2019 Vol. 36, No. 2
Display Method:
2019, 36(2): 119-132.
doi: 10.1007/s00376-018-8028-3
Abstract:
We use the Wind Farm Parameterization (WFP) scheme coupled with the Weather Research and Forecasting model under multiple resolution regimes to simulate turbulent wake dynamics generated by a real onshore wind farm and their influence at the local meteorological scale. The model outputs are compared with earlier modeling and observation studies. It is found that higher vertical and horizontal resolutions have great impacts on the simulated wake flow dynamics. The corresponding wind speed deficit and turbulent kinetic energy results match well with previous studies. In addition, the effect of horizontal resolution on near-surface meteorology is significantly higher than that of vertical resolution. The wake flow field extends from the start of the wind farm to downstream within 10 km, where the wind speed deficit may exceed 4%. For a height of 150 m or at a distance of about 25 km downstream, the wind speed deficit is around 2%. This indicates that, at a distance of more than 25 km downstream, the impact of the wind turbines can be ignored. Analysis of near-surface meteorology indicates a night and early morning warming near the surface, and increase in near-surface water vapor mixing ratio with decreasing surface sensible and latent heat fluxes. During daytime, a slight cooling near the surface and decrease in the near-surface water vapor mixing ratio with increasing surface sensible and latent heat fluxes is noticed over the wind farm area.
We use the Wind Farm Parameterization (WFP) scheme coupled with the Weather Research and Forecasting model under multiple resolution regimes to simulate turbulent wake dynamics generated by a real onshore wind farm and their influence at the local meteorological scale. The model outputs are compared with earlier modeling and observation studies. It is found that higher vertical and horizontal resolutions have great impacts on the simulated wake flow dynamics. The corresponding wind speed deficit and turbulent kinetic energy results match well with previous studies. In addition, the effect of horizontal resolution on near-surface meteorology is significantly higher than that of vertical resolution. The wake flow field extends from the start of the wind farm to downstream within 10 km, where the wind speed deficit may exceed 4%. For a height of 150 m or at a distance of about 25 km downstream, the wind speed deficit is around 2%. This indicates that, at a distance of more than 25 km downstream, the impact of the wind turbines can be ignored. Analysis of near-surface meteorology indicates a night and early morning warming near the surface, and increase in near-surface water vapor mixing ratio with decreasing surface sensible and latent heat fluxes. During daytime, a slight cooling near the surface and decrease in the near-surface water vapor mixing ratio with increasing surface sensible and latent heat fluxes is noticed over the wind farm area.
2019, 36(2): 133-142.
doi: 10.1007/s00376-018-8133-3
Abstract:
Extreme heat over the North China Plain is typically induced by anomalous descending flows associated with anticyclonic circulation anomalies. However, an extreme heat event that happened in the North China Plain region on 12-13 July 2015, with maximum temperature higher than 40°C at some stations, was characterized by only a weak simultaneous appearance of an anomalous anticyclone and descending flow, suggesting that some other factor(s) may have induced this heat event. In this study, we used the forecast data produced by the Beijing Rapid Updated Cycling operational forecast system, which predicted the heat event well, to investigate the formation mechanism of this extreme heat event. We calculated the cumulative heat in the mixed-layer air column of North China to represent the change in surface air temperature. The cumulative heat was composed of sensible heat flux from the ground surface and the horizontal heat flux convergence. The results indicated that the horizontal heat flux in the mixed layer played a crucial role in the temporal and spatial distribution of high temperatures. The horizontal heat flux was found to be induced by distinct distributions of air temperatures and horizontal winds at low levels during the two days, implying a complexity of the low-level atmosphere in causing the extreme heat.
Extreme heat over the North China Plain is typically induced by anomalous descending flows associated with anticyclonic circulation anomalies. However, an extreme heat event that happened in the North China Plain region on 12-13 July 2015, with maximum temperature higher than 40°C at some stations, was characterized by only a weak simultaneous appearance of an anomalous anticyclone and descending flow, suggesting that some other factor(s) may have induced this heat event. In this study, we used the forecast data produced by the Beijing Rapid Updated Cycling operational forecast system, which predicted the heat event well, to investigate the formation mechanism of this extreme heat event. We calculated the cumulative heat in the mixed-layer air column of North China to represent the change in surface air temperature. The cumulative heat was composed of sensible heat flux from the ground surface and the horizontal heat flux convergence. The results indicated that the horizontal heat flux in the mixed layer played a crucial role in the temporal and spatial distribution of high temperatures. The horizontal heat flux was found to be induced by distinct distributions of air temperatures and horizontal winds at low levels during the two days, implying a complexity of the low-level atmosphere in causing the extreme heat.
2019, 36(2): 143-159.
doi: 10.1007/s00376-018-7307-3
Abstract:
Diurnal variations in amount, frequency and intensity of warm-season hourly precipitation (HP) at seven levels, which are defined as HP ≥ 0.1, 0.5, 1, 5, 10, 20 and 50 mm, are revealed based on no less than 30 years of hourly rain-gauge observations at national stations over central and eastern China (CEC). This study investigates the variations, relationships, differences and similarities of total, stratiform, convective and extreme HP over the entire CEC and various subregions. Results indicate that the variations in the amount and frequency of HP at the seven levels over the entire CEC all display a bimodal feature. For various regions, the variations of total HP mostly feature two peaks, while convective HP mainly occurs in the late afternoon and determines the diurnal variation of total HP intensity. On the basis of the primary peak time periods of HP frequency at all levels over different subregions, the variations can be classified into three main categories: late-afternoon primary peak, nocturnal primary peak, and time-shifting primary peak. However, the variations over some coastal regions like the Liaodong Peninsula, the Shandong Peninsula, and the coastal regions of Guangdong, distinctly differ from those over their corresponding larger regions. Overall, the normalized diurnal variation amplitude of amount and frequency increases with the increasing HP intensity; convective precipitation can be represented by HP ≥ 10 mm; and the intensity of HP ≥ 50 mm is slightly larger during the nighttime than during the daytime over the entire CEC. In northern China, diurnal variation in HP ≥ 5 mm can represent well that in convective precipitation.
Diurnal variations in amount, frequency and intensity of warm-season hourly precipitation (HP) at seven levels, which are defined as HP ≥ 0.1, 0.5, 1, 5, 10, 20 and 50 mm, are revealed based on no less than 30 years of hourly rain-gauge observations at national stations over central and eastern China (CEC). This study investigates the variations, relationships, differences and similarities of total, stratiform, convective and extreme HP over the entire CEC and various subregions. Results indicate that the variations in the amount and frequency of HP at the seven levels over the entire CEC all display a bimodal feature. For various regions, the variations of total HP mostly feature two peaks, while convective HP mainly occurs in the late afternoon and determines the diurnal variation of total HP intensity. On the basis of the primary peak time periods of HP frequency at all levels over different subregions, the variations can be classified into three main categories: late-afternoon primary peak, nocturnal primary peak, and time-shifting primary peak. However, the variations over some coastal regions like the Liaodong Peninsula, the Shandong Peninsula, and the coastal regions of Guangdong, distinctly differ from those over their corresponding larger regions. Overall, the normalized diurnal variation amplitude of amount and frequency increases with the increasing HP intensity; convective precipitation can be represented by HP ≥ 10 mm; and the intensity of HP ≥ 50 mm is slightly larger during the nighttime than during the daytime over the entire CEC. In northern China, diurnal variation in HP ≥ 5 mm can represent well that in convective precipitation.
2019, 36(2): 160-172.
doi: 10.1007/s00376-018-8087-5
Abstract:
Assimilation configurations have significant impacts on analysis results and subsequent forecasts. A squall line system that occurred on 23 April 2007 over southern China was used to investigate the impacts of the data assimilation frequency of radar data on analyses and forecasts. A three-dimensional variational system was used to assimilate radial velocity data, and a cloud analysis system was used for reflectivity assimilation with a 2-h assimilation window covering the initial stage of the squall line. Two operators of radar reflectivity for cloud analyses corresponding to single- and double-moment schemes were used. In this study, we examined the sensitivity of assimilation frequency using 10-, 20-, 30-, and 60-min assimilation intervals. The results showed that analysis fields were not consistent with model dynamics and microphysics in general; thus, model states, including dynamic and microphysical variables, required approximately 20 min to reach a new balance after data assimilation in all experiments. Moreover, a 20-min data assimilation interval generally produced better forecasts for both single- and double-moment schemes in terms of equitable threat and bias scores. We conclude that a higher data assimilation frequency can produce a more intense cold pool and rear inflow jets but does not necessarily lead to a better forecast.
Assimilation configurations have significant impacts on analysis results and subsequent forecasts. A squall line system that occurred on 23 April 2007 over southern China was used to investigate the impacts of the data assimilation frequency of radar data on analyses and forecasts. A three-dimensional variational system was used to assimilate radial velocity data, and a cloud analysis system was used for reflectivity assimilation with a 2-h assimilation window covering the initial stage of the squall line. Two operators of radar reflectivity for cloud analyses corresponding to single- and double-moment schemes were used. In this study, we examined the sensitivity of assimilation frequency using 10-, 20-, 30-, and 60-min assimilation intervals. The results showed that analysis fields were not consistent with model dynamics and microphysics in general; thus, model states, including dynamic and microphysical variables, required approximately 20 min to reach a new balance after data assimilation in all experiments. Moreover, a 20-min data assimilation interval generally produced better forecasts for both single- and double-moment schemes in terms of equitable threat and bias scores. We conclude that a higher data assimilation frequency can produce a more intense cold pool and rear inflow jets but does not necessarily lead to a better forecast.
2019, 36(2): 173-188.
doi: 10.1007/s00376-017-8089-8
Abstract:
During 1979-2015, the intensity of the Siberian high (SH) in November and December-January (DJ) is frequently shown to have an out-of-phase relationship, which is accompanied by opposite surface air temperature and circulation anomalies. Further analyses indicate that the autumn Arctic sea ice is important for the phase reversal of the SH. There is a significantly positive (negative) correlation between the November (DJ) SH and the September sea ice area (SIA) anomalies. It is suggested that the reduction of autumn SIA induces anomalous upward surface turbulent heat flux (SHF), which can persist into November, especially over the Barents Sea. Consequently, the enhanced eddy energy and wave activity flux are transported to mid and high latitudes. This will then benefit the development of the storm track in northeastern Europe. Conversely, when downward SHF anomalies prevail in DJ, the decreased heat flux and suppressed eddy energy hinder the growth of the storm track during DJ over the Barents Sea and Europe. Through the eddy-mean flow interaction, the strengthened (weakened) storm track activities induce decreased (increased) Ural blockings and accelerated (decelerated) westerlies, which makes the cold air from the Arctic inhibited (transported) over the Siberian area. Therefore, a weaker (stronger) SH in November (DJ) occurs downstream. Moreover, anomalously large snowfall may intensify the SH in DJ rather than in November. The ensemble-mean results from the CMIP5 historical simulations further confirm these connections. The different responses to Arctic sea ice anomalies in early and middle winter set this study apart from earlier ones.
During 1979-2015, the intensity of the Siberian high (SH) in November and December-January (DJ) is frequently shown to have an out-of-phase relationship, which is accompanied by opposite surface air temperature and circulation anomalies. Further analyses indicate that the autumn Arctic sea ice is important for the phase reversal of the SH. There is a significantly positive (negative) correlation between the November (DJ) SH and the September sea ice area (SIA) anomalies. It is suggested that the reduction of autumn SIA induces anomalous upward surface turbulent heat flux (SHF), which can persist into November, especially over the Barents Sea. Consequently, the enhanced eddy energy and wave activity flux are transported to mid and high latitudes. This will then benefit the development of the storm track in northeastern Europe. Conversely, when downward SHF anomalies prevail in DJ, the decreased heat flux and suppressed eddy energy hinder the growth of the storm track during DJ over the Barents Sea and Europe. Through the eddy-mean flow interaction, the strengthened (weakened) storm track activities induce decreased (increased) Ural blockings and accelerated (decelerated) westerlies, which makes the cold air from the Arctic inhibited (transported) over the Siberian area. Therefore, a weaker (stronger) SH in November (DJ) occurs downstream. Moreover, anomalously large snowfall may intensify the SH in DJ rather than in November. The ensemble-mean results from the CMIP5 historical simulations further confirm these connections. The different responses to Arctic sea ice anomalies in early and middle winter set this study apart from earlier ones.
2019, 36(2): 189-205.
doi: 10.1007/s00376-018-8049-y
Abstract:
In this study, an east-moving Tibetan Plateau vortex (TPV) is analyzed by using the ERA-5 reanalysis and multi-source satellite data, including FengYun-2E, Aqua/MODIS and CALIPSO. The objective is to demonstrate: (i) the usefulness of multi-spectral satellite observations in understanding the evolution of a TPV and the associated rainfall, and (ii) the potential significance of cloud-top quantitative information in improving Southwest China weather forecasts. Results in this study show that the heavy rainfall is caused by the coupling of an east-moving TPV and some low-level weather systems [a Plateau shear line and a Southwest Vortex (SWV)], wherein the TPV is a key component. During the TPV's life cycle, the rainfall and vortex intensity maintain a significant positive correlation with the convective cloud-top fraction and height within a 2.5° radius away from its center. Moreover, its growth is found to be quite sensitive to the cloud phases and particle sizes. In the mature stage when the TPV is coupled with an SWV, an increase of small ice crystal particles and appearance of ring- and U/V-shaped cold cloud-top structures can be seen as the signature of a stronger convection and rainfall enhancement within the TPV. A tropopause folding caused by ageostrophic flows at the upper level may be a key factor in the formation of ring-shaped and U/V-shaped cloud-top structures. Based on these results, we believe that the supplementary quantitative information of an east-moving TPV cloud top collected by multi-spectral satellite observations could help to improve Southwest China short-range/nowcasting weather forecasts.
In this study, an east-moving Tibetan Plateau vortex (TPV) is analyzed by using the ERA-5 reanalysis and multi-source satellite data, including FengYun-2E, Aqua/MODIS and CALIPSO. The objective is to demonstrate: (i) the usefulness of multi-spectral satellite observations in understanding the evolution of a TPV and the associated rainfall, and (ii) the potential significance of cloud-top quantitative information in improving Southwest China weather forecasts. Results in this study show that the heavy rainfall is caused by the coupling of an east-moving TPV and some low-level weather systems [a Plateau shear line and a Southwest Vortex (SWV)], wherein the TPV is a key component. During the TPV's life cycle, the rainfall and vortex intensity maintain a significant positive correlation with the convective cloud-top fraction and height within a 2.5° radius away from its center. Moreover, its growth is found to be quite sensitive to the cloud phases and particle sizes. In the mature stage when the TPV is coupled with an SWV, an increase of small ice crystal particles and appearance of ring- and U/V-shaped cold cloud-top structures can be seen as the signature of a stronger convection and rainfall enhancement within the TPV. A tropopause folding caused by ageostrophic flows at the upper level may be a key factor in the formation of ring-shaped and U/V-shaped cloud-top structures. Based on these results, we believe that the supplementary quantitative information of an east-moving TPV cloud top collected by multi-spectral satellite observations could help to improve Southwest China short-range/nowcasting weather forecasts.
2019, 36(2): 206-218.
doi: 10.1007/s00376-018-8018-5
Abstract:
A three-dimensional transformed Eulerian-mean (3D TEM) equation under a non-hydrostatic and non-geostrophic assumption is deduced in this study. The vertical component of the 3D wave activity flux deduced here is the primary difference from previous studies, which is suitable to mesoscale systems. Using the 3D TEM equation, the energy propagation of the inertia-gravity waves and how the generation and dissipation of the inertia-gravity waves drive the mean flow can be examined. During the mature stage of a heavy precipitation event, the maximum of the Eliassen-Palm (EP) flux divergence is primarily concentrated at the height of 10-14 km, where the energy of the inertia-gravity waves propagates forward (eastward) and upward. Examining the contribution of each term of the 3D TEM equation shows that the EP flux divergence is the primary contributor to the mean flow tendency. The EP flux divergence decelerates the zonal wind above and below the high-level jet at the height of 10 km and 15 km, and accelerates the high-level jet at the height of 12-14 km. This structure enhances the vertical wind shear of the environment and promotes the development of the rainstorm.
A three-dimensional transformed Eulerian-mean (3D TEM) equation under a non-hydrostatic and non-geostrophic assumption is deduced in this study. The vertical component of the 3D wave activity flux deduced here is the primary difference from previous studies, which is suitable to mesoscale systems. Using the 3D TEM equation, the energy propagation of the inertia-gravity waves and how the generation and dissipation of the inertia-gravity waves drive the mean flow can be examined. During the mature stage of a heavy precipitation event, the maximum of the Eliassen-Palm (EP) flux divergence is primarily concentrated at the height of 10-14 km, where the energy of the inertia-gravity waves propagates forward (eastward) and upward. Examining the contribution of each term of the 3D TEM equation shows that the EP flux divergence is the primary contributor to the mean flow tendency. The EP flux divergence decelerates the zonal wind above and below the high-level jet at the height of 10 km and 15 km, and accelerates the high-level jet at the height of 12-14 km. This structure enhances the vertical wind shear of the environment and promotes the development of the rainstorm.
2019, 36(2): 219-230.
doi: 10.1007/s00376-018-8091-9
Abstract:
Horizontal velocity spirals with a clockwise rotation (downward looking) rate of 1.7 m-1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation, the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10-4 m2 s-1 on average. An eddy diffusivity of 2870 m2 s-1 is required for dynamic balance by estimating the residual term. This obtained value of 10-4 m2 s-1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m2 s-1, respectively, which supports a closed budget for velocity rotation.
Horizontal velocity spirals with a clockwise rotation (downward looking) rate of 1.7 m-1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation, the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10-4 m2 s-1 on average. An eddy diffusivity of 2870 m2 s-1 is required for dynamic balance by estimating the residual term. This obtained value of 10-4 m2 s-1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m2 s-1, respectively, which supports a closed budget for velocity rotation.
2019, 36(2): 231-247.
doi: 10.1007/s00376-018-8001-1
Abstract:
This paper preliminarily investigates the application of the orthogonal conditional nonlinear optimal perturbations (CNOPs)-based ensemble forecast technique in MM5 (Fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model). The results show that the ensemble forecast members generated by the orthogonal CNOPs present large spreads but tend to be located on the two sides of real tropical cyclone (TC) tracks and have good agreements between ensemble spreads and ensemble-mean forecast errors for TC tracks. Subsequently, these members reflect more reasonable forecast uncertainties and enhance the orthogonal CNOPs-based ensemble-mean forecasts to obtain higher skill for TC tracks than the orthogonal SVs (singular vectors)-, BVs (bred vectors)- and RPs (random perturbations)-based ones. The results indicate that orthogonal CNOPs of smaller magnitudes should be adopted to construct the initial ensemble perturbations for short lead-time forecasts, but those of larger magnitudes should be used for longer lead-time forecasts due to the effects of nonlinearities. The performance of the orthogonal CNOPs-based ensemble-mean forecasts is case-dependent, which encourages evaluating statistically the forecast skill with more TC cases. Finally, the results show that the ensemble forecasts with only initial perturbations in this work do not increase the forecast skill of TC intensity, which may be related with both the coarse model horizontal resolution and the model error.
This paper preliminarily investigates the application of the orthogonal conditional nonlinear optimal perturbations (CNOPs)-based ensemble forecast technique in MM5 (Fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model). The results show that the ensemble forecast members generated by the orthogonal CNOPs present large spreads but tend to be located on the two sides of real tropical cyclone (TC) tracks and have good agreements between ensemble spreads and ensemble-mean forecast errors for TC tracks. Subsequently, these members reflect more reasonable forecast uncertainties and enhance the orthogonal CNOPs-based ensemble-mean forecasts to obtain higher skill for TC tracks than the orthogonal SVs (singular vectors)-, BVs (bred vectors)- and RPs (random perturbations)-based ones. The results indicate that orthogonal CNOPs of smaller magnitudes should be adopted to construct the initial ensemble perturbations for short lead-time forecasts, but those of larger magnitudes should be used for longer lead-time forecasts due to the effects of nonlinearities. The performance of the orthogonal CNOPs-based ensemble-mean forecasts is case-dependent, which encourages evaluating statistically the forecast skill with more TC cases. Finally, the results show that the ensemble forecasts with only initial perturbations in this work do not increase the forecast skill of TC intensity, which may be related with both the coarse model horizontal resolution and the model error.
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