2014 Vol. 20, No. 4
2014, 20(4): 297-307.
Abstract:
Tropical cyclones (TCs) Lionrock, Kompasu, and Namtheun were formed successively within 40 hours in 2010. Over the next several days afterwards, these TCs exhibited unusual movements which made operational prediction difficult. Verifications are performed on the forecasts of the tracks of these TCs with six operational models, including three global and three regional models. Results showed that the trends of TC tracks could be reproduced by these models, whereas trajectory turning points and landfall locations were not simulated effectively. The special track of Lionrock should be associated with its direct interaction with Namtheun, according to a conceptual model of binary TC interaction. By contrast, the relation between Kompasu and Namtheun satisfied the criteria for a semi-direct interaction. Numerical experiments based on the Global and Regional Assimilation and Prediction System-Tropical Cyclone forecast Model (GRAPES-TCM) further confirmed the effect of Namtheun on the unusual tracks of Lionrock and Kompasu. Finally, the physical mechanism of binary TC interaction was preliminarily proposed.
Tropical cyclones (TCs) Lionrock, Kompasu, and Namtheun were formed successively within 40 hours in 2010. Over the next several days afterwards, these TCs exhibited unusual movements which made operational prediction difficult. Verifications are performed on the forecasts of the tracks of these TCs with six operational models, including three global and three regional models. Results showed that the trends of TC tracks could be reproduced by these models, whereas trajectory turning points and landfall locations were not simulated effectively. The special track of Lionrock should be associated with its direct interaction with Namtheun, according to a conceptual model of binary TC interaction. By contrast, the relation between Kompasu and Namtheun satisfied the criteria for a semi-direct interaction. Numerical experiments based on the Global and Regional Assimilation and Prediction System-Tropical Cyclone forecast Model (GRAPES-TCM) further confirmed the effect of Namtheun on the unusual tracks of Lionrock and Kompasu. Finally, the physical mechanism of binary TC interaction was preliminarily proposed.
2014, 20(4): 308-313.
Abstract:
Based on the Regional Spectral Model (RSM) re-analysis data from Japan Meteorological Agency (JMA) with a horizontal resolution of 20 km and a time interval of 6 h, this study works on the outer and inner core size of 2174 samples of tropical cyclones (TCs) occurring over the western North Pacific between 2001 and 2007. Some conclusions have been drawn on the basis of preliminary analysis of the TC inner core size and outer size and their relationship with TC intensity. First, the outer size increase (decrease) helps TCs intensify (weaken). Second, the enlargement (shrinking) of the inner core size helps TCs intensify (weaken) if TCs have a large inner core (with radius of maximum winds larger than 120 km). Contrarily, when TCs have small inner core (with radius of maximum winds smaller than 120 km), the enlargement (shrinking) of the inner core is good for weakening (intensifying) of TCs.
Based on the Regional Spectral Model (RSM) re-analysis data from Japan Meteorological Agency (JMA) with a horizontal resolution of 20 km and a time interval of 6 h, this study works on the outer and inner core size of 2174 samples of tropical cyclones (TCs) occurring over the western North Pacific between 2001 and 2007. Some conclusions have been drawn on the basis of preliminary analysis of the TC inner core size and outer size and their relationship with TC intensity. First, the outer size increase (decrease) helps TCs intensify (weaken). Second, the enlargement (shrinking) of the inner core size helps TCs intensify (weaken) if TCs have a large inner core (with radius of maximum winds larger than 120 km). Contrarily, when TCs have small inner core (with radius of maximum winds smaller than 120 km), the enlargement (shrinking) of the inner core is good for weakening (intensifying) of TCs.
2014, 20(4): 314-322.
Abstract:
The interaction between the typhoons Fengshen and Fung-wong over the Western Pacific in 2002 is studied with the Conditional Nonlinear Optimal Perturbation (CNOP) method. The study discovered that the CNOP method reveals the process of one-way interaction between Fengshen and Fung-wong. Moreover, if the region of Fung-wong was selected for verification, the sensitivity area was mainly located in the region of Fengshen and presented a half-ring structure; if the region of Fengshen was selected for verification, most of the sensitivity areas were located in the region between the Fengshen and the subtropical high, far away from Fung-wong. This indicated that Fung-wong is mainly steered by Fengshen, but Fengshen is mainly affected by the subtropical high. The sensitivity experiment showed that the initial errors in the CNOP-identified sensitive areas have larger impacts on the verification-area prediction than those near the typhoon center and their developments take a large proportion in the whole domain. This suggests that the CNOP-identified sensitive areas do have large influence on the verification-area prediction.
The interaction between the typhoons Fengshen and Fung-wong over the Western Pacific in 2002 is studied with the Conditional Nonlinear Optimal Perturbation (CNOP) method. The study discovered that the CNOP method reveals the process of one-way interaction between Fengshen and Fung-wong. Moreover, if the region of Fung-wong was selected for verification, the sensitivity area was mainly located in the region of Fengshen and presented a half-ring structure; if the region of Fengshen was selected for verification, most of the sensitivity areas were located in the region between the Fengshen and the subtropical high, far away from Fung-wong. This indicated that Fung-wong is mainly steered by Fengshen, but Fengshen is mainly affected by the subtropical high. The sensitivity experiment showed that the initial errors in the CNOP-identified sensitive areas have larger impacts on the verification-area prediction than those near the typhoon center and their developments take a large proportion in the whole domain. This suggests that the CNOP-identified sensitive areas do have large influence on the verification-area prediction.
2014, 20(4): 323-333.
Abstract:
Severe typhoon Damrey moved across Hainan Island from 00:00 UTC 25 September to 00:00 UTC 27 September in 2005 and gave rise to a significant rain process during its 48-h passage. The precipitation intensity on the southern part of the island is stronger than that on the northern, showing obvious asymmetric distribution. Using Tropical Rainfall Measuring Mission (TRMM) data, the associated mesoscale characteristics of the precipitation were analyzed and the formation of asymmetric rainfall distribution was investigated in the context of a subsynoptic scale disturbance, vertical wind shear and orographic factors. The results are shown as follows. (1) The subsynoptic scale system provided favorable dynamic conditions to the genesis of mesoscale rain clusters and rainbands. (2) The southern Hainan Island was located to the left of the leeward direction of downshear all the time, being favorable to the development of convection and leading to the asymmetric rainfall distribution. (3) Mountain terrain in the southern Hainan Island stimulated the genesis, combination and development of convective cells, promoting the formation of mesoscale precipitation systems and ultimately resulting in rainfall increase in the southern island.
Severe typhoon Damrey moved across Hainan Island from 00:00 UTC 25 September to 00:00 UTC 27 September in 2005 and gave rise to a significant rain process during its 48-h passage. The precipitation intensity on the southern part of the island is stronger than that on the northern, showing obvious asymmetric distribution. Using Tropical Rainfall Measuring Mission (TRMM) data, the associated mesoscale characteristics of the precipitation were analyzed and the formation of asymmetric rainfall distribution was investigated in the context of a subsynoptic scale disturbance, vertical wind shear and orographic factors. The results are shown as follows. (1) The subsynoptic scale system provided favorable dynamic conditions to the genesis of mesoscale rain clusters and rainbands. (2) The southern Hainan Island was located to the left of the leeward direction of downshear all the time, being favorable to the development of convection and leading to the asymmetric rainfall distribution. (3) Mountain terrain in the southern Hainan Island stimulated the genesis, combination and development of convective cells, promoting the formation of mesoscale precipitation systems and ultimately resulting in rainfall increase in the southern island.
2014, 20(4): 334-341.
Abstract:
Based on the simple ocean data assimilation (SODA) reanalysis dataset from the University of Maryland and the method of Empirical Orthogonal Functions (EOF), the characteristics of interannual and interdecadal variabilities of the equatorial Pacific subsurface oceanic temperature anomaly (SOTA) are captured. The first and second modes of the equatorial Pacific SOTA in the interannual and interdecadal variations are found respectively and the effect of the second mode on the ENSO cycle is discussed. Results show that the first mode of SOTA’s interannual and interdecadal variabilities exhibit a dipole pattern, indicating that the warm and cold temperature anomalies appear simultaneously in the equatorial subsurface Pacific. The second mode shows coherent large-scale temperature anomalies in the equatorial subsurface Pacific, which is a dominant mode in the evolution of ENSO cycle. The temporal series of the second mode has a significant lead correlation with the Niño-3.4 index, which can make a precursory prediction signal for ENSO. The function of this prediction factor in SOTA is verified by composite and case analyses.
Based on the simple ocean data assimilation (SODA) reanalysis dataset from the University of Maryland and the method of Empirical Orthogonal Functions (EOF), the characteristics of interannual and interdecadal variabilities of the equatorial Pacific subsurface oceanic temperature anomaly (SOTA) are captured. The first and second modes of the equatorial Pacific SOTA in the interannual and interdecadal variations are found respectively and the effect of the second mode on the ENSO cycle is discussed. Results show that the first mode of SOTA’s interannual and interdecadal variabilities exhibit a dipole pattern, indicating that the warm and cold temperature anomalies appear simultaneously in the equatorial subsurface Pacific. The second mode shows coherent large-scale temperature anomalies in the equatorial subsurface Pacific, which is a dominant mode in the evolution of ENSO cycle. The temporal series of the second mode has a significant lead correlation with the Niño-3.4 index, which can make a precursory prediction signal for ENSO. The function of this prediction factor in SOTA is verified by composite and case analyses.
2014, 20(4): 342-348.
Abstract:
In this study, the relationship between the subsystems of Asian summer monsoon is analyzed using U.S. National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis and Climate Prediction Center Merged Analysis of Precipitation monthly mean precipitation data. The results showed that there is significant correlation between the subsystems of Asian summer monsoon. The changes of intensity over the same period show that weak large-scale Asian monsoon, Southeast Asia monsoon and South Asian monsoon are associated with strong East Asian monsoon and decreasing rainfall in related areas. And when the large-scale Asian monsoon is strong, Southeast Asia and South Asia monsoons will be strong and precipitation will increase. While the Southeast Asia monsoon is strong, the South Asia monsoon is weak and the rainfall of South Asia is decreasing, and vice versa. The various subsystems are significantly correlated for all periods of intensity changes.
In this study, the relationship between the subsystems of Asian summer monsoon is analyzed using U.S. National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis and Climate Prediction Center Merged Analysis of Precipitation monthly mean precipitation data. The results showed that there is significant correlation between the subsystems of Asian summer monsoon. The changes of intensity over the same period show that weak large-scale Asian monsoon, Southeast Asia monsoon and South Asian monsoon are associated with strong East Asian monsoon and decreasing rainfall in related areas. And when the large-scale Asian monsoon is strong, Southeast Asia and South Asia monsoons will be strong and precipitation will increase. While the Southeast Asia monsoon is strong, the South Asia monsoon is weak and the rainfall of South Asia is decreasing, and vice versa. The various subsystems are significantly correlated for all periods of intensity changes.
2014, 20(4): 349-359.
Abstract:
Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and the upper-level objective analysis data provided by the Meteorological Information Comprehensive Analysis and Process System (MICAPS), the feature of the spatio-temporal variation of the East Asian jet stream (EAJS) in persistent snowstorm and freezing rain processes over southern China in January 2008 have been investigated. Each of the storm events was closely linked with the extraordinarily abnormal variations of East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ) at that time. The stronger EASJ with abnormally northward position of the jet axis corresponded to the more intense storm event with broader ranges and longer duration time. The heavy freezing-rain-and-snow event occurred over the region where a strong southerly wind of EASJ prevailed. Meanwhile, the westerly and northerly winds of the EAPJ were significantly intensified, which were also closely related to the beginning, enhancement, and ending of the heavy snowfall. The meridional component of the EAPJ was dominated by the northerly wind during the snowstorm. Thus, the intensification of the snowstorm was attributed to both the strengthening of the meridional wind of EAPJ and the southerly wind of EASJ. Further analysis indicated that wind speed and the zonal wind of the two jets exhibited precursory signals about half a month prior to this extreme event, and the precursory signals were found in the meridional components of the two jets about 20 days preceding the event. The sudden weakening of the meridional component of EASJ and the zonal component of EAPJ signified the ending of this persistent snowstorm.
Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and the upper-level objective analysis data provided by the Meteorological Information Comprehensive Analysis and Process System (MICAPS), the feature of the spatio-temporal variation of the East Asian jet stream (EAJS) in persistent snowstorm and freezing rain processes over southern China in January 2008 have been investigated. Each of the storm events was closely linked with the extraordinarily abnormal variations of East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ) at that time. The stronger EASJ with abnormally northward position of the jet axis corresponded to the more intense storm event with broader ranges and longer duration time. The heavy freezing-rain-and-snow event occurred over the region where a strong southerly wind of EASJ prevailed. Meanwhile, the westerly and northerly winds of the EAPJ were significantly intensified, which were also closely related to the beginning, enhancement, and ending of the heavy snowfall. The meridional component of the EAPJ was dominated by the northerly wind during the snowstorm. Thus, the intensification of the snowstorm was attributed to both the strengthening of the meridional wind of EAPJ and the southerly wind of EASJ. Further analysis indicated that wind speed and the zonal wind of the two jets exhibited precursory signals about half a month prior to this extreme event, and the precursory signals were found in the meridional components of the two jets about 20 days preceding the event. The sudden weakening of the meridional component of EASJ and the zonal component of EAPJ signified the ending of this persistent snowstorm.
2014, 20(4): 360-367.
Abstract:
Based on the air-sea interface heat fluxes and related meteorological variables datasets recently released by Objectively Analyzed Air-Sea Fluxes (OA Flux) Project of Woods Hole Oceanographic Institution, as well as the outgoing longwave radiation and surface wind datasets from National Oceanic and Atmospheric Administration, the seasonal dependence of local air-sea interaction over the tropical western Pacific warm pool (referred to the region (1o-6oN, 144o-154oE)) is revealed and the probable impacts of remote forcing on the air-sea interaction are examined. The results indicated the dominance of oceanic forcing with the significant impact of ENSO in March and that of atmospheric feedback without notable influence of remote forcing in June. While the interannual variability of sea surface temperature anomaly (SSTA) is larger than that of SSTA tendency when oceanic forcing is dominant, the opposite is true when atmospheric feedback is dominant. The magnitude of the oceanic forcing of the atmosphere tends to decrease in March with the occurrence of ENSO, though ENSO has little influence on the atmospheric feedback to the ocean in June. The local air-sea interaction is substantially the same before and after the removal of the effect of Indian Oceanic Dipole. The reduction of shortwave radiation fluxes into the western Pacific warm pool, due to the enhanced overlaying convection in March associated with ENSO, leads to the decline of SST tendency that will weaken the oceanic forcing of the atmosphere.
Based on the air-sea interface heat fluxes and related meteorological variables datasets recently released by Objectively Analyzed Air-Sea Fluxes (OA Flux) Project of Woods Hole Oceanographic Institution, as well as the outgoing longwave radiation and surface wind datasets from National Oceanic and Atmospheric Administration, the seasonal dependence of local air-sea interaction over the tropical western Pacific warm pool (referred to the region (1o-6oN, 144o-154oE)) is revealed and the probable impacts of remote forcing on the air-sea interaction are examined. The results indicated the dominance of oceanic forcing with the significant impact of ENSO in March and that of atmospheric feedback without notable influence of remote forcing in June. While the interannual variability of sea surface temperature anomaly (SSTA) is larger than that of SSTA tendency when oceanic forcing is dominant, the opposite is true when atmospheric feedback is dominant. The magnitude of the oceanic forcing of the atmosphere tends to decrease in March with the occurrence of ENSO, though ENSO has little influence on the atmospheric feedback to the ocean in June. The local air-sea interaction is substantially the same before and after the removal of the effect of Indian Oceanic Dipole. The reduction of shortwave radiation fluxes into the western Pacific warm pool, due to the enhanced overlaying convection in March associated with ENSO, leads to the decline of SST tendency that will weaken the oceanic forcing of the atmosphere.
2014, 20(4): 368-375.
Abstract:
By statistical research on the occurrence pattern of severe convective weather in Jiangsu province under the influence of tropical cyclones within a 10-year period (from 2001 to 2010), this paper discovers that among different severe convective weather, the occurrence frequency of short-range heavy precipitation is the highest, thunderstorms and gales come in second, and general thunderstorms rarely happen, while hailstorms and tornadoes never occur. The statistical results also showed that within the research period there are 21 tropical cyclones (TCs) affecting the Jiangsu area and most of them are in the stage of weakening to tropical depressions. Moreover, through studying indices for relevant cases of severe convection, it is discovered that their thresholds are lower than that of previous research, which indicated that convective instability and energy accumulation can easily lead to severe convective weather eventually due to the influence of TCs.
By statistical research on the occurrence pattern of severe convective weather in Jiangsu province under the influence of tropical cyclones within a 10-year period (from 2001 to 2010), this paper discovers that among different severe convective weather, the occurrence frequency of short-range heavy precipitation is the highest, thunderstorms and gales come in second, and general thunderstorms rarely happen, while hailstorms and tornadoes never occur. The statistical results also showed that within the research period there are 21 tropical cyclones (TCs) affecting the Jiangsu area and most of them are in the stage of weakening to tropical depressions. Moreover, through studying indices for relevant cases of severe convection, it is discovered that their thresholds are lower than that of previous research, which indicated that convective instability and energy accumulation can easily lead to severe convective weather eventually due to the influence of TCs.
2014, 20(4): 376-385.
Abstract:
Due to the existence of thermal offsets, global solar irradiances measured by pyranometers are smaller than actual values, and errors are larger in the daytime. Until now, there is no universally-recognized correction method for thermal offset errors. Therefore, it is imperative to identify a convenient and effective correction method. Five correction methods were evaluated based on the data measured from a field experiment from 23 January to 15 November, 2011. Results have shown: 1) Temporal variation characteristics of thermal offsets in the four tested pyranometers are consistent. 2) Among the five methods, non-dimensional quantity method is suggested for use to correct thermal offsets, because it is convenient and no modification of instruments is required. If collocated net longwave radiation and wind speed data are available and their uncertainties are small, the historical solar radiation datasets can also be corrected. And correction effects by the method are better.
Due to the existence of thermal offsets, global solar irradiances measured by pyranometers are smaller than actual values, and errors are larger in the daytime. Until now, there is no universally-recognized correction method for thermal offset errors. Therefore, it is imperative to identify a convenient and effective correction method. Five correction methods were evaluated based on the data measured from a field experiment from 23 January to 15 November, 2011. Results have shown: 1) Temporal variation characteristics of thermal offsets in the four tested pyranometers are consistent. 2) Among the five methods, non-dimensional quantity method is suggested for use to correct thermal offsets, because it is convenient and no modification of instruments is required. If collocated net longwave radiation and wind speed data are available and their uncertainties are small, the historical solar radiation datasets can also be corrected. And correction effects by the method are better.
2014, 20(4): 386-390.
Abstract:
Based on a current fog detection theory, a multiband threshold method for MODIS data was put forward to detect daytime fog in the South China Sea. It used Bands 1, 2, 18, 20 and 31 of MODIS data to separate fog from the cloud and the sea surface. The digital detection indexes were as follows. If RB1<20%, RB2<20% and RB1>RB2, the pixel was identified to be the sea surface. If RB1>55%, RB2>55% and TB31<273 K, the pixel was identified to be a middle- and high-level cloud. If IFC>20, the pixel was classified to be sea fog. The method was verified with sea fog data observed from the coastal region of Guangdong during January-April 2011. Out of the 13 samples of satellite detection, nine were consistent with the surface observations, three were identified to be low-level the cloud according to the satellite detection but fog according to the surface observations, and only one sample was identified to be the ocean surface by the satellite detection but fog by the surface observations. Because the MODIS data cannot penetrate the cloud or fog, the model was designed for a single field of view which had only one layer of cloud or fog. It can accurately distinguish fog which is not covered by the cloud, but it identifies fog as cloud if the former is covered by a cloud. Generally speaking, the model is effective and feasible.
Based on a current fog detection theory, a multiband threshold method for MODIS data was put forward to detect daytime fog in the South China Sea. It used Bands 1, 2, 18, 20 and 31 of MODIS data to separate fog from the cloud and the sea surface. The digital detection indexes were as follows. If RB1<20%, RB2<20% and RB1>RB2, the pixel was identified to be the sea surface. If RB1>55%, RB2>55% and TB31<273 K, the pixel was identified to be a middle- and high-level cloud. If IFC>20, the pixel was classified to be sea fog. The method was verified with sea fog data observed from the coastal region of Guangdong during January-April 2011. Out of the 13 samples of satellite detection, nine were consistent with the surface observations, three were identified to be low-level the cloud according to the satellite detection but fog according to the surface observations, and only one sample was identified to be the ocean surface by the satellite detection but fog by the surface observations. Because the MODIS data cannot penetrate the cloud or fog, the model was designed for a single field of view which had only one layer of cloud or fog. It can accurately distinguish fog which is not covered by the cloud, but it identifies fog as cloud if the former is covered by a cloud. Generally speaking, the model is effective and feasible.
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