The rain season in Hong Kong this year has begun. Showers and thunderstorms occurred every now and then, sometimes affecting only isolated area and sometimes more intense and widespread. This to some extent affected the daily life of the public. Very often, forecasting heavy rain is a major challenge faced by the Observatory forecasters.
Nowadays, when forecasting the weather for the next few days, the meteorological community mainly relies on numerical weather prediction models run on computers, which simulate the evolution of atmosphere based on physical laws. Despite the advancement in science and technology, computer model predictions are still not perfectly accurate. It is especially challenging to predict weather phenomena such as thundery showers that change rapidly and affect relatively small areas. Generally speaking, the shorter the forecast lead time, the more accurate the computer model predictions will be. Therefore, for the weather forecast for a particular day, forecasters have to continuously assess the latest results of computer models as the subject day gets closer and closer, and consider whether or not to adjust the forecast.
We are usually able to predict environmental factors that favour the occurrence of heavy rain a few days in advance, such as whether the atmosphere will be unstable, and whether a trough of low pressure or an upper-air disturbance will come close to Hong Kong. Yet there is still a degree of randomness as to which hours of the day and at where the heavy rain will eventually occur, or whether the heavy rain will just narrowly miss Hong Kong. As such, for the forecast for the current day, forecasters still need to make reference to a vast amount of actual observation data, including those from automatic weather stations, weather radar and satellite to complement the forecast from the computer models; and if deemed necessary, immediately issue updates to provide a more accurate and detailed forecast to the public. This happens more often during spring and summer. Besides, another reason to update the forecast during the course of the day is that as the day progresses, the period covered by the forecast will shorten from 24 hours at the beginning to a few hours. The weather situation in the weather forecasts will naturally be different and therefore requires updates appropriately with time.
Heavy rain in spring and summer evolves rapidly. The rainfall distribution is also highly uneven. Coupled with the small geographical size of Hong Kong, any small deviations in the location and movement of rain areas can lead to drastically different local weather. As shown in Figure 1, there were two episodes of heavy rain in April 2016, which fell on our neighboring Shenzhen and the waters south of Hong Kong respectively, yet no heavy rain or just localized rain in Hong Kong. These speak for themselves on the difficulty in forecasting heavy rain.
Figure 1 Radar imageries showing heavy rain over Shenzhen and waters south of Hong Kong (areas in yellow and red) but hardly any over Hong Kong.
My advice to all of you before going out in the rain season is: not to judge the weather condition elsewhere simply based on how heavy the rain is at where you are. Instead, please keep track of the weather forecasts of the Observatory, check the location and development of rain areas using radar imageries, and pay attention to the latest 'Special Weather Tips' issued by the Observatory. In doing so, you can grasp the latest situation and get better prepared.
Finally, I would also like to take this opportunity to remind everyone that thunderstorms could bring not only heavy rain but also severe gust, squall line, hail and even tornado. Given the rain and typhoon season has begun, those who work in sectors including marine, logistics, transport and construction should take precautionary measures to avoid objects being blown down by strong winds in heavy rain. Members of the public should also pay attention to the relevant weather warnings and 'Special Weather Tips' issued by the Observatory to ensure their own safety.
 Thunderstorms and squalls
 Severe Weather in Hong Kong
 How is a tornado rated?
We may still recall, after the speculation in the social media on the intense cold surge affecting Hong Kong in late January 2016, rumours of another intense cold surge in March were doing the rounds in mid-February, attracting much public and media interest. Based on the information captured on the social media websites, the intense cold surge "forecast" appeared to hinge on two things: (1) most of the ensemble members of the forecasts by US National Centers for Environmental Prediction (NCEP) predicting the Arctic Oscillation Index (AOI) to become negative again (around -2 to -3), with one of the members forecasting AOI to fall below -4 on 2 March; and (2) AOI also falling below -4 about a week before the intense cold surge reaching southern China in late January 2016. Yet, it turned out that the AOI on 2 March was only about -0.6 and the lowest value in early March was around -2 (Figure 1). As for the cold surge predicted, temperatures did drop to 10oC at the Observatory on 10 and 11 March, but nothing too intense as the figure only ranked about 90th among the daily minimum temperatures for March.
Figure 1 Daily AOI (source: NOAA)
The lesson learnt was that it was never too wise to attach undue importance to the "face value" of long range forecasts produced by computer models and to jump to conclusions about the future weather conditions at a specific location such as Hong Kong. NCEP correctly forecast the trend of AOI changing from positive in mid-February to negative in early March. However, most of the ensemble forecast members provided a quantitative forecast that deviated rather significantly from the actual value of AOI in early March. Just like our regular reminder for users of the Observatory's 9-day forecast, uncertainty and forecast errors normally increased as we made prediction further into the future and as such, the forecasts in terms of such long-term trends should be interpreted objectively and used judiciously for maximum benefits.
Online discussion has also revealed some misunderstanding about the linkage between negative AOI and intense cold surge affecting Hong Kong (or other places in southern China). While negative AOI is a good precursor for the likely southward incursion of cold air from the north, to what extent any particular region would be affected depends on the actual trajectory of the cold air and other location-specific contributing factors involved. Typically, it takes around two to three days for the cold air to reach the south China coast, and the minimum temperature associated with a cold surge is normally recorded around two days after surge arrival. Based on daily AOI data (since 1950) from the US National Oceanic and Atmospheric Administration (NOAA), we have identified days in December - March with AOI at or below -4 and looked at the minimum temperatures at the Observatory in the ensuing seven days (denoted by Tm). As shown in the histogram in Figure 2, Tm was at or below 7oC (very cold weather) for only about 7% of the cases and was actually above 12oC (cold weather) for more than 40% of the cases. It can therefore be readily discerned that there is no general rule linking negative AOI with intense cold surge in Hong Kong.
Figure 2 Minimum temperature at the Observatory in the seven days following a day with AOI ≤ -4 (AOI data source: NOAA)
S M Lee
In late January 2016, an intense cold surge swept across southern China, bringing bitterly cold weather to Hong Kong. The minimum temperature at the Observatory on 24 January was only 3.1 degrees, the lowest record since 1957. Very rare phenomena of icing and freezing rain with ice pellets also occurred in Hong Kong that day, attracted a lot of interest from the media and the public, especially on different types of winter precipitation. There were quite some discussions on "mix of rain and ice pellets" and "mix of rain and snow" (both being referred to as "sleet" in English). In this blog, we try to explain the differences between various types of winter precipitation and the choice of terminology.
As discussed in previous blog articles on winter phenomena[1, 2], the winter precipitation mentioned above are very rare in Hong Kong. Nevertheless, most of the precipitation seen in winter starts out as snowflakes in the clouds high up in the sky, it is the variation in the temperatures all the way from the upper atmosphere to the ground that determines what type of precipitation reaching the ground, be it in the form of liquid, solid, or a mix of them. Different types of winter precipitation and the typical temperature profiles favourable for their formation are described below.
Whether in winter or summer, rain is the most common type of precipitation in southern China. When the temperatures are above 0oC, i.e. freezing point of water, throughout most of the air column above the ground, all the snowflakes formed in clouds above the freezing level (the height where temperatures fall to 0oC) will be warmed and then melt while falling. They will completely change into raindrops before reaching the ground (Figure 1).
Figure 1 Typical vertical temperature (T) profile for rain
Snow is a type of solid precipitation formed from tiny ice crystals clinging together at temperatures well below freezing point to become snowflakes. For snow making its way to the ground, usually the layer of air from ground to where snowflakes are formed has to be at or below 0oC, so that snowflakes remain unmelted on their way down (Figure 2). However there are exceptional cases. If the ambient air near the ground is sufficiently dry, some partially melted or unmelted snowflakes can still reach the ground even though the air temperature near the ground is slightly above freezing point, resulting in snowfall. Snow can also be classified into "dry" and "wet" according to the size and characteristics of snowflakes. As shown in Figure 3, snowflakes falling through dry air which is well below freezing temperature will be small and powdery. This type of snow is usually called "dry snow" which is suitable for snow sports. If the snowflakes fall through a layer which is close to or just above freezing point, the snowflakes will melt around the edges and stick together to form larger and heavier snowflakes which are commonly called "wet snow" and good for making snow balls or snow men but more slippery to pedestrians.
Figure 2 Snow in Beijing on 12 November 2009
Figure 3 Typical vertical temperature profiles for dry (left) and wet (right) snow
Sleet - "mix of rain and ice pellets" or "mix of rain and snow"
In English, "sleet" is a general weather term to describe a mix of solid and liquid precipitation, yet this term does not have any standard definition internationally. The solid precipitation in sleet comes in two distinct forms, namely ice pellets (Figure 4) and partially melted snow, as tabulated in Table 1. As the appearance and formation condition of ice pellets and partially melted snow are different (Figure 5), it may be more intuitive to describe the corresponding precipitation form directly to avoid misunderstanding, viz., "rain and ice pellets" or "rain and snow".
Figure 4 Ice pellets observed at Tai Mo Shan Radar Station on 24 January 2016 (Courtesy of Ms Y S Li)
Figure 5 Typical vertical temperature profiles for mix of rain and ice pellets (left) as well as mix of rain and partially melted snow (right)
The vertical temperature profile which favours freezing rain is similar to that of ice pellets. However, the warm layer aloft is much deeper and the sub-freezing layer near the ground is much shallower (Figure 6). Firstly, snowflakes descend into the deep warmer layer and completely melt into raindrops. Then, raindrops fall through a shallow sub-freezing layer near the ground which is too shallow to allow the raindrops (freezing rain) to immediately refreeze. These super-cooled droplets instantly refreeze when they come into contact with cold objects, forming icicles or glaze of ice on the ground, trees, or power lines (Figure 7). Icy roads caused by freezing rain are dangerous to pedestrians and drivers (Figure 8), while the weight of accumulating ice may damage power lines and tree trunks.
Figure 6 Typical vertical temperature profile for freezing rain
Figure 7 Icing due to freezing rain at Kadoorie Farm, Tai Po, on 24 January 2016 (Courtesy of Mr K W Li)
Figure 8 Slippery icy roads at Tai Mo Shan brought by freezing rain on 24 January 2016 (Courtesy of Ms Y S Li)
Global warning may continue to bring extreme weather. The occurrence of special winter precipitation in Hong Kong may attract members of the public to witness and take pictures of such events, however please beware of the potential dangers brought by such special phenomena, the very cold condition and slippery icy roads. The "Weather Information for Outdoor Photography Webpage" recently launched by the Observatory (http://www.hko.gov.hk/out_photo/outdoor_photo.htm) provides information of temperature, humidity, wind speed and wind direction in the lower atmosphere for the public to understand the weather conditions in different areas and on high ground and to be better prepared.
C K Ho and T C Lee
 Hong Kong at Freezing Point - Observation Perspective
 Hong Kong at Freezing Point - Historical Perspective
 The rate of snowflake melting as it falls
 UKMO, How is snow formed?
 NOAA, National Weather Services, Glossary
 Cleveland Abbe, Jr., 1916 : American definition of "sleet", Monthly Weather Review, 44(5), 281-286.
 UKMO, What is sleet ?
 Bureau of Meteorology, Glossary