Analysis of the Temporal and Spatial Development of Cold Waves in Fujian Province over the Last 58 Years

The escalation in the risk of extreme weather events due to climate system warming has caused a distinct alteration in the frequency and intensity of cold waves in China. Moreover, these changes exhibit distinct regional variations in their altering characteristics. Using daily temperature data from 70 weather stations in Fujian Province of China from 1961 to 2018, this study looks at how cold waves move through the province and how they change over time. It does this by combining linear regression trend, spatial interpolation, and spatial autocorrelation analyses. The findings indicate that the occurrence frequency of cold waves in the designated study area is notably higher in the western and northern regions, while it is comparatively lower in the eastern coastal cities. Furthermore, there is an overall downward trend in the frequency of cold waves, with a declining frequency from 1961 to 2000, followed by a significant increase in frequency after 2000. Furthermore, the number of cold waves in the study area increased significantly after 2010.


INTRODUCTION
A cold wave is characterised by the rapid intensification and extensive intrusion of cold air from high latitudes into the middle and low latitudes under suitable atmospheric circulation conditions at high altitudes.This intrusion occurs when the cold air reaches a specific threshold of cooling standards, which in turn causes a region-wide spectrum of extreme temperatures, high winds, precipitation, and snowfall.Extremely frigid air struck numerous locations in northern China in late 2009, causing severe economic losses.Extreme chilling events transpired in Northeast China, North China, as well as the Yellow and Huaihuai Rivers.In the latter part of 2020, the nation of China A widespread cold wave weather event impacted several regions of the country, resulting in significant economic, industrial, and societal repercussions for China.The complicated elements contributing to the production of cold waves in China primarily include the Arctic Oscillation, North Atlantic Oscillation, Cold Air, Asian Polar Vortex Intensity Index, and East Asian Trough Intensity.China is not an exception to the trend of decreasing frequency and intensity of land-based extreme cold events in the context of global warming (IPCC, 2021) [1].However, Wang and Gong [2] was discovered that while the occurrence rate of frigid air events in China declined as a consequence of global warming, it subsequently resumed its upward trend through the year 2000.The consolidation and revision of meteorological data have revealed novel attributes in the frequency and intensity of cold waves in China.Conversely, the dynamics of cold wave transformations at the regional level exhibit distinct characteristics [3].
However, more scholars have focused on northern China as a typical cold wave study area and neglected some southern regions.Based on kriging interpolation and spatial autocorrelation analysis of weather data from the last 58 years, this study looked at how cold waves have moved and changed over time and space in Fujian Province.Enhancing the capacity to forecast cold waves and extremely low temperatures, the findings of this research can furnish pertinent information for the surveillance, prediction, and evaluation of the consequences of cold wave conditions in Fujian Province.

STUDY AREA
Fujian Province is in the southeastern coastal area of China.It is between 23°31′ and 28°18′ north latitude and 115°50′ and 120°43′ east longitude.The mountainous and hilly region comprises approximately 90% of the province's total land area.Fujian Province's topography is distinguished by its elevated northwest and lower southeast terrain.The weather is subtropical maritime monsoon, warm and humid.

DATA SOURCE
For this study, we selected the day-to-day temperature data from 70 meteorological stations in Fujian Province spanning the years 1961 to 2018.It is important to note that the data underwent rigorous review and quality control measures.A cold wave is defined in this study as a decrease in temperature of over 8℃ over the course of 24 hours.Additionally, the minimum temperature must fall below -5℃.Furthermore, to adhere to the pertinent empirical evidence, it is mandated that any three or more consecutive days that satisfy the conditions for a cold wave are accumulated as a single cold wave event.

METHODS
Kriging interpolation is based on the theory of spatial statistics, and the unbiased and linear optimal interpolation estimation of regional spatial variables using the variation function of geographic elements and the structural characteristics of the original data, which can provide a good visualization of the frequency of occurrence of cold wave and the difference map of the lowest mean temperature in Fujian Province during 1961-2018.
To examine the degree of correlation between the frequencies of cold waves detected at each meteorological station in the study area and to determine whether a significant spatial distribution pattern exists between the number of cold waves, spatial autocorrelation analysis was implemented, utilising both global and local spatial autocorrelation analyses [4].

CHARACTERIZATION OF COLD WAVE ACTIVITY FREQUENCY CLUSTERING IN FUJIAN PROVINCE
This study found a value of 0.692 for world autocorrelation (p>0.01).The spatial autocorrelation plot shows four outcomes: high value clustering (HH), low value clustering (LL), high value elements surrounded by low value elements (HL), and low value elements surrounded by high value elements (LH) [5].Based on the findings of local spatial autocorrelation analysis depicted in Figure 1(b), cold wave weather is prevalent in the northern and northwestern regions of the study area, whereas the frequency of cold waves does not cluster significantly in the central and southwestern areas.Cold waves occur with the lowest frequency in the southern, southeastern, and southwestern littoral regions.

SPATIAL DISTRIBUTION CHARACTERISTICS OF COLD WAVE IN FUJIAN PROVINCE
Figure 3(a) shows how the minimum temperature difference during the cold wave spread across Fujian Province from 1961 to 2018.The average minimum temperature difference during the cold wave has an amplitude of 8 °C to 15.8 °C and is mostly found in the west and south of the study area.Taking the highest value of 15.8°C in the distribution of the lowest temperature difference in the study area from 1961 to 2018 as the lower limit of the extreme temperature difference, 55 stations with the extreme temperature difference were screened out and analyzed spatially visually.It was discovered that the cold wave's extreme temperature difference in the study area changed over space, rising from the region's centre to the west and falling to the east (Figure 3b).The overall range of the extreme temperature difference was 15.9 to 18.3°C.Low-value areas were spread out in the southeast of Fuzhou City and along the coast.High-value areas were in the western and northern parts of the study area.In terms of the general extreme temperature difference, areas with high cold wave frequencies don't always have extreme cold waves.On the other hand, areas with low cold wave frequencies may also have extreme cold waves.As a consequence, the cold wave weather in the studied area varies considerably across space, with the variation being most pronounced in December.

CHARACTERISTICS OF TEMPORAL CHANGES IN THE COLD WAVE
An overall time series analysis of the frequency of cold waves in the study area during 1961-2018 (Fig. 4) showed an overall linear decreasing trend, with a significant decreasing trend during 1961-2000, and a gradual increase in the frequency of cold waves in the study area after 2000, with a peak in 2010 followed by a decrease in the number of cold waves.A temporal spatial analysis was also carried out by random station screening of the weather station sites in the study area (Fig. 5), and it was found that, as with the results of the overall time series analysis, all the sites showed a linear decreasing trend between 1961 and 2018.The inaccuracy of conclusions derived from national and regional analyses of cold waves regarding the actual situation at a specific weather station in a particular region is due to the influence of numerous uncertain factors.Furthermore, variations in the measurement and analysis of cold wave frequency statistics may result from the use of distinct cold wave standards, statistical methodologies, and study time scales.

Figure 1
Figure 1 Cumulative frequency-local autocorrelation analysis (a) and Moran's I scatterplot of cold waves (b) in Fujian Province, 1951-2018

Figure 2 Figure 3
Figure 2 Spatial distribution of minimum mean temperature difference (a)and cold wave extreme temperature difference (b) in Fujian Province, 1961-2018