2003 Vol. 9, No. 1
2003, 9(1): 1-8.
Abstract:
The important effects of local land-sea thermodynamic contrast between the South China Sea (SCS) and Indochina Peninsula on SCS summer monsoon onset are preliminarily studied by using two sets of SSTA tests and two ideal tests in p -s regional climate model. The result shows that warm SST in the SCS in winter and spring is favorable for the formation of monsoon circulation throughout all levels of the atmosphere over the sea, which hastens the onset of SCS summer monsoon. The effects of cold SST are generally the opposite. The local land-sea contrast in the SCS is one of the possible reasons for SCS summer monsoon onset. Superposed upon large-scale land-sea thermodynamic differences, it facilitates the formation of out-breaking onset characteristics of SCS summer monsoon in the SCS area.
The important effects of local land-sea thermodynamic contrast between the South China Sea (SCS) and Indochina Peninsula on SCS summer monsoon onset are preliminarily studied by using two sets of SSTA tests and two ideal tests in p -s regional climate model. The result shows that warm SST in the SCS in winter and spring is favorable for the formation of monsoon circulation throughout all levels of the atmosphere over the sea, which hastens the onset of SCS summer monsoon. The effects of cold SST are generally the opposite. The local land-sea contrast in the SCS is one of the possible reasons for SCS summer monsoon onset. Superposed upon large-scale land-sea thermodynamic differences, it facilitates the formation of out-breaking onset characteristics of SCS summer monsoon in the SCS area.
2003, 9(1): 9-18.
Abstract:
The climatic jump of the western Pacific warm pool and its climatic effects were analyzed using the SST (COADS, NCEP) datasets. The results show that the warm pool has significant interdecadal variability with periods of 10 to 20 years and four major climatic jumps with periods of 40 to 50 years for a time dating back for more than 100 years, which happened in the 1910’s, 1930’s, 1950’s and 1980’s. The warm pool jumps have important climatic effects. After the jumps，the SST increases by about 0.5°C in central and eastern tropical Pacific; the Pacific subtropical high at 500 hPa strengthens and goes southwestward, the precipitation bands tend to be in southern China. The results also show that El Niño event will happen more often than the La Niña event when the warm pool anomalously develops, and the La Niña event will happen more often than the El Niño event when the warm pool anomalously declines. Over the recent 40-to-50 year period，the warm pool is still in its strengthening stage and it is possible that the drought in northern China will continue in the first decade of 21st century.
The climatic jump of the western Pacific warm pool and its climatic effects were analyzed using the SST (COADS, NCEP) datasets. The results show that the warm pool has significant interdecadal variability with periods of 10 to 20 years and four major climatic jumps with periods of 40 to 50 years for a time dating back for more than 100 years, which happened in the 1910’s, 1930’s, 1950’s and 1980’s. The warm pool jumps have important climatic effects. After the jumps，the SST increases by about 0.5°C in central and eastern tropical Pacific; the Pacific subtropical high at 500 hPa strengthens and goes southwestward, the precipitation bands tend to be in southern China. The results also show that El Niño event will happen more often than the La Niña event when the warm pool anomalously develops, and the La Niña event will happen more often than the El Niño event when the warm pool anomalously declines. Over the recent 40-to-50 year period，the warm pool is still in its strengthening stage and it is possible that the drought in northern China will continue in the first decade of 21st century.
2003, 9(1): 19-30.
Abstract:
Using historical data from 1951 to 1996, this paper makes statistical study and elaborate comparisons of tropical cyclone (TC) activity in middle and low latitudes. Some useful results have been achieved. For example, about 65% (90% in May) of the low-latitude tropical cyclones can move north into middle latitudes; TCs in middle latitudes move by about 60° more to the east and 10 km/h faster than in low latitudes; about 60% of the TCs dissipate in middle latitudes; the mean intensity is the maximum near the line dividing the middle and low latitude systems. The work paves the way for more work on revealing characteristics of interactions between middle and low latitude circulation systems.
Using historical data from 1951 to 1996, this paper makes statistical study and elaborate comparisons of tropical cyclone (TC) activity in middle and low latitudes. Some useful results have been achieved. For example, about 65% (90% in May) of the low-latitude tropical cyclones can move north into middle latitudes; TCs in middle latitudes move by about 60° more to the east and 10 km/h faster than in low latitudes; about 60% of the TCs dissipate in middle latitudes; the mean intensity is the maximum near the line dividing the middle and low latitude systems. The work paves the way for more work on revealing characteristics of interactions between middle and low latitude circulation systems.
2003, 9(1): 31-40.
Abstract:
The 500-hPa geopotential height data used in this paper are from NCEP/NCAR data set for the period from 1979 to 1996 (from March to July). Using pentad average, we define the intensity, westernmost ridge point and mean latitude of the subtropical high ridge. Then the wavelet transform and EOF analysis are performed. It is found that there mainly exist three interseasonal abrupt change processes, which correspond to the onset time of the South China Sea Summer Monsoon (SCSSM), the beginning and the end of the Mei-yu respectively. The interannual variation of the subtropical high in late spring and early summer presents quasi-4-year and 8-year periods.
The 500-hPa geopotential height data used in this paper are from NCEP/NCAR data set for the period from 1979 to 1996 (from March to July). Using pentad average, we define the intensity, westernmost ridge point and mean latitude of the subtropical high ridge. Then the wavelet transform and EOF analysis are performed. It is found that there mainly exist three interseasonal abrupt change processes, which correspond to the onset time of the South China Sea Summer Monsoon (SCSSM), the beginning and the end of the Mei-yu respectively. The interannual variation of the subtropical high in late spring and early summer presents quasi-4-year and 8-year periods.
2003, 9(1): 41-48.
Abstract:
Ensemble forecasting of tropical cyclone (TC) motion was studied using a primitive equation barotropic model by perturbing initial position and structure for 1979 –C 1993 TC. The results show that TC initial position perturbation affects its track, but the ensemble mean is close to control forecast. Experiments was also performed by perturbing TC initial parameters which were used to generate TC initial field, and more improvement can be obtained by taking ensemble mean of selective member than selecting members randomly. The skill of 60 % –C 70 % of all cases is improved in selective ensemble mean. When the ambient steering current is weak, more improvement can be obtained over the control forecast.
Ensemble forecasting of tropical cyclone (TC) motion was studied using a primitive equation barotropic model by perturbing initial position and structure for 1979 –C 1993 TC. The results show that TC initial position perturbation affects its track, but the ensemble mean is close to control forecast. Experiments was also performed by perturbing TC initial parameters which were used to generate TC initial field, and more improvement can be obtained by taking ensemble mean of selective member than selecting members randomly. The skill of 60 % –C 70 % of all cases is improved in selective ensemble mean. When the ambient steering current is weak, more improvement can be obtained over the control forecast.
2003, 9(1): 49-56.
Abstract:
With NCEP/NCAR reanalysis daily data and SST for 1998, the paper investigates the features of summer monsoon low-frequency oscillation (LFO) over the South China Sea (SCS). Results show that SCS summer monsoon onset is enhanced because of its LFO. Low-frequency (LF) low-level convergence (divergence) region of SCS is in the LF positive (negative) rainfall area. LFO of the SCS region migrates from south to north in the meridian and from west to east in zonal direction. LF divergence of SCS is vertically compensating to each other between high and low level
With NCEP/NCAR reanalysis daily data and SST for 1998, the paper investigates the features of summer monsoon low-frequency oscillation (LFO) over the South China Sea (SCS). Results show that SCS summer monsoon onset is enhanced because of its LFO. Low-frequency (LF) low-level convergence (divergence) region of SCS is in the LF positive (negative) rainfall area. LFO of the SCS region migrates from south to north in the meridian and from west to east in zonal direction. LF divergence of SCS is vertically compensating to each other between high and low level
2003, 9(1): 64-73.
Abstract:
With the singular value decomposition (SVD), correlation analysis has been conducted between the Pacific Ocean sea surface temperature (SST) and northwestern China precipitation over March –C May (MAM). The result shows that there is good relationship between the North Pacific and spring precipitation in northwestern China. When the SST is of the peak El Niño phase, precipitation is less over this part of the country except for the Qinghai-Tibetan Plateau; when the SST for the months DJF is of the mature El Niño phase, precipitation is more over the region in the subsequent March –C May; when the North Pacific SST for DJF is of the La Niña pattern, precipitation is less over the plateau in the subsequent March –C May. For the Pacific SST, the westerly drift, kuroshio current, Californian current and north equatorial current are all significantly correlating with the March –C May precipitation in northwestern China. Specifically, the SST in DJF over the kuroshio current region is out of phase with the precipitation in northern Xinjiang, i.e. when the former is low, the latter is more. In northwestern China, regions in which March –C May precipitation response to the variation of SST in the Pacific Ocean are northern Xinjiang, the Qinghai-Tibetan Plateau and areas off its northeastern part, the desert basin and western part of the Corridor of the Great Bend of Yellow River valley (Corridor).
With the singular value decomposition (SVD), correlation analysis has been conducted between the Pacific Ocean sea surface temperature (SST) and northwestern China precipitation over March –C May (MAM). The result shows that there is good relationship between the North Pacific and spring precipitation in northwestern China. When the SST is of the peak El Niño phase, precipitation is less over this part of the country except for the Qinghai-Tibetan Plateau; when the SST for the months DJF is of the mature El Niño phase, precipitation is more over the region in the subsequent March –C May; when the North Pacific SST for DJF is of the La Niña pattern, precipitation is less over the plateau in the subsequent March –C May. For the Pacific SST, the westerly drift, kuroshio current, Californian current and north equatorial current are all significantly correlating with the March –C May precipitation in northwestern China. Specifically, the SST in DJF over the kuroshio current region is out of phase with the precipitation in northern Xinjiang, i.e. when the former is low, the latter is more. In northwestern China, regions in which March –C May precipitation response to the variation of SST in the Pacific Ocean are northern Xinjiang, the Qinghai-Tibetan Plateau and areas off its northeastern part, the desert basin and western part of the Corridor of the Great Bend of Yellow River valley (Corridor).
2003, 9(1): 74-79.
Abstract:
The GMS-5 infrared cloud imagery for two yearly first raining seasons in 1998 and 1999 are used to study the relationship between brightness temperature and surface rain rates. The result shows that it is likely to have large probability of heavy precipitation with the decrease of brightness temperature and the gradual increase of rainfall intensity; for areas of low temperature, the brightness temperature is better determined for atmosphere above rain gauge stations with multiple points sampling than with single point one; for the yearly first raining season, the threshold brightness temperature is set at –C4.6℃ for indication of heavy precipitation in the Fujian area.
The GMS-5 infrared cloud imagery for two yearly first raining seasons in 1998 and 1999 are used to study the relationship between brightness temperature and surface rain rates. The result shows that it is likely to have large probability of heavy precipitation with the decrease of brightness temperature and the gradual increase of rainfall intensity; for areas of low temperature, the brightness temperature is better determined for atmosphere above rain gauge stations with multiple points sampling than with single point one; for the yearly first raining season, the threshold brightness temperature is set at –C4.6℃ for indication of heavy precipitation in the Fujian area.
2003, 9(1): 80-85.
Abstract:
In this paper, time and space distribution regularity of meso-scale heavy rains in five selected typhoons which landed at Fujian from 1996 to 1998 has been analyzed. Besides, with hourly digitized satellite infrared imagery, the features of the mesoscale are revealed for the genesis and evolution of mesoscale convective systems in typhoons. It indicates that the intensity of mesoscale storms is closely connected with the temperature and the area of the coldest cloud cluster. The heavy rainfall usually emerges on the eastern side of the mesoscale convective cloud clusters, where the cloud mass is developing and with a dense gradient and big curvature of isoline of the cloud top temperature.
In this paper, time and space distribution regularity of meso-scale heavy rains in five selected typhoons which landed at Fujian from 1996 to 1998 has been analyzed. Besides, with hourly digitized satellite infrared imagery, the features of the mesoscale are revealed for the genesis and evolution of mesoscale convective systems in typhoons. It indicates that the intensity of mesoscale storms is closely connected with the temperature and the area of the coldest cloud cluster. The heavy rainfall usually emerges on the eastern side of the mesoscale convective cloud clusters, where the cloud mass is developing and with a dense gradient and big curvature of isoline of the cloud top temperature.
2003, 9(1): 86-94.
Abstract:
The circulation of β-gyres in tropical cyclones is studied using numerical simulations. As shown in the result, there is clear circulation of β-gyres in the deviation flow field of the middle layer of the model，i.e. there is cyclone current west of the vortex center but anticyclone current east of it. The theory analysis shows that the circulation of β-gyres is formed by the advection of geostrophic vorticity.
The circulation of β-gyres in tropical cyclones is studied using numerical simulations. As shown in the result, there is clear circulation of β-gyres in the deviation flow field of the middle layer of the model，i.e. there is cyclone current west of the vortex center but anticyclone current east of it. The theory analysis shows that the circulation of β-gyres is formed by the advection of geostrophic vorticity.
2003, 9(1): 95-104.
Abstract:
With a simple tropical coupled ocean-atmosphere model, this paper presents an analysis aiming to understand the relative role of the meridional and zonal wind stresses in the tropical unstable air-sea interaction. The roles of the zonal wind stress, the meridional wind stress and the both are considered respectively into the coupled system. It is demonstrated that the meridional component of the wind stress does not lead to any instability under the local thermal balance assumption, but it does lead to a weak instability under the sea surface temperature advection assumption. Unstable air-sea interaction is dominated by the zonal component of the wind stress, suggesting that ignoring the meridional wind stress is approximately feasible in studying the tropical unstable air-sea interaction.
With a simple tropical coupled ocean-atmosphere model, this paper presents an analysis aiming to understand the relative role of the meridional and zonal wind stresses in the tropical unstable air-sea interaction. The roles of the zonal wind stress, the meridional wind stress and the both are considered respectively into the coupled system. It is demonstrated that the meridional component of the wind stress does not lead to any instability under the local thermal balance assumption, but it does lead to a weak instability under the sea surface temperature advection assumption. Unstable air-sea interaction is dominated by the zonal component of the wind stress, suggesting that ignoring the meridional wind stress is approximately feasible in studying the tropical unstable air-sea interaction.
2003, 9(1): 105-112.
Abstract:
By statistic and dynamic analyses, we have come to the following conclusions: (1) The ECMWF medium-term numerical forecast can forecast medium-term activity of subtropical high, and the accuracy rate of forecast cannot have large improvement by translational corrections. (2) The important cause for the ECMWF medium-term numerical forecast to have errors in 1998 is that the astronomical tide is not included in the model. (3) Two indexes are found from which it can be judged that ECMWF medium-term numerical forecast will have errors if the astronomical tide is ignored in the model : ① When the 54.7° line under the moon of the nodical month astronomical singularities coincides with the trough-line of the subtropical jet flow from 50°E to 150°E on the 500 hPa level at 2000 L.T. of the same day, and is approximately vertical (α>60°) with the isotherm, then the day 0 –C 2 days after the appearance of the nodical month astronomical singularities is defined as the initial day. Then in three successive days after the initial day, ECMWF medium-term numerical forecast of the northern latitude of the 588 line at 120 °E will have continuous errors as large as two latitudes (7/9). Otherwise, it won't have continuous errors (13/13). ② Otherwise, if the 54.7 ° line is in the range of a low pressure between two high pressures, then there is a dispersive error on the day of the nodical month astronomical singularities (5/7). There is not any error (6/6) otherwise.
By statistic and dynamic analyses, we have come to the following conclusions: (1) The ECMWF medium-term numerical forecast can forecast medium-term activity of subtropical high, and the accuracy rate of forecast cannot have large improvement by translational corrections. (2) The important cause for the ECMWF medium-term numerical forecast to have errors in 1998 is that the astronomical tide is not included in the model. (3) Two indexes are found from which it can be judged that ECMWF medium-term numerical forecast will have errors if the astronomical tide is ignored in the model : ① When the 54.7° line under the moon of the nodical month astronomical singularities coincides with the trough-line of the subtropical jet flow from 50°E to 150°E on the 500 hPa level at 2000 L.T. of the same day, and is approximately vertical (α>60°) with the isotherm, then the day 0 –C 2 days after the appearance of the nodical month astronomical singularities is defined as the initial day. Then in three successive days after the initial day, ECMWF medium-term numerical forecast of the northern latitude of the 588 line at 120 °E will have continuous errors as large as two latitudes (7/9). Otherwise, it won't have continuous errors (13/13). ② Otherwise, if the 54.7 ° line is in the range of a low pressure between two high pressures, then there is a dispersive error on the day of the nodical month astronomical singularities (5/7). There is not any error (6/6) otherwise.
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