Kai-Hon Lau

Observed structure and propagation characteristics of tropical summertime synoptic scale disturbances

Abstract The three-dimensional structure and propagation characteristics of tropical synoptic scale transients during the northern summer we studied with twice daily ECMWF global gridded analyses for the 1980–1987 period. Regions of enhanced variability in relative vorticity at 850 mb are identified in the western Pacific, eastern Pacific, Bay of Bengal/northern India and eastern Atlantic/western Africa sectors. Dominant spectral peaks with time scales ranging from 3 to 8 days are noted in the power spectra for these locations. The lag-correlation and regression statistics of tropical fluctuations with synoptic time scales are examined. Strong teleconnectivity and temporal coherence are found over all of the active sites with enhanced vorticity variance, as well as over the western Atlantic/Caribbean and the Indochinese Peninsula. These results indicate that a substantial amount of synoptic scale variability in the tropics is associated with propagating wavelike disturbances that remain coherent over seve...

The Energetics and Propagation Dynamics of Tropical Summertime Synoptic-Scale Disturbances

Abstract Periods of enhanced synoptic activity in the tropical western Pacific, Bay of Bengal-northeastern India, and African-Atlantic regions are identified by extended empirical orthogonal function analysis. Composite mete-orological fields for such active periods at various sites are constructed using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses for the northern summers of 1980–1987. These composite data form the basis for evaluating the contributions of different dynamical processes to local balances of beat, moisture, vorticity, enstrophy, and energy, so that the propagation dynamics and principal energy sources of the tropical disturbances may be studied in detail. In all three tropical regions considered here, the westward propagation of the synoptic-scale disturbances is attributed mostly to vorticity advection by both the time-mean flow and the transient fluctuations. In the western Pacific and Indian sectors, condensation heating associated with cumulus convection is seen ...

Visual Analysis of the Air Pollution Problem in Hong Kong

We present a comprehensive system for weather data visualization. Weather data are multivariate and contain vector fields formed by wind speed and direction. Several well-established visualization techniques such as parallel coordinates and polar systems are integrated into our system. We also develop various novel methods, including circular pixel bar charts embedded into polar systems, enhanced parallel coordinates with S-shape axis, and weighted complete graphs. Our system was used to analyze the air pollution problem in Hong Kong and some interesting patterns have been found.

A Numerical Study of a Mesoscale Convective System over the Taiwan Strait

On 7 June 1998, a mesoscale convective system (MCS), associated with a mesoscale cyclone, was initiated on the south side of a mei-yu front near Hong Kong and developed over the Taiwan Strait. In this study, numerical simulations for this event are performed using the fifth-generation Pennsylvania State University‐NCAR Mesoscale Model (MM5). The model captures the evolution of the MCS, including the shapes of clouds and the rainfall rate. In the mature phase of the simulated MCS, the MCS is composed of several meso-b- and meso-g-scale convective clusters possessing commalike shapes similar to that of a midlatitude occluded cyclone. The cluster at the head of the ‘‘comma’’ consists of convective clouds that are decaying, while the tail of the comma is made up of a leading active convective line. A mesoscale cyclone, associated with a mesolow, at the tailing region of the leading convective line is well developed below 500 hPa. At 850 hPa, a mesoscale low-level jet (mLLJ) is located on the south side of the mesolow, and is directed toward the comma-shaped convective clusters. At 300 hPa, a mesohigh develops over the leading cluster. A mesoscale upper-level jet (mULJ) is located on the east side of this mesohigh. Relative streamline and trajectory analyses show that the mLLJ, associated with low ue and sinking air motion, is a rear-inflow jet, while the mULJ is the outflow jet of the MCS. Monsoon air from the boundary layer in front of the MCS feeds deep convection within the MCS. Momentum budget calculations are performed in the regions of the mLLJ and mULJ, at the developing and mature stage of the MCS. The pressure gradient force and the horizontal advection are the main contributors to the development of mLLJ in the developing stage. Although the effects of the pressure gradient force are weakened considerably when the MCS reaches maturity, the horizontal advection continues to accelerate the mLLJ. Vertical advection tends to decelerate the mLLJ both in its developing and mature stages. The pressure gradient force and vertical advection are responsible for generating the mULJ in the early stages, and maintaining the mULJ in its mature stage. Strong convective upward motion, which carries the horizontal momentum upward, from the exit of the mLLJ to the entrance of the mULJ, is crucial in the vertical coupling of the mLLJ and mULJ.

Numerical Study of a Typhoon with a Large Eye: Model Simulation and Verification

Abstract Typhoon Winnie (1997) was the fourth supertyphoon in the western North Pacific in 1997. In its mature stage, an outer eyewall, consisting of deep convection with a diameter of 370 km, was observed by satellite and radar. Within this unusually large outer eyewall existed an inner eyewall, which consisted of a ring of shallow clouds with a diameter of ∼50 km. In this study, Typhoon Winnie is simulated using a nested-grid version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with an inner grid length of 9 km. The model reproduces an outer cloud eyewall with a diameter of ∼350 km. The simulated radar reflectivity and hourly precipitation are verified with satellite microwave, infrared, and cloud brightness temperature images. Analysis of the model results indicates that the large outer eyewall in many ways possesses the structure of a typical hurricane eyewall. This includes strong tangential winds and radial inflow out...

A new way of using MODIS data to study air pollution over Hong Kong and the Pearl River Delta

Aerosols are the main air pollutant in Asia. In this paper, the MODIS level 2 aerosol optical depth (AOD) products derived by NASA were validated with in situ sun-photometer observations over Hong Kong (HK). The MODIS AOD values were correlated with mass concentrations of respirable suspended particulates (RSP) measured at air quality monitoring stations over HK and Macau. Correlation between RSP and AOD were found to be statistically significant, suggesting that the satellite data is very useful for aerosol-related air pollution studies. Compared with concentrations measured from ground-based air quality monitoring networks, the AOD data cover a much larger area and have much better spatial resolution. Combining with meteorological information, the AOD data also proved to be very useful for the understanding of RSP variations at air quality monitoring stations. An example of using AOD data to help understand a pollution event over the PRD will be presented. Finally, monthly-mean distributions of AOD over Eastern China showed a distinct local maximum over the PRD, separated from high AOD areas to the north, suggesting that the aerosol problem over the PRD are mostly regional. Remote-sensing from space has provided a new and powerful way to study air pollution. To fully utilize this technique for air quality studies, the combination of a lidar and an X-band satellite receiver (for the MODIS data) is recommended. The AOD fields are vertically integrated products, together with the vertical profiles of extinction coefficients provided by a lidar, the surface distribution of aerosol could be derived.

Numerical simulation on mesoscale convective system along Mei-Yu front in Southern China

A mesoscale convective system (MCS) was generated over the South China Sea at around 115° E, 21°N on 0000 UTC 7 June 1998. Riding along the Mei-Yu front, the system moved through the Taiwan Strait and finally hit the southern part of the Taiwan Island, produced over 300 mm of rainfall over parts of Taiwan in the next 24 hours. It was found that the Penn State-NCAR Mesoscale Model Version 5 (MM5) did quite well in simulating the evolution of the MCS. Diagnostic studies on the mesoscale structure of MCS, moisture concentration process and the momentum budget were based on the model output. The concept model of the MCS in the mature phase can be concluded as the following: At the 850 hPa level, the mesoscale low level jet (mLLJ) was found to the southwest of the MCS, which was also associated with a mesoscale low generated by convection, the mesoscale upper level jet (mULJ) was found due east of the MCS (and an upper-level mesoscale high) at the 300 hPa level. Horizontal advection of momentum acted to accelerate the exit of the mLLJ but decelerate the entrance of the mLLJ where the pressure gradient force acted to compensate this lost and maintain its strength. For maintenance of the mULJ, vertical advection of momentum and the pressure gradient force were both found to be important.

Research on air pollution in Beijing and its surroundings with MODIS aerosol products

In this paper the MODIS Level2 aerosol products over Beijing are validated by comparison with the observations from sun-photometer at Peking University (PKU) at first. The MODIS aerosol optical depths (AOD) also correlates well with the averaged mass concentration of respirable suspended particulate (RSP, PM10) calculated from released API data. The relations between the visibilities and the AOD values with distinct aerosol scale heights in different seasons are estimated. The cases described by the AOD distributions over Beijing and its surroundings show some dramatic processes. The analysis assisted by the relevant changes of meteorological variables can help us find the corresponding answer to each air pollution episodes. The seasonal AOD variations in Beijing show the mean AOD value is highest in summer, and has a decreasing trend from summer to autumn, and then to winter. It has a rapid increasing from winter to spring due to spring dust in North China. Finally we gain the seasonal mean visibility distributions of Beijing and its surroundings. The mean air quality gets worst in winter due to increased pollutant source in winter and bad dispersion conditions, and becomes the best in spring because strong wind provides fast dispersion in spite of occasional dust weathers. The results indicate the air pollution in Beijing urban area is both contributed by local urban emission and regional transport. Remote sensing from space has provided us a new view to study the air pollution.