest two kinds of negative feedback during nIOD events: the western boundary reflection and the easterly wind bursts. During the strong 1998–99 nIOD event, the downwelling anomalies in the eastern Indian Ocean are terminated by persistent and strong upwelling Kelvin waves from the western boundary, which are reflected from the wind-forced equatorial Rossby waves over the southern central Indian Ocean. During the 1996–97 nIOD, however, the reflection of upwelling anomalies at the western boundary is terminated by the arrival of downwelling equatorial Rossby waves from the eastern boundary reflection in early 1997. Therefore, the negative feedback of this nIOD event is not provided by the western boundary reflection. The downwelling anomalies in the eastern basin during the 1996–97 nIOD event are terminated by easterly wind anomalies over the equatorial Indian Ocean in early 1997. The disclosed equatorial wave dynamics are important to the simulation and prediction of IOD evolution.
Abstract
The equatorial wave dynamics of sea level variations during negative Indian Ocean dipole (nIOD) events are investigated using the LICOM ocean general circulation model forced with the European Centre for Medium-Range Weather Forecast reanalysis wind stress and heat flux from 1990 to 2001. The work is a continuation of the study by Yuan and Liu, in which the equatorial wave dynamics during positive IOD events are investigated. The model has reproduced the sea level anomalies of satellite altimeter data well. Long equatorial waves extracted from the model output suggest two kinds of negative feedback during nIOD events: the western boundary reflection and the easterly wind bursts. During the strong 1998–99 nIOD event, the downwelling anomalies in the eastern Indian Ocean are terminated by persistent and strong upwelling Kelvin waves from the western boundary, which are reflected from the wind-forced equatorial Rossby waves over the southern central Indian Ocean. During the 1996–97 nIOD, however, the reflection of upwelling anomalies at the western boundary is terminated by the arrival of downwelling equatorial Rossby waves from the eastern boundary reflection in early 1997. Therefore, the negative feedback of this nIOD event is not provided by the western boundary reflection. The downwelling anomalies in the eastern basin during the 1996–97 nIOD event are terminated by easterly wind anomalies over the equatorial Indian Ocean in early 1997. The disclosed equatorial wave dynamics are important to the simulation and prediction of IOD evolution.
Abstract
In this study, the boundary layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer, is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal-wind balance, with eyewall convection coming into moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies, as found recently based on idealized numerical simulations. An ad hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic Ocean and eastern and western North Pacific Ocean. As compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as seen in observations.
Abstract
In this study, the boundary layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer, is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal-wind balance, with eyewall convection coming into moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies, as found recently based on idealized numerical simulations. An ad hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic Ocean and eastern and western North Pacific Ocean. As compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as seen in observations.
Abstract
The sea surface temperature anomalies (SSTA) in the Maritime Continent (MC) region are mainly related to local variability, ENSO, and the Indian Ocean dipole. Using the reanalysis data from NOAA and NCEP–NCAR, by employing the empirical orthogonal function (EOF) analysis, we have explored the principal mode of ENSO-independent summertime SSTA in the MC and its associations with regional climate anomalies. After ENSO signals have been removed, the leading mode of SSTA in the MC exhibits a uniformly signed pattern, which mainly varies on an interannual time scale. The maintenance mechanisms of the ENSO-independent SSTA are different in different subregions, especially over the region south of Java and the tropical northwestern Pacific. When the time coefficient of the first leading EOF mode (EOF1) is positive, warmer SSTAs are observed in the area south of Java. The oceanic dynamic heating there facilitates the warmer SSTA. Thus, the Gill-type response of the atmosphere is found over the region south of Java. The diabatic cooling in the atmosphere is dominant over the tropical northwestern Pacific where the warmer SSTA is maintained by the absorption of solar radiation due to less cloud cover there. A tilted vertical circulation is hence formed, linking the tropical southeastern Indian Ocean with the tropical northwestern Pacific. The anomalous circulations in the Asian–Australian monsoon region are affected by this ENSO-independent SSTA mode, resulting in decreased summer rainfall anomaly in the region near the southeast coast of China and increased winter precipitation anomaly over the extratropical region of Australia.
Abstract
The sea surface temperature anomalies (SSTA) in the Maritime Continent (MC) region are mainly related to local variability, ENSO, and the Indian Ocean dipole. Using the reanalysis data from NOAA and NCEP–NCAR, by employing the empirical orthogonal function (EOF) analysis, we have explored the principal mode of ENSO-independent summertime SSTA in the MC and its associations with regional climate anomalies. After ENSO signals have been removed, the leading mode of SSTA in the MC exhibits a uniformly signed pattern, which mainly varies on an interannual time scale. The maintenance mechanisms of the ENSO-independent SSTA are different in different subregions, especially over the region south of Java and the tropical northwestern Pacific. When the time coefficient of the first leading EOF mode (EOF1) is positive, warmer SSTAs are observed in the area south of Java. The oceanic dynamic heating there facilitates the warmer SSTA. Thus, the Gill-type response of the atmosphere is found over the region south of Java. The diabatic cooling in the atmosphere is dominant over the tropical northwestern Pacific where the warmer SSTA is maintained by the absorption of solar radiation due to less cloud cover there. A tilted vertical circulation is hence formed, linking the tropical southeastern Indian Ocean with the tropical northwestern Pacific. The anomalous circulations in the Asian–Australian monsoon region are affected by this ENSO-independent SSTA mode, resulting in decreased summer rainfall anomaly in the region near the southeast coast of China and increased winter precipitation anomaly over the extratropical region of Australia.
Abstract
The origin of mid-high-latitude intraseasonal oscillations (ISOs) was investigated through both observational and theoretical studies. It was found that maximum intraseasonal variability centers appear near 60°N, which is at odds with maximum synoptic variability centers that are located along the upper-tropospheric jet stream (~45°N). The ISO along 60°N is characterized by a typical zonal wavelength of 7700km and a westward phase speed of −3 m s−1. A marked feature of the mid-high-latitude ISO is a tight coupling among moisture and precipitation and circulation. Motivated by this observational discovery, a moist baroclinic theoretical model was constructed. The analysis of this model indicates that under a realistic background mean state, the model generates the most unstable mode along the reference latitude (60°N), which has a preferred zonal wavelength of 7000km, a westward phase speed of about −3 m s−1, a westward tilted vertical structure, and a zonal structure of perturbation moisture/precipitation being located to the east of the low-level trough, all of which resemble the observed. The cause of the instability arises primarily from the moisture-precipitation-circulation feedback under a moderate background vertical shear.
Abstract
The origin of mid-high-latitude intraseasonal oscillations (ISOs) was investigated through both observational and theoretical studies. It was found that maximum intraseasonal variability centers appear near 60°N, which is at odds with maximum synoptic variability centers that are located along the upper-tropospheric jet stream (~45°N). The ISO along 60°N is characterized by a typical zonal wavelength of 7700km and a westward phase speed of −3 m s−1. A marked feature of the mid-high-latitude ISO is a tight coupling among moisture and precipitation and circulation. Motivated by this observational discovery, a moist baroclinic theoretical model was constructed. The analysis of this model indicates that under a realistic background mean state, the model generates the most unstable mode along the reference latitude (60°N), which has a preferred zonal wavelength of 7000km, a westward phase speed of about −3 m s−1, a westward tilted vertical structure, and a zonal structure of perturbation moisture/precipitation being located to the east of the low-level trough, all of which resemble the observed. The cause of the instability arises primarily from the moisture-precipitation-circulation feedback under a moderate background vertical shear.
