Yuanjian Yang

Effects of super typhoons on cyclonic ocean eddies in the western North Pacific: A satellite data‐based evaluation between 2000 and 2008

A composite time series of the merged satellite altimeters sea surface height anomaly (SSHA) data and satellite-observed sea surface temperature (SST) data were used to identify eddies in the Western North Pacific Ocean (WNPO), where there were numbers of intense typhoons. This study systematically investigated 15 super typhoons during the period of 2000-2008 in the WNPO to study their impacts on the pre-typhoon ocean features, e.g., the cyclonic ocean eddy (COE) feature (closed contours of SSHA < −6 cm) and neutral condition (SSHA between −6 and 6 cm). Two new COEs are generated by two super typhoons, and 18 pre-existing COEs are intensified by 13 super typhoons. 5 of the 13 super typhoons each influenced two pre-exisiting COEs. Although the typhoon-induced maximum cooling centers had a right bias along the tracks due to wind conditions, pre-existing COEs also play a significant role in determining the strength and location of large SST cooling. Three possible factors (maximum wind speed, typhoon translation speed and the typhoon forcing time, Tf) are employed to explain the interactions. Above all, the changes of the COE geometric and physical parameters (e.g., effective radius, area, SST, SSHA, and eddy kinetic energy) were mostly related to the typhoon forcing time, Tf. This is because Tf is a parameter that is a combination of the typhoons translation speed, intensity and size. Although the typhoons may significantly impact COEs, such samples were not commonly observed. Thus, the impact of typhoon on the strength of COEs is generally inefficient.

The biophysical responses of the upper ocean to the typhoons Namtheun and Malou in 2004

The responses of the upper ocean to typhoons were investigated by the observations of sea surface wind (SSW), sea surface temperature (SST), sea surface height anomaly (SSHA), chlorophyll-a (Chl-a) and Argo floats. Typhoon Namtheun had notable impacts on the upper ocean along its track from July to August 2004. The local processes (entrainment and upwelling) dominated the upper ocean responses in the regions of the pre-existing cold eddy and beneath the typhoon track, where the observed locations of upwelling, SSHA changes, SST cooling, and Chl-a enhancement were consistent with each other. Besides, there were cold tongues extending from the cold centres. The trajectories of Argo floats, along with the cold tongues, indicated that the surface advections induced such non-local responses. On the other hand, the following weak typhoon Malou had few impacts on the upper ocean. Finally, the mechanisms of the Chl-a concentration enhancement were sketched as the effects of both the local upwelling and the non-local advection. This study implies that some non-local processes, e.g. horizontal advections, may play a notable role in the upper ocean responses to the typhoons.The responses of the upper ocean to typhoon Namtheun in July 2004 are investigated by sea surface measurements and vertical profiles. Pre-typhoon ocean environment played an important role in this case. There were two extreme cooling regions located at cyclonic eddy A and typhoon wake B. Although the magnitudes of SST cooling at A and B were similar, other physical and biophysical responses were quite different. Combining multi-satellite data with vertical profile data, it is found that the upwelling dominated the responses at A and the vertical mixing dominated the responses at B. This study implies that to insight into the ocean surface responses to typhoon, the subsurface dynamics need to be analyzed via both the in situ and satellite-based observations, and the physical and biological models.

The most typical shape of oceanic mesoscale eddies from global satellite sea level observations

In this research, we normalized the characteristics of ocean eddies by using satellite observation of the Sea Level Anomaly (SLA) data to determine the most typical shape of ocean eddies. This normalization is based on modified analytic functions with nonlinear optimal fitting. The most typical eddy is the Taylor vortex (∼50%), which exhibits a Gaussian-shaped exp(−r2) SLA and a vorticity distribution of (1 − r2)exp(−r2) as a function of the normalized radii r. The larger the amplitude of the eddy, the more likely the eddy is to be Gaussian-shaped. Furthermore, approximately 40% of ocean eddies are combinations of two Gaussian eddies with different parameters, but the composition of these types of eddies is more like a quadratic eddy than a Gaussian one. Only a small portion of oceanic eddies are pure quadratic eddies (<10%) with the same vorticity distribution as a Rankine vortex. We concluded that the Taylor vortex is a good approximation of the typical shape of ocean eddies.

Monsoon and eddy forcing of chlorophyll-a variation in the northeast South China Sea

The physical and biological environments of the northeast South China Sea (SCS) were investigated using 11 year satellite and reanalysis data, including ocean chlorophyll-a (Chl-a) concentrations, sea surface wind (SSW) values, sea surface temperatures (SSTs), sea surface height anomalies (SSHAs), etc. The findings reveal that ocean Chl-a concentrations west of the Luzon Strait have the most significant annual cycles in the SCS. The dominant forcing mechanisms are monsoon winds and mesoscale eddies studied by multiple regression analysis. In the offshore regions, strong winds directly caused the enhancements by local vertical mixing and entrainment. As in the near-shore regions, the alongshore winds indirectly caused the enhancements by inducing coastal upwelling. Although SST is highly correlated with Chl-a concentration, SST cooling is difficult to observe in the bloom region. It is considered a consequence of monsoon and eddy forcings. Other mechanisms, such as local Ekman pumping by the wind stress curl and the geostrophic potential vorticity, have little effect on Chl-a seasonal variations.

Recent Trends of Summer Convective and Stratiform Precipitation in Mid-Eastern China.

Many studies have reported on the trends of precipitation in Mid-Eastern China (EC). However, the trends of convective and stratiform precipitation are still unknown. Here, we examine the trends of summer convective and stratiform precipitation in EC from 2002 to 2012 on the basis of the TRMM observations. Results revealed that the rain frequency (RF) for both convective and stratiform precipitation increased in majority regions of Southern EC (SEC), but decreased in Northwest part of Northern EC (NEC). The decreasing rate of RF for stratiform precipitation in NEC is twice as much as that for convective precipitation, while the increase of convective precipitation in SEC is more evident than stratiform precipitation. The rain rate (RR) exhibited a decreasing trend in most portions of EC for both convective and stratiform precipitation. In SEC, neither PW nor WVT has good ability in explaining the precipitation variability. However, in NEC, PW is closely correlated to convective RF and WVT is more closely related to stratiform RF.

Climatological characteristics of summer precipitation over East Asia measured by TRMM PR: A review

Precipitation is an important indicator of climate change and a critical process in the hydrological cycle, on both the global and regional scales. Methods of precipitation observation and associated analyses are of strategic importance in global climate change research. As the first space-based radar, the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has been in operation for almost 17 years and has acquired a huge amount of cloud and precipitation data that provide a distinctive view to help expose the nature of cloud and precipitation in the tropics and subtropics. In this paper we review recent advances in summer East Asian precipitation climatology studies based on long-term TRMM PR measurements in the following three aspects: (1) the three-dimensional structure of precipitation, (2) the diurnal variation of precipitation, and (3) the recent precipitation trend. Additionally, some important prospects regarding satellite remote sensing of precipitation and its application in the near future are discussed.

Impacts of the binary typhoons on upper ocean environments in November 2007

Abstract. Using multiple satellite observations, Argo floats profiles, and one-dimensional (1-D) ocean mixed layer model, this study systematically investigated the impacts of the binary typhoons Hagibis and Mitag [which coexisted respectively in the South China Sea (SCS) and western North Pacific (WNP) during November, 22 to 26, 2007] on upper ocean environments. It was observed that intense Ekman pumping and two mesoscale cold, cyclonic eddies, which, induced by long forcing time of strong wind stress curls, appeared respectively in two certain areas instead of after the binary typhoons’ trails. Both cyclonic eddies retained for ∼ 39 days, accompanied with maximum sea surface height anomaly (SSHA) reduction of ∼ 25     cm induced by Hagibis and of ∼ 44     cm induced by Mitag, respectively. The largest sea surface temperature (SST) drop of 7°C and 2°C, the maximum chlorophyll a (Chl- a ) enhancement respectively was > 20 times and ∼ 3 times in these two eddies’ regions induced by Typhoon Hagibis and Mitag, respectively. The results of the 1-D ocean mixed layer model showed that, given its 84 h forcing time, the simulated MLT cooling and mixed layer deepening induced by Hagibis were ∼ - 2.8 ° C and 45 m, respectively, ∼ - 0.5 ° C and 25 m for Mitag at its 66 h forcing time. This work provides convincing evidences that typhoons, which appear frequently in the SCS and the WNP, play a notable role in the activities of mesoscale eddies in these areas.

Grid-cell aerosol direct shortwave radiative forcing calculated using the SBDART model with MODIS and AERONET observations: An application in winter and summer in eastern China

Taking winter and summer in eastern China as an example application, a grid-cell method of aerosol direct radiative forcing (ADRF) calculation is examined using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model with inputs from MODIS and AERONET observations and reanalysis data. Results show that there are significant seasonal and regional differences in climatological mean aerosol optical parameters and ADRF. Higher aerosol optical depth (AOD) occurs in summer and two prominent high aerosol loading centers are observed. Higher single scattering albedo (SSA) in summer is likely associated with the weak absorbing secondary aerosols. SSA is higher in North China during summer but higher in South China during winter. Aerosols induce negative forcing at the top of the atmosphere (TOA) and surface during both winter and summer, which may be responsible for the decrease in temperature and the increase in relative humidity. Values of ADRF at the surface are four times stronger than those at the TOA. Both AOD and ADRF present strong interannual variations; however, their amplitudes are larger in summer. Moreover, patterns and trends of ADRF do not always correspond well to those of AOD. Differences in the spatial distributions of ADRF between strong and weak monsoon years are captured effectively. Generally, the present results justify that to calculate grid-cell ADRF at a large scale using the SBDART model with observational aerosol optical properties and reanalysis data is an effective approach.摘要本文利用SBDART模式, 结合MODIS和AERONET提供的气溶胶数据以及再分析数据, 并以中国东部冬、夏季节为例, 提出了一种模拟计算格点尺度上气溶胶直接辐射强迫的方法.结果表明, 气溶胶光学参数和直接辐射强迫存在明显的季节变化和区域差异.对于光学厚度而言, 夏季的值要明显高于冬季, 且夏季时中国东部存在两个明显的高值区. 对于单次散射反照率来说, 夏季的值也高于冬季, 这主要与夏季时弱吸收性的二次气溶胶增多有关, 且从空间分布上来看, 夏季时中国北方的单次散射反照率值高于南方, 冬季时正相反. 对于中国东部冬、夏季而言, 气溶胶在大气顶和地面层都会造成负的辐射强迫, 且在地面的辐射强迫为大气顶辐射强迫的四倍. 气溶胶的辐射强迫对地面层有明显的降温增湿效果. 除季节变化外, 气溶胶光学厚度和辐射强迫也都表现出明显的年际变化, 且夏季时年际变化比冬季时剧烈. 但是受到单次散射反照率影响, 辐射强迫的年际变化情况并不总是与光学厚度的完全对应. 本文还研究了气溶胶辐射强迫与季风活动的关系, 结果表明气溶胶辐射强迫的空间分布在季风强弱年时存在明显差异. 总体而言, 本文的研究结果表明在较大尺度的区域内, 利用SBDART模式并结合气溶胶观测资料和再分析资料进行格点尺度上气溶胶直接辐射强迫模拟计算的方法是有效的.

Satellite-Based Investigation and Evaluation of the Observational Environment of Meteorological Stations in Anhui Province, China

In this paper, by using multi-temporal and high resolution Landsat data and geographic information system techniques, the land use/land cover (LULC) in the 2-km buffer zone of 52 meteorological stations in the Anhui province of China is retrieved and categorized into three types: vegetation (including farmland, forest and grass land), water (including lakes, rivers and pools), and construction (including buildings and roads). Besides, the land surface temperature (LST) in the buffer zone of these stations is also obtained from thermal infrared data. The normalized LST index (NLI) and the heat effect contribution index (HECI) of different LULC types are calculated. Via case studies and statistical analysis, the LULC and thermal environment’s temporal-spatial variance in the 2-km buffer zone of these stations are surveyed, and their impacts on the observational environment are investigated. The study shows that the observational environments of the meteorological stations in Anhui province have been greatly influenced by rapid urbanization. The study proposes two new methods to classify the stations’ observational environment into three types (urban, sub-urban, and rural). One uses the NLI and the other uses the HECI. The NLI method needs only LST information. The HECI method combines both LULC and LST information and, hence, is considered more reliable. The evaluation methods and criteria can be used conveniently, effectively, and quantitatively, and are especially useful when analyzing observational data from meteorological stations in weather and climate research and when choosing a location for a new meteorological station.

The responses of cyclonic and anticyclonic eddies to typhoon forcing: The vertical temperature-salinity structure changes associated with the horizontal convergence/divergence

The responses of the cyclonic eddies (CEs) and anti-cyclonic eddies (AEs) to typhoon forcing in the Western North Pacific Ocean (WNPO) are analyzed using Argo profiles. Both CEs and AEs have the primary cooling at the surface (0-10 m depth), and deep upwelling from the top of thermocline (200 m depth) down to deeper ocean shortly after typhoon forcing. Due to the deep upwelling, part of warm and fresh water at the top of AEs move out of the eddy, which leads to a colder and saltier subsurface in the AEs after the passage of the typhoon. In contrast, the inflow of warm and fresh water heats and freshens the subsurface in the CEs to compensate the cooling induced by the typhoon. This explains why the observed strong SST cooling were much less than modeled, since the AEs are more frequent than CEs in the WNPO. It indicates that there is divergence (convergence) of warm and fresh water in the surface of AEs (subsurface of CEs). The divergence-convergence effects of the AEs and CEs lead to the secondary cooling center locate at a shallow layer of 100 m in the AEs and a much deeper layer of 350 m in the CEs. This shallow divergence-convergence flow could lead a shallow overturning flow in upper oceans, which may potentially influences the large-scale ocean circulations and climates.