Abstract:A two-dimensional two-layer barotropic quasi-equilibrium ocean model considering Rayleigh dissipation is built, there are two kinds of situations in the model: One is the west coast of the ocean considered only, the other is the east and west coasts of the ocean taken into account. And both analytic solutions of the free vortex wave are given. The results show that, the forms of the solutions are wave packet, when only the west coast of the ocean is considered, whose carrier frequency is continuous spectrum; while both the east and west coasts of the oceans are taken into account, whose carrier frequency is discrete spectrum. And both cases have the same characteristics that: When carrier frequency is higher (period is shorter), the horizontal scale of wave packet is greater. For the excessive low-frequency fluctuations, the quasi-equilibrium assumption will be no longer applied. The period of wave package carrier frequency in the model is about 26 d to 24 a. Because friction is taken into account, the amplitude of wave packets decay exponentially with time, but the size of the coefficient of friction affects only the extent of wave packets decay without altering spatial structure. Finally, amplitude of the wave packet tends to 0, so the solution of the two-layer barotropic ocean model tends to particular solution of atmospheric wind field to forcing. In the model, carrier frequency of wave package spread westward; when the frequency is high, then the westward propagation is faster, and the characteristics and deformation of wave packets are obvious; while the frequency is low, then the westward propagation is slowly, and the waveform is close to plane simple harmonic wave. In the two-layer barotropic model, the flow of the upper flow field is similar to the barotropic model’s, while the lower ocean flow, whose velocity is the same as the upper ocean, the direction opposite. In the model, the nature of fluctuations is the quasi-equilibrium (quasi-non-divergence) vortex wave, when the friction is not too large while horizontal scale of fluctuations is greater than 10 km, the nature is quasi geostrophic Rossby waves.

Abstract:The anomalously low Surface Air Temperature (SAT) and the corresponding atmospheric circulation anomalies in the Northern Hemisphere during the winter (DJF) of 2009/2010 are investigated. Surface Air Temperature Anomalies (SATA) were zonally distributed with a "positive-negative-positive" pattern from tropical to polar region which was characterized by positive SATA in low latitudes, negative SATA in middle latitudes, and positive SATA in high latitudes. The coldest SATA were located in Eurasia and eastern USA, and in some parts SATA exceeded -4 ℃. SAT in middle latitudes decreased by nearly 1 ℃ compared to the average of the last 15 years, and in Eurasia SAT even decreased by -8 ℃. The anomalous feathers of horizontal circulation was that, both sea level pressure and geopotential height fields exhibited positive anomalies in high latitudes and negative anomalies in low latitudes, associated with anticyclonic circulation anomalies in high latitudes and cyclonic circulation anomalies in middle latitude, and both high levels and low levels exhibited this feather. The anomalous feathers of zonally mean meridional circulation indicated that, Ferrel cell was weakened, with anomalous ascending motion in middle latitudes and anomalous descending motion in high latitudes, at the same time, atmosphere temperature of troposphere in middle latitudes was anomalously low and that in high latitudes was warmer, with a stronger jet in subtropics and a weaker jet in the polar region. Composite and correlation analysis of the Northern Hemisphere Annular Mode (NAM) and SATA shows that, during positive (negative) phase of NAM, there was a positive (negative) band in middle latitudes, and it was most significant in Eurasia and eastern USA, with nearly 2 ℃ warming (cooling) in some regions. Analysis of air temperature equation shows that meridional temperature advection anomalies associated with anomalous meridional wind play a very important role in the variability of local temperature, and its distribution closely resembled the SATA in 2009/2010 winter. With composite and correlation analysis, the mechanism for the relationship between NAM and SATA is investigated. During the positive phase of NAM, the Ferrel cell is anomalously strong, the mass between two latitude zones exchanges, in sea level pressure positive anomalies is in middle latitudes and negative anomalies is in high latitudes, which forces southerly surface wind anomalies in the middle latitude. As a result, the meridional temperature advection is enhanced, which results in warmer SAT in middle latitudes. The situations is opposite during negative phase of NAM. The above-mentioned physical mechanism reveals that surface air temperature in mid-latitude is modulated by NAM as a natural variability.

Abstract:To simulate the urban dispersion of denser-than-air gases, an urban denser-than-air dispersion model known as SLAB_URBAN model was developed. The model is based on the shallow layer theory. A new urban boundary and dispersion coefficients parameterization representing the urban wind and turbulence features is applied to the model. Tests were performed during the field campaign Urban 2000 which was held in October 2000 in Salt Lake City. The tracer data sets are adopted to demonstrate the SLAB_URBAN model performance.The focus of the comparisons is on the maximum concentration, on a given monitoring arc, normalized by the emission rate. At Urban 2000, for all samples and release times, the predictions of the SLAB_URBAN model are shown to agree with observations. Furthermore, SLAB_URBAN model is evaluated by comparison with other urban dispersion models. The SLAB_URBAN model predictions are shown to perform well in all urban dispersion models.

Abstract:The OAFlux (Objectively Analyzed air-sea Fluxes) database, provided by the Woods Hole Oceanographic Institution, and widely applied in the validation of climate models, is tested by using the TOGA COARE and KAWJEX measurements which are two serial survey databases of NOAA ETL(Environmental Technology Laboratory). The result shows that the latent heat flux of OAFlux is systematically higher than that of ETL, and the difference between them is comparatively large when the heat flux is large itself. Wind velocity plays a dominant role on the change of latent heat when the influence of humidity difference is minimal. In addition, when the sea surface temperature, air temperature, and wind direction do not vary much, the latent difference between OAflux and direct measurements is significant under medium to large wind speed condition and is minimal under the low wind condition. Meanwhile, the sensible heat exchange is very weak and the values are still mainly controlled by wind speed. However, the values of the measured sensible heat flux are very close to the instrument error, leading to difficulty in comparison analysis. The result also shows that the interaction between air and sea mainly depended on the wind velocity when the humidity difference between sea surface and air did not change much during the experimental periods.

Abstract:Dual polarization X-band radar data and ground-based automatic rain gauge data were analyzed for precipitation estimation in summer of Beijing in 2009. Firstly, the low pass filtering and attenuation correction methods were used for controlling the quality of radar data. Then two relationships of X-band radar parameters and rain rate were established, i.e.Z_h=159R^1.37 and R =13.9K_dp ^0.81, after the specific differential propagation phase (K_dp) was extracted. Finally the comparison analysis was made between the rain rates estimated from the two relationships and the corresponding ground data. The results indicate that the rain rates estimated from the two relationships have a good consistency with the ground data. In addition, when the rain rates are greater than 10 mm·h^-1, the estimated rain rates from K_dp-R are more accurate and stable than that from Z_h-R. Also, the average standard deviation analysis shows that the estimation accuracy of K_dp-R is significantly higher than Z_h-R.

Abstract:The regional and seasonal variations of surface O₃ over East Asia from 2002 to 2007 and the reason that the maximum of the O₃ monthly mean values occurred at different time at different sites are analysed by using the data of Acid Deposition Monitoring Network in East Asia,NCEP/NCAR,NOAA, and Global Precipiation Climatology Project. The results showed that the highest value occurred in spring and the lowest occurred in summer over East Asia and most midlatitude clean background areas in the Northern Hemisphere. Because of the East Asian monsoon advancement process the time when the monthly mean value in spring reaches the maximum are slightly different at different sites. AT Ogasawara and Hedo,they reached the highest values in March, while at Rishiri, Happo, Oki, and Cheju,they reached the highest values in April or May. This is mainly because of the push of East Asia summer monsoon. The O₃ concentration is higher than the climate mean in the spring of 2007. This is mainly because of the delay of subtropical high establishing and the block of the pushing. In 2007, the O₃ concentration reached the maximum in May at Cheju. The O₃ concentration in the spring 2004 at Hedo was significantly lower than average because of the earlier establishing of East Asia summer monsoon.

Abstract:In the winter of 2009 to 2010, a rare severe drought struck Yunan Province. This severe drought is due to the lack of rainfall for a long period. The statistical features of the atmospheric circulation and the sea surface temperature for anomalous precipitation in winter over Yunan Province are studied. Furthermore, the severe drought in the winter of 2009 to 2010 is analyzed. Study shows that the occurrence of this severe drought is related to the anomalies of the atmospheric circulation systems in the westerly belt. Positive height anomalies control Lake Baikal and negative height anomalies are sustained over the coastal areas of East Asia, which weakens the northwesterly and force of cold air from the north. Thus cold air can not southward extend into Yunan Province. Especially the Middle East jet stream is weakened. Because a high pressure ridge develops from East Europe to the Caspian Sea, the perturbations of the westerly belt could hardly reach East Asia. Over the Tibetan Plateau there is a high pressure ridge, which leads to the dry northwesterly prevails over Yunan Province. Although the sea surface temperature anomalies in the Pacific can influence the winter precipitation over Yunan Province, it does not give cause for the drought in winter of 2009 to 2010.

Abstract:The influence of the sea surface temperature in the Eastern Indian Ocean (EIOSST) in winter on the synchronous rainfall in the southern part of China was studied by using the NCEP reanalysis monthly data, sea surface temperature data, and the Chinese monthly precipitation data at 160 stations. It shows that EIOSST is closely connected with the precipitation in the southern part of China. As EIOSST in wintertime was warmer than usual, there would be more precipitation in the southern part of China. EIOSST affected the rainfall in the southern part of China by the perturbation in the area of the southern branch trough and moisture transportation. As the eastern Indian Ocean was warmer than usual, the local convection and ascending motion enhanced. Thus, the perturbation in the area of the southern branch trough intensified. As the intensified perturbation moved eastward, the water vapor transport from the Bay of Bengal to the southern part of China strengthened, leading to more precipitation in that area. Further analysis shows that the influence of EIOSST on the southern branch trough and the precipitation in the southern part of China was consistent with ENSO, but it was weaker and was independent of ENSO. If the effects of EIOSST and ENSO were consistent in one year, there would be a distinctly anomalous precipitation in the southern part of China.

Abstract:In order to address the possible change of the Hadley circulation in response to global warming in the future, the ability of coupled climate models in simulating the temporal and spatial features of Hadley circulation is assessed by a comparison with the observation during 1970-1999. On this basis, three models which can well reproduce the spatial structure and the temporal change of the intensity and expansion of Hadley circulation are selected as the ensemble to project its possible change under the A1B scenario. The projection results show that the Northern Hemispheric Hadley circulation tends to weaken in four seasons but with weaker change in spring during the late 21st century(2070-2099) as compared to the period 1970-1979. The Southern Hemispheric Hadley circulation will also weaken in winter and summer, while its change in spring and autumn is not significant. In addition, except a southward shift in summer, the Hadley circulation in the Northern Hemisphere will exhibit a northward expansion in the other three seasons during the late 21st century. The Hadley circulation in the Southern Hemisphere will move poleward in all seasons. Moreover, the upward shift of the Hadley circulation in the vertical may appear in both hemispheres in a warmer climate.

Abstract:This study aims to investigate the importance of local thermal factors contributing to urban heat island (UHI) for the cities of Changchun, Yanji, and Linjiang in Northeast China over the past 50 years. Changchun is a large city, the capital of Jilin Province, and Yanji is a middle one, while Linjiang which locates in a lap of hills, is a small town situate in the down of heavy industry centre of Northeast China. Statistics shows that the rank order of their urbanization rates (URT) is the same as the city sizes. The observation shows that their minimum temperature trends (MNTT) are much greater than their maximum counterparts (noted as MXTT), and the winter trends are greater than their summer counterparts. The rank order of the MNTT is contrast to the MXTT and approximately the same as the URT, while the difference of MNTT and MXTT is proportional to the URT. The land use, anthropogenic heating, and air pollution are three major factors for UHI, and the later two are proportional to the URT. Inter-comparison reveals that the air pollution and anthropogenic heating are key factors for the UHI, because their combination could explain the major characteristics of the trends and orders, while the land use doesn’t. The results also manifest the importance of local geographical environment in Linjiang, which lead to a weak wind climate that has the thermal factors localized. The localization may be one of the causes that produced the strongest MNTT and MXTT in Lingjiang in wintertime. Nevertheless, the difference of the MNTT and MXTT can reduce, to some extent, the influence of the geographic factors on the temperature trends because it is proportional to the URT. Hence, the difference can be regarded as a good climate index in representing urbanization. Besides, the results also imply that the important impact of air pollution (aerosols) on long-wave radiation should be considered not only in observation, but also in numerical model of atmosphere. A suggestion is to increase green land, and meanwhile to reduce air pollution and anthropogenic heating for mitigating city climate warming.

Abstract:Extreme events of weather and climate happened frequently over the world in 2011. The longest cold snap since 1945 impacted the Korean peninsula in January. At the same time, the strong winter storm with snowy weather impacted 100 million people across the United States. Storms and heavy monsoonal rains from July to October contributed to the worst flooding in Thailand since 1942. Crippling drought gripped East Africa and Cuba in South America. A freak sandstorm driven by extremely dry conditions and strong winds swept across the town of Rostock in the northern part of Germany on 8 April. Arctic sea-ice volume was estimated at a new lowest record in September. The number of high temperature days in China during the period of May to September in 2011 ranked the second since 1961 and the maximum temperature records were broken in many stations. The annual precipitation is the lowest in recent 60 years, which led to the extreme drought in many regions of China. Severe autumn flood happened in western China and Huanghe River-Huaihe River basins.

Abstract:An intercomparison by interpolation is made between two types of reanalysis data (NCEP/NCAR and ERA-Interim) and seven months (May 2008 to December 2008) measurements of high vertical resolution GPS radiosonde (RS92) at Shouxian station (32.56°N, 116.78°E), China. The correlation coefficient (R), bias (B_ias), and mean absolute deviation (M_ad)of temperature (T), zonal wind (U), meridional wind (V), and relative humidity (RH) are presented at the standard pressure levels of two reanalysis data sets from 1000 hPa to 20 hPa. The results indicate better correlation of RS92 observations with the ERA-Interim data than that with the NCEP/NCAR data. T, U, and V have higher correlation coefficients than RH at all levels. R of T is close to 1.0 from 1000 hPa to 250 hPa and decreases gradually above 250 hPa. The absolute values of bias for T is mostly less than 0.3 ℃ between ERA-Interim and RS92, but about two times between NCEP and RS92 at all levels. R of U is larger in middle and upper troposphere than in lower troposphere and lower stratosphere. R of V increases with height in the troposphere and decreases dramatically in lower stratosphere. R of RH decreases with height. It is shown that B_ias of RH ranges from 10% to 20% through 400 hPa to 100 hPa and becomes smaller than 10% in the upper layers, which is in part attributed to the limitation of measurement precision.