Climate and Tropical Cyclone

Prof. Johnny CHAN
Tel: 3442-7820

Email address
Large Errors in Tropical Cyclone Track Forecasts: Causes and Possible Reduction

Tropical cyclones, commonly known as typhoons in Asia, affect many coastal regions in the world. Better forecasting of their movement (track forecast) is therefore crucial in reducing the fatalities and damage to properties when the cyclones make landfall. During the past few decades, improvement of technologies have led to a significant decrease in the track forecast errors. However, there still exist many cases in which the errors are extremely large. Clearly, the number of such large-error cases should be reduced. To investigate the possible reasons for these large-error cases, so as to improve the accuracy of track prediction, therefore forms the main objective of this proposed study. There are at least two types of situations in which large track forecast errors have been found. The project will examine the cases in which the forecast errors are extremely large and identify the possible reasons for such large errors through analyses of the predicted fields and the characteristics of the cyclones. Results of these analyses will provide a basis for hypothesizing the reasons for the forecast failures. These hypotheses will be tested by running sensitivity experiments with a prediction model.

The findings in this study will contribute significantly to prediction model developers who can modify these models to improve the track forecasts. In addition, more optimal observation systems can be proposed through analyzing the deficiency in the initial conditions of these models to produce more accurate track forecasts. From an operational forecasting point of view, forecasters can take advantage of the results of this study to determine the conditions under which they have to be cautious in interpreting the ensemble tracks.

Tropical Cyclone Size: Climatology, Physics and Prediction

Tropical cyclones (TCs), generally known as typhoons, affect many parts of Asia every year, causing loss of lives and property. In issuing warnings to the public when a TC is approaching, a weather agency will need to predict not only the strongest winds associated with the TC but also the extent of the damaging winds, such as gale-force winds. However, the predictions of the latter, commonly known as TC size, are currently done empirically because of our limited understanding of the physical processes responsible for causing changes in the size. A recently completed project by the PI, through the support of the RGC, has identified one possible process and generated the climatology of TC size for two ocean basins. However, the results and the conclusions are based on a dataset that has only 11 years of data, although it is already the largest dataset available. To ascertain these results, identify other possible physical processes, establish more robust size climatologies for all ocean basins, search for possible interannual or even interdecadal variations of size, it is necessary to use a dataset that spans a much longer period of time. This proposal is therefore to address these objectives by using such a dataset. The second part of this project is to investigate the possibility of using an operational numerical-weather-prediction model for real-time prediction of TC size, and verify the predictions against this dataset. Such predictions have never been attempted before. Specifically, the project has the following objectives:

1) to validate the use of the new size estimates against existing observational datasets
2) to establish the spatial climatology of TC size in all ocean basins
3) to examine the seasonal, interannual and interdecadal variations as well as possible trends in TC size for all ocean basins
4) to verify the dynamic mechanisms proposed in previous studies and identify other mechanisms (especially thermodynamic) related to TC size change
5) to explore the possibility of real-time prediction of TC size and to evaluate the accuracy of such predictions, and
6) to make detailed diagnostics of model outputs to further identify possible causes of TC size change
Results of this study will therefore be of tremendous benefit to the operational community in terms of real-time forecasts of TC size, and to the scientific community in terms of an improved understanding of the physical processes responsible for TC size changes.

An Observational Study of the Rainfall Distributions Associated with Tropical Cyclone Landfall

During the period 2008-2013, a total of 47 tropical cyclones (TCs) made landfall in China. Data related to all these TCs have been archived by the Shanghai Typhoon Institute of the China Meteorological Administration. In addition, radar data from two radars and meteorological data of over 10 TCs have also been archived by the Hong Kong Observatory. A combination of all these data represents probably the largest dataset ever assembled for the study of meteorological conditions related to landfalling TCs in Asia. This project is to identify temporal changes in the rainfall distributions associated with TC landfall under different environmental conditions and different TC characteristics of a large number of cases through analyses of observations from a variety of platforms. This observational analyses will provide a basis for future studies of the physical mechanisms responsible for the convection and rainfall distributions associated with TC landfall under various environmental conditions.

Dr. Wen ZHOU
Tel: 3442-7816

Email address
Biases of North Atlantic Storm Track and Ural Blocking Influencing on East Asian Winter Monsoon Projection

Projection of East Asian winter climate under a global warming scenario is very challenging because unusual and sudden cold weather over Eurasia has become very frequent recently, even though people expect warmer winters to accompany global warming. The frequent unexpected cold winters of recent decades actually highlight the need to improve our understanding and prediction abilities. Anomalous East Asian winter monsoons bring extreme winter weather to South China, including Hong Kong. For example, the Hong Kong Observatory recorded 24 consecutive cold days from 24 January to 16 February 2008, which was the longest cold spell since 1968. In boreal winter, the eastward extension of the North Atlantic storm track and the frequency of atmospheric blocking centered over the Ural Mountains and western Siberia are interrelated. This interrelationship is also closely tied to the occurrence of extreme cold spells in East Asia and the intensity of the East Asian winter monsoon. However, most general circulation models (GCMs) do not simulate the spatio-temporal variability of the storm track and blocking very well. It is important to understand the biases in the North Atlantic storm track and Ural blocking in order to evaluate the uncertainty in projecting the East Asian winter monsoon. In this study, we propose novel analyses of the relationship between the North Atlantic storm track and Ural blocking using reanalysis datasets and the GCMs participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). First, we will diagnose the relationship between Ural blocking and the North Atlantic storm track using observations and CMIP5 GCMs. The major energetic and dynamic factors contributing to the biases in this relationship will be highlighted. Second, we will select the GCMs that can best simulate the blockingˇV storm track relationship, and then project the future changes in RCP4.5 and 8.5 runs. We will explore projection uncertainties in each run. Finally, we will carry out numerical experiments with different SST conditions over the North Atlantic to verify our hypothesis.

Through this study, we will better understand the inter-model spread of storm track and Ural blocking simulations and evaluate their impacts on future changes in the East Asian winter climate. These results should also be important for risk assessment of extreme winter conditions in Hong Kong under a changing climate.

Teleconnections and Future Changes in the East Asian Winter Monsoon under Arctic Amplification

After the severe and prolonged snowstorms in South China in January 2008, several extreme cold spells hit different parts of the Northern Hemisphere in the six winters between 2008/09 and 2013/14. Because the intensity and duration of these cold spells were exceptional in the past few decades, more research should be devoted to understanding the underlying physical mechanisms of cold extremes under a changing climate in order to better predict their occurrence in the future. In East Asia, the occurrence and persistence of extreme cold spells are closely related to an anomalous East Asian winter monsoon. These cold spells are preceded by blocking events near the Ural Mountains and the western Pacific. The occurrence of blocking events is linked with low-frequency atmospheric oscillations, such as the North Atlantic Oscillation and the El Nino/Southern Oscillation. Estimations of future Ural and western Pacific blocking frequency and characteristics are crucial for projecting change in the East Asian winter monsoon.

Since 1990, a significant winter cooling trend has occurred in the midlatitudes, including western Siberia, where cold air activity in East Asia originates. Simultaneously, a pronounced warming trend has been observed over the polar region accompanying the reduction of Arctic sea ice, which is known as the Arctic amplification. Arctic amplification has thus occurred alongside cold midlatitude winters in recent years, in contrast to cold winters in earlier periods. Under a global warming trend, Arctic amplification may exert an impact on the large-scale teleconnections of the East Asian winter monsoon with blocking in the midlatitudes, but the underlying mechanisms are not yet fully understood.

One area of particular concern is the effect that climate change and the substantial reduction of Arctic sea ice might have on the winter monsoon circulation over East Asia. How the East Asian winter monsoon will respond to a warmer planet is not totally clear. One possibility is that anomalous blocking events (frequency, location, intensity, duration) due to Arctic amplification and sea ice loss may enhance extreme cold spells, but the mechanisms potentially involved in such changes are still under discussion. For example, the Hong Kong Observatory recorded 24 consecutive cold days from 24 January to 16 February 2008, which is the longest cold spell since 1968. Due to the huge economic loss this prolonged cold spell caused in South China, it is important to better prepare for the impact of cold extremes under the global warming trend.