http://dspace.cityu.edu.hk:80/handle/2031/6071 Fri, 31 May 2013 06:11:59 GMT 2013-05-31T06:11:59Z http://dspace.cityu.edu.hk:80/handle/2031/6528 Title: Anomalous east Asian winter monsoon in relation to symbolic Eurasian blocking patterns Authors: Cheung, Ho Nam ( 張皓嵐) Abstract: This study attempts to examine the anomalous state of the East Asian Winter Monsoon (EAWM) in relation to spatial and temporal features of atmospheric blocking over Eurasia at the upstream of the climatological Siberian high region. Atmospheric blocking is identified by geopotential height gradients (zonal indices) over the extratropics. Surface air temperature is used to assess the impact of blocking on the EAWM region. Generally, there are two spatially independent blocking patterns of an omega shape with the ridge centered over the Ural-Siberia region and the European continent respectively. The impact is significant if the downstream cyclonic vortex of the blocking system forms a dynamic contact with the Siberian high, which is more confined to the former case. On the whole, the impact can be viewed from the perspective of a single event and a whole season. Establishment of a blocking high over Ural-Siberia involves interaction between the Siberian high and an upstream cyclone. Three temporal features of the blocking high are investigated: duration, intensity and extension. Firstly, the tropospheric warm-core structure is maintained by which the kinetic energy of the thermal ridge is converted to the potential energy of the blocking ridge. The warm ridge extending poleward induces cold advection to promote sustained development of the Siberian high. Secondly, the amplification of the blocking ridge is dependent on the amount of incoming anticyclonic vorticity advection. The advection can be determined by the zonal pressure gradient between the upstream cyclone and the Siberian high, which tightens when the cyclone is located right to the west of the Siberian high. However, this dynamic factor is not deterministic for the thermodynamic evolution of the Siberian high. Thirdly, the extension may be related to the size of pre-existing cold anomalies over western Siberia. Intense cold air masses tend to tighten the pressure gradient and to amplify the upper-tropospheric trough aloft the upstream cyclone. The thermodynamic feedback from Siberia perhaps supports the blocking high to stay for longer time. Therefore, a long-lasting cold period may take place as a consequence of a long-lasting blocking event. The blocking-EAWM relationship is close (weak) when outstanding blocking frequency is over Ural-Siberia (Europe). These upstream blocking activities may be regarded as a response to a combined signal of the Arctic Oscillation (AO) and El Niño/Southern Oscillation (ENSO). Weakened (strengthened) meridional flow in the positive (negative) phase of the AO is unfavorable (favorable) for blocking high formations. As the AO shows a close relationship with the North Atlantic Oscillation (NAO), the teleconnection between the AO and the Eurasian blocking activity may exist in the form of an eastward propagating wave-train signal generated over the North Atlantic Ocean. Be that as it may, the transmission of a signal across East Asia may be disturbed by the external effect of the ENSO, which probably suppresses (enhances) the sinking motion near Siberia in its positive (negative) phase. In short, the blocking-EAWM linkage is stronger (weaker) when the AO and ENSO are in phase (out of phase). If both the AO and ENSO attain their positive (negative) phase, the blocking frequency is distinctly low (high) over Ural-Siberia and uniform warming (cooling) would be observed in East Asia. Rather, if they are out of phase, the blocking signal would not be clear over Ural-Siberia and the monsoonal flow in northern (southern) East Asia would be stronger in negative AO (negative ENSO). Notes: CityU Call Number: QC939.M7 C45 2011; xx, 193 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 169-183) Sat, 01 Jan 2011 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6528 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6527 Title: Atmospheric nitrogen dioxide measurements in Hong Kong using long path differential optical absorption spectroscopy Authors: Chan, Ka Lok ( 陳嘉樂) Abstract: Long term measurements of atmospheric nitrogen dioxide (NO2) are performed using a Fibre Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument since December 2009 in Hong Kong. The instrument is using a high power blue LED with peak intensity at 450 nm as the light source to sample a path about 30 m to 50 m above the street level. Two setups with total absorption paths of 550 m and 3820 m are used in this study. The measurements are used to give better insights in surface air pollution conditions in Hong Kong. This thesis presents a detailed description of the instrument setup and the automated data retrieval. The LP-DOAS measurements were performed in Kowloon Tong and Mong Kok district in Hong Kong. The measurement results are compared to the concentration from a nearby monitoring station, operated by the Hong Kong Environmental Protection Department. Hourly averages of coinciding measurements are in reasonable agreement (R=0.73 on average). Additionally, NO2 coinciding measurements using the Ozone Monitoring Instrument (OMI) are compared to the LP-DOAS data. Monthly averaged LP-DOAS and OMI measurements correlate well (R=0.77). The comparison of mean weekly cycle however, displays different characteristics. The daily and weekly cycles of NO2 measured by the LP-DOAS are used to indentify different sources of NO2 in Hong Kong. Notes: CityU Call Number: TD885.5.N5 C45 2011; xii, 114 leaves : col. ill. 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 102-113) Sat, 01 Jan 2011 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6527 2011-01-01T00:00:00Z http://dspace.cityu.edu.hk:80/handle/2031/6153 Title: Rainfall distributions over land associated with tropical cyclones making landfall along the south China coast Authors: Fung, Kit Ying (馮潔瑩) Abstract: Rainfall distributions over land when tropical cyclones (TCs) made landfall along the South China (SC) coast during 1992 - 2007 are studied and changes in rainfall distributions as TCs are at 50 km offshore before, during and 50 km onshore after landfall are also examined, all based on raingauge data. For studying the rainfall distribution over land during landfall, TCs are divided into inner (In) zone with radial distances from TCs centers are less than 100 km and outer (Out) zone with radial distances from TC centers are between100 - 300 km. In examining the variation of rainfall distribution over land when TCs are at different perpendicular distances from the coastline, TCs are divided into left and right quadrant with radius of 300 km. For all situations, the enhancement of rainfall over land is in general found in the right quadrant of TCs. Vertical wind shear is one of the contributors towards the asymmetric rainfall distributions over land. The relationship between the individual vertical wind shear and the rainfall distribution is not well related. However, a composite of the vertical wind shears shows that it can explain the enhancement of rainfall in RF. Most of the TCs are found to experience northeasterly and southeasterly vertical wind shear during landfall and it is related to the environmental flow. In addition, the vertical wind shear shows no significant change in direction when TCs are at 6 hours before, during and 6 hours after landfall but a sharp change is found between the time of 12 hours to 6 hours before landfall. This variation of vertical wind shear with time may be related to the interaction between land and the TC. Land interacts with TC through topography. Convergence of vertically integrated horizontal water vapor flux is also a factor that can enhance the rainfall in RF because the convergence of water vapor dominates in RF, resulting in enhanced condensation and finally the stronger precipitation over there. Since topography is hypothesized to be the contributor to the asymmetric rainfall distributions over land, the Weather Research and Forecasting model is used to simulate its effect. Four groups of TCs are simulated. In each group, two TCs are simulated one with and another without topography. A larger extent of rainfall enhancement is found in RF for TCs with topography. Moreover, the variation of vertical wind shear in the simulations with topography is consistent with that in the observational studies. Finally, there is a positive relationship between terrain height and the extent of rainfall enhancement in RF. Notes: CityU Call Number: QC925.5.C62 H665 2010; xiii, 87 leaves : ill. (some col.) 30 cm.; Thesis (M.Phil.)--City University of Hong Kong, 2010.; Includes bibliographical references (leaves 83-87) Fri, 01 Jan 2010 00:00:00 GMT http://dspace.cityu.edu.hk:80/handle/2031/6153 2010-01-01T00:00:00Z