2013 Vol. 19, No. 4
2013, 19(4): 305-321.
Abstract:
The large-scale and small-scale errors could affect background error covariances for a regional numerical model with the specified grid resolution. Based on the different background error covariances influenced by different scale errors, this study tries to construct a so-called "optimal background error covariances" to consider the interactions among different scale errors. For this purpose, a linear combination of the forecast differences influenced by information of errors at different scales is used to construct the new forecast differences for estimating optimal background error covariances. By adjusting the relative weight of the forecast differences influenced by information of smaller-scale errors, the relative influence of different scale errors on optimal background error covariances can be changed. For a heavy rainfall case, the corresponding optimal background error covariances can be estimated through choosing proper weighting factor for forecast differences influenced by information of smaller-scale errors. The data assimilation and forecast with these optimal covariances show that, the corresponding analyses and forecasts can lead to superior quality, compared with those using covariances that just introduce influences of larger- or smallerscale errors. Due to the interactions among different scale errors included in optimal background error covariances, relevant analysis increments can properly describe weather systems (processes) at different scales, such as dynamic lifting, thermodynamic instability and advection of moisture at large scale, high-level and low-level jet at synoptic scale, and convective systems at mesoscale and small scale, as well as their interactions. As a result, the corresponding forecasts can be improved.
The large-scale and small-scale errors could affect background error covariances for a regional numerical model with the specified grid resolution. Based on the different background error covariances influenced by different scale errors, this study tries to construct a so-called "optimal background error covariances" to consider the interactions among different scale errors. For this purpose, a linear combination of the forecast differences influenced by information of errors at different scales is used to construct the new forecast differences for estimating optimal background error covariances. By adjusting the relative weight of the forecast differences influenced by information of smaller-scale errors, the relative influence of different scale errors on optimal background error covariances can be changed. For a heavy rainfall case, the corresponding optimal background error covariances can be estimated through choosing proper weighting factor for forecast differences influenced by information of smaller-scale errors. The data assimilation and forecast with these optimal covariances show that, the corresponding analyses and forecasts can lead to superior quality, compared with those using covariances that just introduce influences of larger- or smallerscale errors. Due to the interactions among different scale errors included in optimal background error covariances, relevant analysis increments can properly describe weather systems (processes) at different scales, such as dynamic lifting, thermodynamic instability and advection of moisture at large scale, high-level and low-level jet at synoptic scale, and convective systems at mesoscale and small scale, as well as their interactions. As a result, the corresponding forecasts can be improved.
2013, 19(4): 322-330.
Abstract:
The interannual variability of winter and spring precipitation in South China (SC) and its relation to moisture transport are investigated by using the monthly precipitation data of NMIC, NCEP reanalysis datasets and NOAA ERSST analysis datasets from 1960 to 2008. The results show that winter and spring precipitation in SC is less than normal from the 1960s to the start of the 1970s and from the end of the 1990s to the present. Most of rainfall anomalies on the whole regional scale of SC is well in phase during winter and spring, and the frequency of persistent drought is higher than that of persistent flood. Seasonal variations of moisture transport differences of SC between persistent drought and flood events are observed: the differences in winter are characterized by moisture transport from Bay of Bangle (BOB) and South China Sea (SCS), while differences in spring are characterized by that from SCS and North China (NC). There are two types of Niño3.4 sea surface temperature anomaly (SSTA) related to persistent winter and spring drought (flood) events in SC, which are positive SSTA next to Niño4 (Niño3) and negative SSTA next to Niño3 (Niño4). Moreover, the variations of moisture transport from BOB and SC have important effects on persistent drought/flood in SC when the Niño3.4 index is in the positive phase, while those from western North Pacific (WNP)-SC in winter and those from Philippine Sea (PHS)-SC and NC in spring primarily contribute to persistent drought/flood events in SC when the Niño3.4 index is in the negative phase, and these stronger (weaker) moisture transports are observed in persistent flood (drought) during winter and spring regardless of the Niño3.4 index. In conclusion, with the correlation between variations and distributions of Niño3.4 SSTA and persistent drought/flood events in SC, moisture transport is responsible for the formation of precipitation anomalies. In addition, the moisture transport from SCS is most significantly correlated with persistent drought/flood events during winter and spring.
The interannual variability of winter and spring precipitation in South China (SC) and its relation to moisture transport are investigated by using the monthly precipitation data of NMIC, NCEP reanalysis datasets and NOAA ERSST analysis datasets from 1960 to 2008. The results show that winter and spring precipitation in SC is less than normal from the 1960s to the start of the 1970s and from the end of the 1990s to the present. Most of rainfall anomalies on the whole regional scale of SC is well in phase during winter and spring, and the frequency of persistent drought is higher than that of persistent flood. Seasonal variations of moisture transport differences of SC between persistent drought and flood events are observed: the differences in winter are characterized by moisture transport from Bay of Bangle (BOB) and South China Sea (SCS), while differences in spring are characterized by that from SCS and North China (NC). There are two types of Niño3.4 sea surface temperature anomaly (SSTA) related to persistent winter and spring drought (flood) events in SC, which are positive SSTA next to Niño4 (Niño3) and negative SSTA next to Niño3 (Niño4). Moreover, the variations of moisture transport from BOB and SC have important effects on persistent drought/flood in SC when the Niño3.4 index is in the positive phase, while those from western North Pacific (WNP)-SC in winter and those from Philippine Sea (PHS)-SC and NC in spring primarily contribute to persistent drought/flood events in SC when the Niño3.4 index is in the negative phase, and these stronger (weaker) moisture transports are observed in persistent flood (drought) during winter and spring regardless of the Niño3.4 index. In conclusion, with the correlation between variations and distributions of Niño3.4 SSTA and persistent drought/flood events in SC, moisture transport is responsible for the formation of precipitation anomalies. In addition, the moisture transport from SCS is most significantly correlated with persistent drought/flood events during winter and spring.
2013, 19(4): 331-339.
Abstract:
The response of non-uniformity of precipitation extremes over China to doubled CO2 has been analyzed using the daily precipitation simulated by a coupled general circulation model, MIROC_Hires. The major conclusions are as follows: under the CO2 increasing scenario (SRES A1B), the climatological precipitation extremes are concentrated over the southern China, while they are uniformly distributed over the northern China. For interannual variability, the concentration of precipitation extremes is small over the southern China, but it is opposite over the northern China. The warming effects on the horizontal and vertical scales are different over the northern and southern part of China. Furthermore, the atmospheric stability is also different between the two parts of China. The heterogeneous warming is one of the possible reasons for the changes in non-uniformity of precipitation extremes over China.
The response of non-uniformity of precipitation extremes over China to doubled CO2 has been analyzed using the daily precipitation simulated by a coupled general circulation model, MIROC_Hires. The major conclusions are as follows: under the CO2 increasing scenario (SRES A1B), the climatological precipitation extremes are concentrated over the southern China, while they are uniformly distributed over the northern China. For interannual variability, the concentration of precipitation extremes is small over the southern China, but it is opposite over the northern China. The warming effects on the horizontal and vertical scales are different over the northern and southern part of China. Furthermore, the atmospheric stability is also different between the two parts of China. The heterogeneous warming is one of the possible reasons for the changes in non-uniformity of precipitation extremes over China.
2013, 19(4): 340-348.
Abstract:
The daily precipitation data of 740 stations in China (1958-2001) and the daily upper air data of European Centre for Medium-Range Weather Forecasts reanalysis dataset (1958-2001) are used to define an East Asian summer monsoon (EASM) index based on dynamic and thermal factors. The index is used to represent the front (or leading edge) of EASM to describe and characterize the advance and retreat of EASM objectively. During 1958-2001, the EASM movement underwent three interdecadal abrupt shifts in 1965, 1980 and 1994, respectively. During 1958-1964, the front primarily concentrated in South China and North China, while it stayed at the mid- and lower-Yangtze River for a short period. During 1965-1979, the front was located in South China and the lower reach of Yellow River for a long time. During 1980-1993, the time in which the front of EASM stayed at the mid- and lower-Yangtze River was much longer, but it settled in North China for just a short time. During 1994-2001, the front generally concentrated in the south of the mid- and lower-Yangtze River. The three interdecadal shifts of EASM directly resulted in rainfall anomalies, as well as frequent disasters of flood and drought in East China.
The daily precipitation data of 740 stations in China (1958-2001) and the daily upper air data of European Centre for Medium-Range Weather Forecasts reanalysis dataset (1958-2001) are used to define an East Asian summer monsoon (EASM) index based on dynamic and thermal factors. The index is used to represent the front (or leading edge) of EASM to describe and characterize the advance and retreat of EASM objectively. During 1958-2001, the EASM movement underwent three interdecadal abrupt shifts in 1965, 1980 and 1994, respectively. During 1958-1964, the front primarily concentrated in South China and North China, while it stayed at the mid- and lower-Yangtze River for a short period. During 1965-1979, the front was located in South China and the lower reach of Yellow River for a long time. During 1980-1993, the time in which the front of EASM stayed at the mid- and lower-Yangtze River was much longer, but it settled in North China for just a short time. During 1994-2001, the front generally concentrated in the south of the mid- and lower-Yangtze River. The three interdecadal shifts of EASM directly resulted in rainfall anomalies, as well as frequent disasters of flood and drought in East China.
2013, 19(4): 349-356.
Abstract:
With temperatures increasing as a result of global warming, extreme high temperatures are becoming more intense and more frequent on larger scale during summer in China. In recent years, a variety of researches have examined the high temperature distribution in China. However, it hardly considers the variation of temperature data and systems when defining the threshold of extreme high temperature. In order to discern the spatio-temporal distribution of extreme heat in China, we examined the daily maximum temperature data of 83 observation stations in China from 1950 to 2008. The objective of this study was to understand the distribution characteristics of extreme high temperature events defined by Detrended Fluctuation Analysis (DFA). The statistical methods of Permutation Entropy (PE) were also used in this study to analyze the temporal distribution. The results showed that the frequency of extreme high temperature events in China presented 3 periods of 7, 10―13 and 16―20 years, respectively. The abrupt changes generally happened in the 1960s, the end of 1970s and early 1980s. It was also found that the maximum frequency occurred in the early 1950s, and the frequency decreased sharply until the late 1980s when an evidently increasing trend emerged. Furthermore, the annual averaged frequency of extreme high temperature events reveals a decreasing-increasing-decreasing trend from southwest to northeast China, but an increasing-decreasing trend from southeast to northwest China. And the frequency was higher in southern region than that in northern region. Besides, the maximum and minimum of frequencies were relatively concentrated spatially. Our results also shed light on the reasons for the periods and abrupt changes of the frequency of extreme high temperature events in China.
With temperatures increasing as a result of global warming, extreme high temperatures are becoming more intense and more frequent on larger scale during summer in China. In recent years, a variety of researches have examined the high temperature distribution in China. However, it hardly considers the variation of temperature data and systems when defining the threshold of extreme high temperature. In order to discern the spatio-temporal distribution of extreme heat in China, we examined the daily maximum temperature data of 83 observation stations in China from 1950 to 2008. The objective of this study was to understand the distribution characteristics of extreme high temperature events defined by Detrended Fluctuation Analysis (DFA). The statistical methods of Permutation Entropy (PE) were also used in this study to analyze the temporal distribution. The results showed that the frequency of extreme high temperature events in China presented 3 periods of 7, 10―13 and 16―20 years, respectively. The abrupt changes generally happened in the 1960s, the end of 1970s and early 1980s. It was also found that the maximum frequency occurred in the early 1950s, and the frequency decreased sharply until the late 1980s when an evidently increasing trend emerged. Furthermore, the annual averaged frequency of extreme high temperature events reveals a decreasing-increasing-decreasing trend from southwest to northeast China, but an increasing-decreasing trend from southeast to northwest China. And the frequency was higher in southern region than that in northern region. Besides, the maximum and minimum of frequencies were relatively concentrated spatially. Our results also shed light on the reasons for the periods and abrupt changes of the frequency of extreme high temperature events in China.
2013, 19(4): 358-366.
Abstract:
Two field experiments were performed in order to dissipate the fog at Wuqing District of Tianjin in November and December of 2009. Hygroscopic particles were seeded to dissipate fog droplets on 6-7 November, 2009. Liquid nitrogen (LN) was seeded into the natural supercooled fog in the experiments of 30 November-1 December, 2009. Significant response was found after seeding. Significant changes were observed in the microstructure of fog in the field experiments. The of fog droplet changed dramatically; it increased first and then decreased after seeding. Remarkable variation also was found in the Liquid Water Content (LWC) and in the size of fog droplet. The Droplet Size Distribution (DSD) of fog broadened during the seeding experiments. The DSD became narrow after the seeding ended. After seeding, the droplets were found to be at different stages of growth, resulting in a transform of DSD between unimodal distribution and bimodal distribution. The DSD was unimodal before seeding and then bimodal during the seeding experiment. Finally, the DSD became unimodally distributed once again.
Two field experiments were performed in order to dissipate the fog at Wuqing District of Tianjin in November and December of 2009. Hygroscopic particles were seeded to dissipate fog droplets on 6-7 November, 2009. Liquid nitrogen (LN) was seeded into the natural supercooled fog in the experiments of 30 November-1 December, 2009. Significant response was found after seeding. Significant changes were observed in the microstructure of fog in the field experiments. The of fog droplet changed dramatically; it increased first and then decreased after seeding. Remarkable variation also was found in the Liquid Water Content (LWC) and in the size of fog droplet. The Droplet Size Distribution (DSD) of fog broadened during the seeding experiments. The DSD became narrow after the seeding ended. After seeding, the droplets were found to be at different stages of growth, resulting in a transform of DSD between unimodal distribution and bimodal distribution. The DSD was unimodal before seeding and then bimodal during the seeding experiment. Finally, the DSD became unimodally distributed once again.
2013, 19(4): 367-374.
Abstract:
Based on MM5, POM, and WW3, a regional atmosphere-ocean-wave coupled system is developed in this work under the environment of Message Passing Interface. The coupled system is applied in a study of two typhoon processes in the South China Sea (SCS). The results show that the coupled model operates steadily and efficiently and exhibits good capability in simulating typhoon processes. It improves the simulation accuracy of the track and intensity of the typhoon. The response of ocean surface to the typhoon is remarkable, especially on the right side of the typhoon track. The sea surface temperature (SST) declines, and the ocean current and wave height are intensified. In the coupling experiment, the decline of SST intensifies and the inertial oscillation amplitude of the ocean current increases when the ocean-wave effect is considered. Therefore, the atmosphere-ocean-wave coupled system can help in the study of air-sea interaction and improve the capability of predicting and preventing weather and oceanic disasters in SCS.
Based on MM5, POM, and WW3, a regional atmosphere-ocean-wave coupled system is developed in this work under the environment of Message Passing Interface. The coupled system is applied in a study of two typhoon processes in the South China Sea (SCS). The results show that the coupled model operates steadily and efficiently and exhibits good capability in simulating typhoon processes. It improves the simulation accuracy of the track and intensity of the typhoon. The response of ocean surface to the typhoon is remarkable, especially on the right side of the typhoon track. The sea surface temperature (SST) declines, and the ocean current and wave height are intensified. In the coupling experiment, the decline of SST intensifies and the inertial oscillation amplitude of the ocean current increases when the ocean-wave effect is considered. Therefore, the atmosphere-ocean-wave coupled system can help in the study of air-sea interaction and improve the capability of predicting and preventing weather and oceanic disasters in SCS.
2013, 19(4): 375-387.
Abstract:
Recent studies indicated that except for the land-sea thermal contrast, there also existed the land-land thermal contrast. The composite analysis and t-test method are used to further study the local thermal contrast variation over the Asian continent, and to discuss the association of seasonal variation of land thermal state with circulation over East Asia, the early summer and summer monsoon activity, and the precipitation anomaly in China in the decadal scale. Results show that the positive meridional temperature anomaly transports downward from upper tropospheric layers in middle-high latitudes north of 25°N in the positive years. In the zonal direction, the Tibetan Plateau heating in the successive spring acts as a force to influence the atmosphere, leading to the rapid temperature warming over eastern Chinese continent, which could increase the land-sea thermal contrast with the negative SSTA. Accordingly, the monsoon activity in early summer over East Asian establishes earlier and the summer monsoon intensity becomes stronger. The early summer precipitation is more-than-normal over the Yangtze River, and the summer precipitation is more-than-normal over the north China and the southwest China. The situation is contrary in the negative years.
Recent studies indicated that except for the land-sea thermal contrast, there also existed the land-land thermal contrast. The composite analysis and t-test method are used to further study the local thermal contrast variation over the Asian continent, and to discuss the association of seasonal variation of land thermal state with circulation over East Asia, the early summer and summer monsoon activity, and the precipitation anomaly in China in the decadal scale. Results show that the positive meridional temperature anomaly transports downward from upper tropospheric layers in middle-high latitudes north of 25°N in the positive years. In the zonal direction, the Tibetan Plateau heating in the successive spring acts as a force to influence the atmosphere, leading to the rapid temperature warming over eastern Chinese continent, which could increase the land-sea thermal contrast with the negative SSTA. Accordingly, the monsoon activity in early summer over East Asian establishes earlier and the summer monsoon intensity becomes stronger. The early summer precipitation is more-than-normal over the Yangtze River, and the summer precipitation is more-than-normal over the north China and the southwest China. The situation is contrary in the negative years.
2013, 19(4): 388-396.
Abstract:
In this paper, the forecasting equations of a 2nd-order space-time differential remainder are deduced from the Navier-Stokes primitive equations and Eulerian operator by Taylor-series expansion. Here we introduce a cubic spline numerical model (Spline Model for short), which is with a quasi-Lagrangian time-split integration scheme of fitting cubic spline/bicubic surface to all physical variable fields in the atmospheric equations on spherical discrete latitude-longitude mesh. A new algorithm of "fitting cubic spline―time step integration―fitting cubic spline―……" is developed to determine their first- and 2nd-order derivatives and their upstream points for time discrete integral to the governing equations in Spline Model. And the cubic spline function and its mathematical polarities are also discussed to understand the Spline Model's mathematical foundation of numerical analysis. It is pointed out that the Spline Model has mathematical laws of "convergence" of the cubic spline functions contracting to the original functions as well as its 1st-order and 2nd-order derivatives. The "optimality" of the 2nd-order derivative of the cubic spline functions is optimal approximation to that of the original functions. In addition, a Hermite bicubic patch is equivalent to operate on a grid for a 2nd-order derivative variable field. Besides, the slopes and curvatures of a central difference are identified respectively, with a smoothing coefficient of 1/3, three-point smoothing of that of a cubic spline. Then the slopes and curvatures of a central difference are calculated from the smoothing coefficient 1/3 and three-point smoothing of that of a cubic spline, respectively. Furthermore, a global simulation case of adiabatic, non-frictional and "incompressible" model atmosphere is shown with the quasi-Lagrangian time integration by using a global Spline Model, whose initial condition comes from the NCEP reanalysis data, along with quasi-uniform latitude-longitude grids and the so-called "shallow atmosphere" Navier-Stokes primitive equations in the spherical coordinates. The Spline Model, which adopted the Navier-Stokes primitive equations and quasi-Lagrangian time-split integration scheme, provides an initial ideal case of global atmospheric circulation. In addition, considering the essentially non-linear atmospheric motions, the Spline Model could judge reasonably well simple points of any smoothed variable field according to its fitting spline curvatures that must conform to its physical interpretation.
In this paper, the forecasting equations of a 2nd-order space-time differential remainder are deduced from the Navier-Stokes primitive equations and Eulerian operator by Taylor-series expansion. Here we introduce a cubic spline numerical model (Spline Model for short), which is with a quasi-Lagrangian time-split integration scheme of fitting cubic spline/bicubic surface to all physical variable fields in the atmospheric equations on spherical discrete latitude-longitude mesh. A new algorithm of "fitting cubic spline―time step integration―fitting cubic spline―……" is developed to determine their first- and 2nd-order derivatives and their upstream points for time discrete integral to the governing equations in Spline Model. And the cubic spline function and its mathematical polarities are also discussed to understand the Spline Model's mathematical foundation of numerical analysis. It is pointed out that the Spline Model has mathematical laws of "convergence" of the cubic spline functions contracting to the original functions as well as its 1st-order and 2nd-order derivatives. The "optimality" of the 2nd-order derivative of the cubic spline functions is optimal approximation to that of the original functions. In addition, a Hermite bicubic patch is equivalent to operate on a grid for a 2nd-order derivative variable field. Besides, the slopes and curvatures of a central difference are identified respectively, with a smoothing coefficient of 1/3, three-point smoothing of that of a cubic spline. Then the slopes and curvatures of a central difference are calculated from the smoothing coefficient 1/3 and three-point smoothing of that of a cubic spline, respectively. Furthermore, a global simulation case of adiabatic, non-frictional and "incompressible" model atmosphere is shown with the quasi-Lagrangian time integration by using a global Spline Model, whose initial condition comes from the NCEP reanalysis data, along with quasi-uniform latitude-longitude grids and the so-called "shallow atmosphere" Navier-Stokes primitive equations in the spherical coordinates. The Spline Model, which adopted the Navier-Stokes primitive equations and quasi-Lagrangian time-split integration scheme, provides an initial ideal case of global atmospheric circulation. In addition, considering the essentially non-linear atmospheric motions, the Spline Model could judge reasonably well simple points of any smoothed variable field according to its fitting spline curvatures that must conform to its physical interpretation.
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