2008 Vol. 22 No. 3
Cover Note
Abstract：The sensitivity of the simulated tropical intraseasonal oscillation or MJO (Madden and Julian oscillation) to different cumulus parameterizations is studied by using an atmospheric general circulation model(GCM)-SAMIL (Spectral Atmospheric Model of IAP LASG). Results show that performance of the modelin simulating the MJO alters widely when using two di erent cumulus parameterization schemes-the moistconvective adjustment scheme (MCA) and the Zhang-McFarlane (ZM) scheme. MJO simulated by the MCAscheme was found to be more realistic than that simulated by the ZM scheme. MJO produced by the ZM scheme is too weak and shows little propagation characteristics. Weak moisture convergence at low levels simulated by the ZM scheme is not enough to maintain the structure and the eastward propagation of the oscillation. These two cumulus schemes produced di erent vertical structures of the heating pro le. The heating profile produced by the ZM scheme is nearly uniform with height and the heating is too weak compared to that produced by the MCA, which maybe contributes greatly to the failure of simulating a reasonable MJO. Comparing the simulated MJO by these two schemes indicate that the MJO simulated by the GCM is highly sensitive to cumulus parameterizations implanted in. The diabatic heating profile plays an important role in the performance of the GCM. Three sensitivity experiments with di erent heating pro les are designed in which modified heating pro les peak respectively in the upper troposphere (UH), middle troposphere (MH), and lower troposphere (LH). Both the LH run and the MH run produce eastward propagating signals on the intraseasonal timescale, while it is interesting that the intraseasonal timescale signals produced by the UH run propagate westward. It indicates that a realistic intraseasonal oscillation is more prone to be excited when the maximum heating concentrates in the middle-low levels, especially in the middle levels, while westward propagating disturbances are more prone to be produced when the maximum heating appears very high.More+
Abstract：This article focuses on the variability of the coupling between surface air temperature (SAT) and northern annular mode (NAM) at various levels. To measure the coupling intensity between the SAT and the NAM anomaly fields, the coupling index has been defined as the leading principal component of the partial least squares regression model of the SAT and NAM anomalies. Both a composite analysis and the coupling index have been used to reveal level-by-level and month-to-month variability of the coupling between the upper anomalous NAM and the SAT in the Northern Hemisphere. The major results are as follows: the January SAT anomaly is more strongly coupled with the January NAM anomaly at the middle-upper tropospheric levels than that at the other levels, while the same is true for the February SAT anomaly with the January NAM anomaly at the lower stratospheric levels. The January NAM anomaly at the middleupper tropospheric levels is most strongly coupled with the January SAT anomaly, and the coupling intensity is successively reduced month by month and becomes trivial after April. The January NAM anomaly at the lower stratospheric levels is more strongly coupled with January, February and March SAT anomalies, but the coupling becomes trivial after April.More+
Abstract：A strong cyclonic wind perturbation generated in the northern South China Sea (SCS) moved northward quickly and developed into a mesoscale vortex in southwest Guangdong Province, and then merged with a southward-moving shear line from mid latitudes in the period of 21-22 May 2006, during which three strong mesoscale convective systems (MCSs) formed and brought about torrential rain or even cloudburst in South China.
With the 1°× 1°NCEP (National Centers for Environment Prediction) reanalysis data and the Weather and Research Forecast (WRF) mesoscale model, a numerical simulation, a potential vorticity inversion analysis, and some sensitivity experiments are carried out to reveal the formation mechanism of this rainfall event. In the meantime, conventional observations, satellite images, and the WRF model outputs are also utilized to perform a preliminary dynamic and thermodynamic diagnostic analysis of the rainstorm systems. It is found that the torrential rain occurred in favorable synoptic conditions such as warm and moist environment, low lifting condensation level, and high convective instability. The moisture transport by strong southerly winds associated with the rapid northward advance of the cyclonic wind perturbation over the northern SCS provided the warm and moist condition for the formation of the excessive rain. Under the dynamic steering of a southwesterly flow ahead of a north trough and that on the southwest side of the West Pacific subtropical high, the mesoscale vortex (or the cyclonic wind perturbation), after its genesis,moved northward and brought about enormous rain in most parts of Guangdong Province through providing certain lifting forcing for the triggering of mesoscale convection. During the development of the mesoscale vortex, heavy rainfall was to a certain extent enhanced by the mesoscale topography of the Yunwu Mountain in Guangdong. The effect of the Yunwu Mountain is found to vary under di erent prevailing wind directions and intensities. The location of the heavy rainfall was in a degree determined by the trumpet-shaped topography of the Zhujiang Delta. It is identified that the topographic effect on precipitation depends on the relative position between the terrain and the mesoscale storm systems. The short distance from the SCS to South China facilitates the moisture transport, which offers ease for the heavy rain to form in South China. Finally, the role played by land-sea contrast in the fast intensification of the MCSs in South China is not yet clear, and the interaction between the MCSs and the mesoscale vortex needs to be clarified as well.More+
Abstract：By using a reverse computation method and the NCEP/NCAR daily reanalysis data from 1960 to 2004, the atmospheric heat source (AHS) was calculated and analyzed. The results show that AHS over the Tibetan Plateau (TP) and its neighboring areas takes on a persistent downtrend in spring and summer during the foregone 50 years, especially the latest 20 years. Snow depth at 50 stations over the TP in winter and spring presents an increase, especially the spring snow depth exhibits a sharp increase in the late 1970s. A close negative correlation exists between snow cover and AHS over the TP and its neighboring areas,as revealed by an SVD analysis, namely if there is more snow over the TP in winter and spring, then the weaker AHS would appear over the TP in spring and summer. The SVD analysis between AHS over the TP in spring and summer and rainfall at 160 stations indicates that the former has a negative correlation with summer precipitation in the middle and lower reaches of the Yangtze River, and a positive correlation with that in South China and North China. The SVD analysis of both snow cover over the TP in winter and spring and rainfall at the same 160 stations indicates that the former has a marked positive correlation with precipitation in the middle and lower reaches of the Yangtze River, and a reversed correlation in South China and North China. On the decadal scale, the AHS and winter and spring snow cover over the TP have a close correlation with the decadal precipitation pattern shift (southern flood and northern drought) in East China. The mechanism on how the AHS over the TP influences rainfall in East China is discussed.The weakening of AHS over the TP in spring and summer reduces the thermodynamic difference between ocean and continent, leading to a weaker East Asian summer monsoon, which brings more water vapor to the Yangtze River Valley and less water vapor to North China. Meanwhile, the weakening of AHS over the TP renders the position of the subtropical high further westward and the rain belt lasting longer in the Yangtze River Valley, which causes more rain there and less rain in North China, thus showing the pattern of "southern flood and northern drought" in the latest 20 years. It is inferred that the increase of snow cover over the TP brings about the reduction of surface temperature and then surface heat source, leading eventually to the weakening of AHS there.More+
2008, 22(3): 317-328.
Abstract：The response of the Asian-African summer monsoon (AASM) to the fast global warming in the 1980s is studied based on several datasets, which span a long time period of nearly 100 yr, with two special periods 1980{1985 and 1990{1995 being focused on. Wavelet analyses are employed to explore the interdecadal variations of the AASM.It is found that after the mid-1980s, the global annual mean surface temperature rises more signi cantly and extensively over most parts of the African Continent, north of the Indian Ocean, and the Eurasian Continent excluding the Tibetan Plateau. Correspondingly, the global precipitation pattern alters with in creased rainfall seen over the Sahel and North China in 1990-1995, though it is not recovered to the level of the rainy period before the mid-1960s.Changes of monsoonal circulations between the pre- and post-1980s periods display that, after the fast global warming of the 1980s, the African summer monsoon intensi es distinctly, the Indian summer monsoon weakens a little bit, and the East Asian summer monsoon remains almost unchanged. The summer precipitation over the Asian-African Monsoon Belt (AAMB) does not change in phase coherently with the variations of the monsoonal circulations.Wavelet analyses of the land-sea thermal contrast and precipitation over North China and the Sahel indicate that interdecadal signals are dominant and in positive phases in the 1960s, leading to an overall enhanced interdecadal variation of the AASM, although the 1960s witnesses a global cooling. In the 1980s, however, in the context of a fast global warming, interdecadal signals are in opposite phases, and they counteract with each other, leading to a weakened interdecadal variation of the AASM.After the mid-1960s, the AASM weakened remarkably, whereas after the mid-1980s, the AASM as a whole did not strengthen uniformly and synchronously, because it is found that the interannual variations of the AASM in the 1980s are stronger than those in the 1960s, and they superimposed on the counteracting interdecadal signals, causing di erent regions of the AAMB behaving di erently. Therefore, the response of the AASM to the accelerated global warming post the mid-1980s is not simply out-of-phase with that after the mid-1960s; it may involve more complicated multiscale physical elements.More+
Abstract：Three kinds of typhoons with distinct tracks are sorted based on a set of typhoon data from 1958 to 1998. The results of composite analyses con rm that different typhoon tracks correspond to different patterns of the subtropical anticyclone over the western Pacific (SAWP). When the tracks are westward, the SAWP is strong, with a zonal form, and stretches westward; when the tracks are recurving, the main body of the SAWP shifts eastward and breaks near 160°E; and when the tracks are northward, the SAWP is located far east of its normal position. Based on the above result, two different initial fields are con gured, one has a zonal and strong SAWP, and the other has a meridional and weak SAWP. By using the GOALS R42L9 climate model, a temperature disturbance is added into these two different initial fields to force the formation of a typhoon. Westward and northward tracked typhoons are well simulated, thus verifying that different patterns of the SAWP have different effects on typhoon tracks. Results also show that typhoons can induce barotropic Rossby waves propagating to the mid and high latitudes. Under different background zonal flows, the wave trains triggered by the typhoons of westward and northward tracks are also different, and their effects on the mid and high latitude circulations and the SAWP are different. Compared to a northward tracked typhoon, a westward tracked typhoon is able to induce positive geopotential height anomaly to its north and northwest, resulting in the SAWP strengthening and developing westward.More+
2008, 22(3): 342-350.
Abstract：Quantitative estimates of liquid water path (LWP) in clouds using satellite measurements are critical to understanding of cloud properties and the assessment of global climate change. In this paper, the relationship between microwave brightness temperature (TB) and LWP in the nonprecipitating clouds is studied by using satellite microwave measurements from the TRMM Microwave Imager (TMI) onboard the Tropical Rainfall Measuring Mission (TRMM), together with a radiative transfer model for microwave radiance calculations. Radiative transfer modeling shows that the sensitivity is higher at both 37.0 and 85.5 GHz horizontal polarization channels for the LWP retrievals. Also, the differences between the retrieved values responding to TBs of various channels and the theoretical values are displayed by the model. Based upon above simulations, with taking into account the factor of resolution and retrieval bias for a single channel, a nonprecipitating cloud LWP in the summer subtropical marine environment retrieval algorithm is formulated by the combination of the two TMI horizontal polarization channels, 37.0 and 85.5 GHz. Moreover, by using TMI measurements (1B11), this algorithm is applied to retrieving respectively LWPs for clear sky,nonprecipitating clouds, and typhoon precipitating clouds. In the clear sky case, the LWP changes from -1 to 1 g m-2, and its mean value is about 10-5 g m-2. It indicates that, using this combination retrieval algorithm, there are no obvious systemic deviations when the LWP is low enough.The LWP values varying from 0 to 1000 g m-2 in nonprecipitating clouds are reasonable, and its distribution pattern is very similar to the detected results in the visible channel of Visible and Infrared Scanner (VIRS) on the TRMM. In typhoon precipitating clouds, there is much more proportion of high LWP in the mature phase than the early stage. When surface rainfall rate is lower than 5 mm h-1, the LWP increases with increasing rainfall rate.More+
2008, 22(3): 351-361.
Abstract：This paper investigated several approaches to remapping and combining multiple-radar reflectivity fields onto a unified 3D Cartesian grid with high spatial and temporal resolutions,and analyzed systematic observation differences among multiple radars. The remapping approaches were evaluated by inspecting the spatial consistency of the reflectivity fields on vertical and horizontal cross sections on the equidistant line of radars, and the intensity change of 1-h accumulated precipitation before and after interpolation. The combining approaches were evaluated by continuity examination. The results show that for remapping schemes,the vertical interpolation with nearest neighbor on the range-azimuth plane is the most reasonable scheme that provides consecutive reflectivity fields and retains the high-resolution structure comparable to that of the raw data; for multiple-radar data mosaics, the distance-exponential-weighted mean scheme provides spatially consistent reflectivity mosaics. The mosaics can mitigate various problems caused by the radar beam geometry such as the cone of silence.More+
2008, 22(3): 362-374.
Abstract：A comparison is made of the high-speed (2000 fps) photographic records in rocket-triggered negative lightning between two techniques. The analysis shows that: the initial speed of upward positive leader (UPL) in altitude-triggered negative lightning (ATNL) is about one order of magnitude less than that in classically triggered negative lightning (CTNL), while the triggering height of ATNL is higher than that of CTNL; the afterglow time of metal-vaporized part of the lightning channel can endure for about 160-170 ms, thus the luminosity of the air-ionized part can reflect the characteristics of the current in the lightning channel better than that of the metal-vaporized part. According to the different characteristics of the luminosity change of the lightning channel, together with the observation of the electric field changes, three kinds of processes after return-stroke (RS) can be distinguished:the continuous decaying type without M component, the isolated type and the continuing type with M component, corresponding to different wave shapes of the continuous current. The geometric mean of the interval of RS with M component is 77 ms,longer than that (37 ms) of RS without M component. And the initial continuous current (ICC) with M component also has a longer duration compared to the ICC without M component. The distinction in the relative luminosity between the lightning channel before RS and that before M component is obvious: the former is very weak or even cannot be observed, while the latter is still considerably luminous.More+
2008, 22(3): 375-382.
Abstract：A new short-term climatic prediction model based on the singular value decomposition (SVD) iteration was designed with solid mathematics and strict logical reasoning. Taking predictors into prediction model, using iteration computation, and substituting the last results into the next computation, we can acquire better results with improved precision.Precipitation prediction experiments were separately done for 16 stations in North China and 30 stations in the mid-lower catchment of the Yangtze River during 1991-2000. Their average mean square errors are 0.352 and 0.312, and the results are very stable. Mean square errors of 9 yr are less than 0.5 while only that of 1 yr is more than 0.5. The mean sign correlation coe cients between forecast and observed summer precipitation during 1991-2000 are 0.575 in North China and 0.623 in the mid-lower catchment of the Yangtze River. Librations of them in North China during the 10 years are small. Only in 1996 the sign correlation coefficient is below 0.5; the others are all over 0.5. But sign correlation coe cients in the mid-lower catchment of the Yangtze River vary obviously. The lowest is only 0.3 in 1992, and the highest is 0.9 in 1998, As the distribution of the forecast precipitation anomaly field in the summer 1998 of is examined,it is known that the model captured the positive and negative anomalyies of precipitation,and also well forecasted the anomaly distributions. But the errors are obvious in quantities between the forecast and the observed precipitation anomalies.Climate characteristics of large scale meteorological elements, such as summer precipitation have obvious differences in spatial distribution. We can forecast better if we divide a big region into many subregions according to the discrepancy of climatic characteristics in the region, and predict in each subregion. The research shows that the model of SVD iteration is a very effective forecast model and has a strongly applicable value.More+
Vol. 22, No. 3, Jun 2025
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