Very heavy rainfall over Punjab, Himachal Pradesh and Haryana during 24-27 September 1988- case study

Heavy to exceptionally very heavy rainfall over Haryana Punjab and Himachal Pradesh during 24-27 September 1988 was associated with the low level easterly trough which was overlain by upper level diffluent westerly trough. The case study of this phenomenon is reported here.

The Cold Avoidance of Typhoons in Their North Turning Over the South China Sea

Based on the typhoon best tracks of the China Meteorological Administration (CMA), ERA5 reanalysis data of ECMWF at 0.25 degrees horizontal resolution, and NOAA optimal interpolated sea surface temperature (OISST V2) data, the dynamical compositing analysis is used to study the north turning at nearly 90 degrees of 4 westward typhoons over the South China Sea (SCS). The composite analysis results show that: (1) As the typhoon goes westward into the SCS, the upper-level westerly trough moves eastward to the vicinity of 110°E in the mainland of China, and the western North Pacific subtropical high (SH) retreats eastward at the same time, which weakens the steering flow of typhoon and slowes down its movement. (2) The cold air guided by the westerly trough invades southwardly into the western part of SCS from the mainland leading to a descending and divergent airflow in the lower-to-middle atmospheric layers and enhancing the eastward pressure gradient force (PGF) in the west quadrant of the typhoon, which blocks and repesl the typhoon from moving any further westward. (3) Due to the cold air intrusion, the vertical atmospheric stratification in the west quadrant of the typhoon becomes static and stable, which may suppress the convection, impeding a typhoon’s westward motion. (4) With the cold air involving to the south of the typhoon, the direction of the PGF on the typhoon switches from eastward to northward, and the SH falling southward enhances the southwesterly airflow on the south of the typhoon at the same time. The remarkable increase of the northward steering airflows of the typhoon results in an abrupt northward turn. (5) In addition, the sea surface temperature (SST) and the ocean heat content (OHC) on the western part of the SCS is also reduced, attributed to the cold air cooling, and the typhoon is likely to avoid the cold ocean and approach a relatively warmer region. This study suggests that cold avoidance during the westward movement of typhoons is worthy of consideration in the operational forecast of typhoon tracks.

The relationship between Pacific SSTA and Autumn Extreme Precipitation Events of China

Based on 610 Meteorological stations daily precipitation datasets in China and NCEP/NCAR reanalysis datasets and NOAA reconstruct sea surface temperature from 1960~2009, the spatial and temporal coupling relationships between the Pacific SSTA and extreme precipitation events (EPE) are analyzed by using Singular Value of Decomposition method. Moreover, possible effect mechanism of Pacific SSTA for EPE is also discussed, the results are as follows: They are remarkable relations between equator middle-east Pacific ocean (EMEPO) SSTA and EPE. In anomaly cold years of EMEPO, compel 500hPa height field to show reverse PNA teleconnection pattern, intensity of west Pacific ocean subtropical high boost down. Meanwhile North Africa subtropical high become weeker compared with the mean. The trough over East Asia is abnormally weaker, and zonal circulation prevails over Asia, which is useful for the northward motion of southwestern water vapor. Meanwhile, the 500hPa geopotential height become weeker in New Siberian, leading to development and eastward motion of westerly trough. It is useful for the southward motion of stronger cold air, which is beneficial to the interaction between cold and warm air in Huang-Huai Basin, benefit the occurrence of EPE. However, wester china influenced by western vapor transportation, and vapor divergence in these areas, not benefit the occurrence of EPE. The opposite is true with positive SSTA in EMEPO.

Influence of air mass downward transport on the variability of surface ozone at Xianggelila Regional Atmosphere Background Station, southwest China

In situ measurements of ozone (O3), carbon monoxide (CO) and meteorological parameters were made from December 2007 to November 2009 at the Xianggelila Regional Atmosphere Background Station (28.006° N, 99.726° E; 3580 m a.s.l.), southwest China. It was found that both O3 and CO peaked in spring while the minima of O3 and CO occurred in summer and winter, respectively. A normalized indicator (marked as "Y") on the basis of the monthly normalized O3, CO and water vapor, is proposed to evaluate the occurrence of O3 downward transport from the upper, O3-rich atmosphere. This composite indicator has the advantage of being less influenced by the seasonal or occasional variations of individual factors. It is shown that the most frequent and effective transport occurred in winter (accounting for 39% of the cases on the basis of a threshold of the Y value larger than 4) and they can make a significant contribution to surface O3 at Xianggelila. A 9.6 ppb increase (21.0%) of surface ozone is estimated based on the impact of deep downward transport events in winter. A case of strong O3 downward transport event under the synoptic condition of a deep westerly trough is studied by the combination of the Y indicator, potential vorticity, total column ozone and trajectory analysis. Asian monsoon plays an important role in suppressing O3 accumulation in summer and fall. The seasonal variation of O3 downward transport, as suggested by the Y indicator at Xianggelila, is consistent with the seasonality of stratosphere-to-troposphere transport and the subtropical jet stream over the Tibetan Plateau.

Influence of airmass transport events on the variability of surface ozone at Xianggelila Regional Atmosphere Background Station, Southwest China

In situ measurements of ozone (O3), carbon monoxide (CO) and meteorological parameters were made from December 2007 to November 2009 at the Xianggelila Regional Atmosphere Background Station (28.006° N, 99.726° E, 3580 m a.s.l.), Southwest China. It is found that both O3 and CO peaked in spring while the valleys of O3 and CO occurred in summer and winter, respectively. A normalized indicator (marked as "Y") of transport events on the basis of the monthly normalized O3, CO, and water vapor, is proposed to evaluate the occurrence of O3 transport events from the upper, O3-rich atmosphere. This composite indicator has the advantage of being less influenced by and seasonal or occasional variations of individual factors. It is shown that the most frequent transport events occurred in winter and they can make a significant contribution to surface O3 at Xianggelila. A case of strong O3 transport event under the synoptic condition of a~deep westerly trough is studied by the combination of the Y indicator, potential vorticity, total column ozone, and trajectory analysis. A 9.6 ppb increase (21.0%) of surface ozone is estimated based on the impacts of deep transport events in winter. Asian Monsoon plays an important role in suppressing O3 accumulation in summer and fall.

Clustering of Tibetan Plateau Vortices by 10–30-Day Intraseasonal Oscillation*

Tibetan Plateau (TP) vortices and the related 10–30-day intraseasonal oscillation in May–September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satellite-retrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30–60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10–30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10–30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30–60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10–30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10–30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10–30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indian monsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10–30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.