A new approach to cold surge classification in East Asia

Evidence showing a strengthening of intense cold surge event (CSE) in East Asia, e.g. CSE of Jan 2016 and Jan–Feb 2008, is focusing attention towards the science of CSE onset prediction. Predicting the onset of such strong CSEs remains elusive as the extent of these surges varies over spatial and temporal scales. Changes in radiative cooling over Siberia in winter as potentially affected by changes in the Arctic are further expected to influence CSE occurrences in East Asia. Moreover, unprecedented and long lasting CSEs in East Asia have a very distinct Jet Stream pattern via their shifts from the climatological mean, influencing the lower troposphere. Here, using modelling framework we propose a new relationship between Jet Stream and Aleutian Low for identifying and characterizing atmospheric process that leads to CSEs in East Asia. Our results reveal new insight into the mechanisms of CSEs occurrences, the absence of which may lead to major constraints on reducing CSE onset prediction error.

Comparison of the influence of two types of cold surge on haze dispersion in eastern China

Cold surge (CS) is considered a favorable weather process to improve air quality and is widely recognized. However, there is no detailed study on the differences in the dispersion ability of different types of CSs in relation to haze days in eastern China (HDEC). This paper uses the hierarchical clustering algorithm to classify the cool-season (November to February of the following year) CSs across eastern China into blocking CSs and wave-train CSs and compares their influences on the number of HDEC from 1980 to 2017. Results show that the wave-train CSs can significantly improve the visibility in eastern China and generally improve air quality for about 2 d longer than the blocking CSs, which indicates that the blocking CSs have a weaker ability to dissipate HDEC compared with the wave-train CSs. The CSs affect the HDEC by changing meteorological elements like thermal inversion potential, horizontal surface wind, sea level pressure (SLP), and surface air temperature (SAT). A period of 4 d after the outbreak of CSs, the variations of thermal inversion potential and horizontal surface wind of two types of CSs tend to be consistent. However, the negative SAT anomalies and the positive SLP anomalies caused by the blocking CSs lasted shorter than those caused by the wave-train CSs, forming favorable conditions for the rapid growth of HDEC. Furthermore, results show that in recent years, especially after the 1990s, the frequency of wave-train CSs has decreased significantly, while the frequency of blocking CSs has slightly increased, indicating that the overall ability of CSs to dissipate HDEC has weakened in general. This work may provide reference for the future formulation of haze control policies in East Asia.

The Impact of the Interaction between Madden-Julian Oscillation and Cold Surge, on Rainfall over Western Indonesia

The Madden-Julian Oscillation and Cold Surge phenomena have been known to cause increased rainfall, with the capacity to trigger hydrometeorological disasters, in western Indonesia. However, further investigations are required regarding the interaction between these phenomena on rainfall pattern. Therefore, this study aims to analyze the interaction between MJO and CS over western Indonesia, particularly by using land-based rainfall observation data from multiple stations, as previous studies were dominated by the use of gridded data from remote observations. This study utilized in-situ observation data obtained from 4329 weather observations and rain stations between 1989 and 2018. Subsequently, quality control performed based on data availability exceeding 70% over a 30-year period resulted in 303 selected stations to be used for further analysis. Meanwhile, the RMM index, as well as reanalysis data of mean sea level pressure and 925 hPa meridional wind, were also applied for MJO and CS identification. According to the composite analysis, the effect of CS on MJO phases tends to increase precipitation by about 50%, over western Indonesia, with maximum increase ranging from 200 to 400% over the northeastern coast of Sumatra, around Karimata Strait (Riau Islands and West Kalimantan), as well as the northern coast of Java. These areas are exposed to the sea and have direct access to the wind-terrain interaction. In addition, the highest rainfall anomaly due to the MJO-CS interaction occurs around Karimata Strait, followed by northern Sumatra and Java, with spatially averaged rainfall anomaly reaching 5 mm/day over the area.

Comparison of influence between two types of cold surge on haze dispersion in Eastern China

Cold surge (CS) is considered as a favorable weather process to improve air quality and is widely recognized. However, there is no detailed study on the differences in the dispersion ability of different types of CSs to haze days in eastern China (HDEC). This paper uses the hierarchical clustering algorithm to classify the cool season (November to February of the following year) CSs across eastern China into blocking and wave-train CSs and compares their influences on the number of HDEC from 1980 to 2017. Results show that the wave-train CS can significantly improve the visibility in eastern China and generally make the high air quality last for about 2 days longer than the blocking CS, which indicates that the blocking CS has a weaker ability to dissipate HDEC compared with the wave-train CS. The CSs affect the HDEC by changing these meteorological elements like thermal inversion potential, horizontal surface wind, sea level pressure (SLP), and surface air temperature (SAT). 4 days after the CSs outbreak, the variations of thermal inversion potential and horizontal surface wind of two types of CSs tend to be consistent. However, the negative SAT anomalies, and the positive SLP anomalies caused by the blocking CSs lasted shorter than those caused by the wave-train CSs, which forms favorable conditions for the rapid growth of HDEC. Furthermore, results show that in recent years, especially after the 1990s, the frequency of wave-train CSs has decreased significantly, while the frequency of blocking CSs has slightly increased, indicating that the overall ability of CSs to dissipate HDEC has weakened in general.

Cold Surge Pathways in East Asia and Their Tropical Impacts

Cold surge occurrences are one of the robust features of winter monsoon in East Asia and are characterized by equatorward outbreaks of cold air from the high latitudes. Beside greatly affecting weather variability across the Far East, cold surges are of importance for Southeast Asian countries because they can propagate far to the tropics and excite convective activities. However, the tropical responses highly depend on the downstream pathways of the surges. To better understand how cold surges influence tropical weather, we investigate 160 cold surges identified using a quantitative approach during 40 winters from 1979/80 to 2018/19, and then classify them into several groups based on their prominent pathways. At the midlatitudes, we find two groups: one for surges that show clear equatorward propagation of cold air to lower latitudes and the other for surges that turn eastward and bring cold air to the North Pacific. These groups arise due to the strength difference of the Siberian high expansion controlled by cold air blocking near the Tibetan Plateau. The tropical impact is evident in the former group. We perform further classification on this group and find four types of surges based on their pathways in the low latitudes: 1) South China Sea (SCS) surges, 2) Philippines Sea (PHS) surges, 3) both SCS and PHS surges, and 4) blocked surges. They exhibit distinct precipitation signatures over the Maritime Continent, which are driven by interactions between the surges and the pre-existing synoptic conditions over the tropics, particularly the Madden–Julian oscillation (MJO).