Sub-seasonal Variations Of Chemical Constitients In The UTLS And The Stratosphere-Troposphere Exchange During The Asian Summer Monsoon Season

Asian summer monsoon is an important component of global monsoon system. Furthermore, Asian summer monsoon region is a significant transport pathway for the boundary layer air entering the stratosphere. Jianghuai Meiyu is a precipitation process during Asian summer monsoon season. To investigate the variations of tropopause over Meiyu area and characteristics of stratosphere-troposphere exchange (STE) during Meiyu season, and how chemical tracers distribute and transport during Asian summer monsoon season are very important. The investigation can also give us theory to further understand interaction between troposphere and stratosphere, to find out global transport mechanism after pollution lifting into stratosphere. By using satellite data such as Microwave Limb Sounder (MLS), Infrared Atmospheric Sounding Interferometer (IASI) and Ozone Monitoring Instrument (OMI), Natinal Center for Enviroment Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, Global Forcasting System (GFS) data, ECMWF ERA reanalysis data, Hybrid Single-Particle Lagrangian Integrated Trajectory model (NOAA HYSPLIT) provided by National Oceanic and Atmospheric Adiministration, as well as Meiyu data from China national climate center, we studied abnormal signals in tropopause and stratosphere before Meiyu onset, and chrateristics of STE during Meiyu season. Then we compared capabilities of limb instrument and nadir mearsurement in analyzing chemical distribution in upper troposphere and lower stratosphere (UTLS) region during Asian summer monsoon season, the suitable datasets are used to study the sub-seasonal scale variability associate with Asian summer monsoon anticyclone. The main conclusions are as follow:1. Using NCEP/NCAR reanalysis and ERA-Interim reanalysis together with and Meiyu data from the National Climate Center of China during 1989-2008, tropopause anomalies and northern hemisphere annual mode (NAM) over Yangtze River and the Huaihe River valleys before Meiyu are analyzed. It is found that the tropopause height over Yangtze River and the Huaihe River valleys decreases 3-5 days before Meiyu onset, and the meridional change of the tropopause height is more significant than that in zonal direction. The decrease of the tropopause before Meiyu onset is proposed to cause by frequent tropopause fold events associated with frequent cold surges from the north, onset of eastern Asia monsoon as well as the northward movement of the westerly jet. The decrease of the tropopause height before Meiyu onset is accompanied by descent of cold air with high potential vorticity (PV) from the stratosphere. Positive PV anomalies can be noted over Yangtze River and the Huaihe River valleys before Meiyu onset, and the establishment and the maintenance of the those PV anomalies are associated with PV anomalies over Lake Baikal, West Siberia and near the Okhotsk. The stratospheric temperatures over the Meiyu area reach a peak value before Meiyu onset while the zonal winds in the stratosphere show a transition from west to east. After the onset of Meiyu, the stratospheric temperatures over the Meiyu area decrease. The total precipitation during the Meiyu season has a consistent positive correlation with NAM index in the upper troposphere and lower stratosphere, which is most significant 15-30 days before Meiyu onset. The results suggest that the NAM index near tropopause region is one of good factors for the forecast of the precipitation in Meiyu season.2. Characteristics of stratosphere-troposphere exchange (STE) during the Meiyu season in Yangtze-Huaihe Valley, China, are investigated using the ERA-Interim reanalysis data, NCEP/NCAR reanalysis data, Meiyu records from the National Climate Center of China, the data from a trajectory model and a general circulation model (GCM). Results show increases in potential vorticity and decreases in specific humidity in the upper troposphere and lower stratosphere (UTLS) before Meiyu onset, suggesting a strong downward transport of air masses around tropopause which can be attributed to frequent tropopause folds over the Meiyu area. The minimum tropopause height occurs 3 days before Meiyu onset and then rises till about 6 days afterwards. The downward cross tropopause mass transport (CTMF) is evidenced before Meiyu onset, which is mainly caused by the sharp meridional gradients in the tropopause pressure over the Meiyu area. After Meiyu onset, the upward cross tropopause transport intensifies due to enhanced convections. The analysis also suggests the strongest upward transport in the UTLS occurs at northeast of the Meiyu region, within the core of the upper tropospheric westerly jet. Results from a trajectory model indicate that the lower stratospheric air intrudes into the troposphere before Meiyu onset. The significant upward movements of the middle tropospheric air are notable after Meiyu onset. As convections are weak and the upper level westerly jet is located at far to the Meiyu area in poor Meiyu years, the upward CTMF over the Meiyu region is weaker during the Meiyu season, compared with that in rich Meiyu years.3. Limb instrument has high vertical resolution but poor horizontal resolution while nadir measurement is good at global coverage but limited vertical information. We compared MLS (limb instrument), OMI (nadir measurement) and IASI (nadir measurement) datasets, also with Fusion data which is combined with MLS and AIRS carbon monoxide (CO) data, to study the differences of them in describing chemical distribution in the UTLS region during Asian summer monsoon season. We compared IASI CO at 16-17 km (12-13 km) and OMI ozone (O3) at layer 17 to MLS data at 100 hPa (215hPa).Although CO distribution of IASI in the southern hemisphere is similar as MLS CO at 100 hPa, they are different in the northern hemisphere. However, IASI and MLS CO at 215 hPa have similar distributions. A significant feature is global distributions of IASI CO at 2 layers we analyzed are almost the same. The averaging kernels (AKs) profiles and degress of freedom for signal (DOFS) distribution show second peak of AKs is around 12 km and DOFS in Asian summer monsoon region is about 1.6, but it is 2 or larger in some areas. Global distribution of OMI O3 shows similar pattern with MLS O3, but the value is smaller than MLS O3. A relative low O3 center can be found within the Asian summer monsoon anticyclone with a little southly. In the area east to 40°E, between 0 and 30° S, OMI data shows a low O3 region, which is possiblely due to differences in the a priori climatology. As a MLS and AIRS combined data, Fusion CO has both high horizontal and vertical resolution in the UTLS region. In addition, the daily Fusion data is convenient when study daily variation of CO distribution and transport. Fusion CO is smaller than MLS CO at 100 hPa and the CO global variation is also smaller, but they show similar pattern. In summary, OMI O3 and Fusion CO are good in studying chemical distribution and transport in the UTLS region during the Asian summer monsoon season. Although IASI CO at 100 hPa is not sensitivity, but that at 200 hPa is much better. We can still use IASI CO data to investigate chemical tracers transport in upper troposphere.4. We use MLS CO, MLS O3, Fusion CO, OMIO3, and GFS data, analyzed daily variations of CO and O3 horizontal distribution at 100 hPa during Asian summer monsoon season, and how the anomalies of them vary within the Asian summer monsoon anticyclone during the period. Different datasets show that CO and O3 oscillate between Tibetan Mode (TM) and Iranian Mode (IM) with the anticyclone propagation. Both MLS and Fusion CO show relative high CO center oscillate from TM to IM several times during the monsoon season. In some cases, TM and IM of CO occur at the same time, which can last for a few days. They also both show the relative high CO region is always southwesterly than the anticyclone. The relative low O3 center also propagates between two modes in MLS data. In contrast, OMI O3 can only illustrate low O3 region keeps within the anticyclone with a little southerly but no low O3 center oscillation. It is very clear in the hovmoller diagram that the anticyclone propagates from TM to IM during the Asian summer monsoon anticyclone. But the two modes also happen at the same time, which also effects chemical distribution. The ozone anomaly from OMI is much better correlated with the geopotential height (GPH), indicating the better horizontal resolution. GPH anomaly, CO anomaly and O3 anomaly all oscillate between TM and IM every 8-12 days. CO and O3 vary with the anticyclone, which means the dynamical and chemical processes are consistent during the Asian summer monsoon season within the anticyclone.