Classification of Synoptic Circulation Patterns for Fog in the Urumqi Airport

The purpose of this study is to investigate the Classification of Synoptic Circulation Patterns for Fog in the Urumqi Airport in Xinjiang, China. By using relevant climate statistical method, the hourly ground observation data and four times per day and NCEP/NCAR reanalysis data (1 ̊ × 1 ̊) from 1985 to 2014 were analyzed. The results showed that: 1) The occurrence of fog significantly increased during 1985 and 2014; There are two stages of the airport fog in the 30 years, less-fog period (1985-2002) and more-fog period (2002-2014), the fog focused occurred in November to March of the following year, most in December and least in March; 2) Based on Lamb-Jenkinson method, the dominant types of fog in Urumqi Airport are C, E, SE, W, A types (Among them, A, C, E, W, SE are the circulation types. C is Cyclone, A is Anti-cyclone, E is East, W is West, SE is Southeast), and the international distribution of each type is also different; 3) The dominant types of persistence fog are A, C, E, SE types, A type appears in the afternoon to the nighttime, in the morning less frequent, on the other hand, C type is just the opposite; 4) Atmospheric circulation characteristics for persistence fog profile can be divided into four series: A, C, E, SE series, and climatic characteristics in different series are different.


Introduction
According to the relevant definition in the Civil Aviation Weather, Part 1: Observations and Reports of the Civil Aviation Industry Standard of the People's Republic of China (MH/T 4016.[1] issued by the Civil Aviation Administration of China (CAAC), fog is near ground water vapor condensation or condensate leaving the visibility reduced to 1000 m below the phenomenon.It is the biggest cause of aviation accidents, severe fog can lead to poor visibility, affecting the aircraft's normal taking-off and landing, increasing the aircraft's ice, bumps, etc., in the clouds [2].A long-time continuing fog causes airport closures, flights canceled, resulting in a large number of passengers stranded, affecting the flight safety and normal operating order.In view of the causes and climatic characteristics of the foggy weather in Urumqi, the researchers have done a corresponding analysis from different perspectives.Researchers [3] [4] [5] analyzed the weather background, temperature and humidity characteristics, stratification conditions, surface wind and other meteorological conditions of the fog weather and indicated that the winter Urumqi fog is occurrence and maintenance under the joint action of a specific weather situation, a certain temperature and humidity condition and atmospheric vertical structure.
Aviation weather staffs have a lot of research on the airport fog.Chen [2] analyzed the daily, seasonal and inter-annual changes in the fog characteristics of the Capital Airport (Beijing, China).Li [6] pointed out the corresponding weather situation at different stages of fog respectively, classified the fog process, and analyzed the temporal characteristics, the background field characteristics and the physical characteristics such as air pressure and wind field before and after the appearance and disappearance of various fogs.In addition, Huang [7] analyzed the reasons on the Capital Airport advection fog formation, and put forward the forecast indicators.Dong [8] gave the summary on the characteristics of the daily and yearly variation characteristics of the fog in the Xianyang Airport (Shaanxi, China).
The Urumqi Airport is located in the fog high incidence area.The winter is the most important impact on flight safety and accuracy of the weather phenomenon.Wang [9] studied the characteristics of uneven distribution in the Urumqi Airport foggy season, but lacked research on circulation classification of fog in the Urumqi Airport.In the weather situation classification, it can be divided into subjective and objective methods.The subjective classification method is very intuitive and easy to explain its physical meaning, but this method mainly depends on experience, varies from person to person, and has obvious subjectivity.Based on the Lamb atmospheric circulation classification method [10], the Jenkinson method [11] (hereinafter referred to as the L-J method) is used to define the objective classification method by defining some objective criteria from qualitative classification.The Lamb-Jenkinson circulation type classification system is applied to obtain circulation information in China on daily basis.For that purpose, the NCEP daily mean sea-level pressure reanalysis This study starts from the weather situation when the fog is formed, and analyzes the weather conditions of the Urumqi airport in fog.The L-J method is used to explore and analyze the possibility of weather classification, and provide the reference for improving the fog forecast in Urumqi.More accurate weather forecast and more timely measures are necessary to reduce the losses caused by foggy weather.

Data
In this study, we use the visibility data of the Urumqi Airport from January 1985 to December 2014, and the 1˚ × 1˚ average sea level pressure reanalysis grid data of NCEP four times daily.

Methods
The Urumqi Diwopu International Airport (hereinafter referred to as "the airport") locates in the northwest suburb of Urumqi.The runway center coordinates of 87˚28'28"E, 43˚54'23"N.In this study, we took the center (16.5˚N, 87.5˚E) of the area 32.5˚N -52.5˚N and 72.5˚N -102.5˚E, and obtained 16 points from every 10 longitude and 5 latitude grid points (Figure 1), covering the Tianshan Mountains fog-prone areas.Then we did the circulation classification In the above area, the six circulant factors u, v, V, ξu, ξv and ξ of the center point (42.5˚N,87.5˚E) are calculated from the monthly mean sea level pressure of 16 grid points by the difference formula.The specific formula is as follows: We take u, v, V, ξ u , ξ v and ξ in Equations ( 1)-( 6) as the six circulation indexes to divide the circulation type.
In the above equation, P(n) is the sea level pressure (n = 1, 2, •••, 16) on the nth grid, which is the latitude value of the center point, A1 and A2 respectively.
Small letters u, v are the zonal and radial components of the ground wind, ξ is the total shear vorticity, ξ u is the meridional gradient of u, ξ v is the zonal gradient of v, and the units of the six circulation indexes are hPa/10˚.From the Equations ( 1), (2) to calculate u, v, the direction of the geostrophic wind is determined by the following formula: According to the geostrophic wind speed, direction and vorticity value, the circulation is divided into three types: airflow type, rotary type and mixed type (

Inter-Monthly Distribution of the Urumqi Airport's Fog Accumulated Time
For the Urumqi Airport, fog accumulated time was 13,225 hours during 1985 to 2014, and showed a clear upward trend (Figure 2).From the histogram, this upward trend is divided into two more obvious periods: before 2002, the cumulative occurrence of fog was very uniform, with an average of 293.5 hours/year; and the cumulative increase in fogging time after 2002 was rapid and averaged the value of 633.5 hours/year, for the high fog period.In addition, the airport fog generally began in November each year, December to next January the most.In early spring season fog began to decrease, and after mid-March rarely appeared.The cumulative fog of the month above accounted 99.3% for the year.In December, fog average annual cumulative time was 87.9 hours, the distance percentage of 76%, followed by fog easy to appear in January, and March at least.E-type fog distribution is more average; in addition to relatively few in November, the remaining four months difference is not big, and the largest is in February.C-type fog is the most uneven distribution of the five months, more in November, followed by March, however, foggy in December, January, February is relatively less.A type appears mainly in December and January.C-type is mainly in November, March, respectively, late autumn and winter.

Persistent Fog Circulation Classification
Persistent fog has a huge impact on the punctual and safe operation of the flight.
As the data used in this study is a per-6-hour reanalysis data, in order to analyze the type characteristics of persistent fog, this study screens the fog weather process with duration of 12 hours or more.After quality control, we select a total of 95 persistent fog process in 1985-2014, which have a total of 312 times as a sample to study the persistent fog for the circulation classification.Persistent fog type distribution is more dispersed.There were only 4 persistent fog types, which were type A (14.6%), type C (14.1%), SE type (13.1%) and E type (12.2%), respectively.And the above four types in the distribution of time also have their  4).Type A appears in the afternoon to night, early morning to midday less appear, and type C is the opposite.The rest, E-type and SE-type distribution is relatively uniform.
According to the classification of 95 persistent fog weathers, we defined the fog process according to the more appeared types associated with other types of circulation situation, then the 95 fog process can be divided into four series and 12 types.Each type of circulation background is as follows: The first category is A-series fog.The first is the A-type fog circulation situa-  has occurred a total of seven times.For the A-type fog, the Mongolian high pressure center east withdraw, while the degree of warp is reduced and the zonal dimension is increased.Most of the northern Xinjiang region is located on the edge of high pressure.The airport is located in the back of the high pressure.In addition to being controlled by high-pressure edges, there are southeast geostrophic wind.Furthermore, there is an A-ASE fog circulation situation (Figure 5(c)).The situation has occurred a total of 10 times.Compared with the A-SE type, high pressure control more (Mean sea level pressure in this airport is 1035 hPa).Mongolian high pressure's main body moves to south, high pressure belt extends to east.There is also an A-E-type fog circulation situation (Figure 5(d)).The situation has a total of five times.High pressure's main body is obviously north, so the airport is at the bottom of the high-pressure periphery and has an eastfacing geostrophic wind.
Summed up all kinds of A-series fog type, we can see that in this series fog Xinjiang were almost all significantly controlled by high pressure, and the airport were mostly in the bottom or back bottom of the high pressure.Under normal circumstances, in about one day before the A-series fog process occurred, there is a low trough invasion of northern Xinjiang, causing a precipitation process.The water vapor is not only condensation and landing, but also to increase the overall humidity and provide better humidity conditions.Then the northern Xinjiang is controlled by the trough after the warm ridge, and the back part of the warm ridge back blow a gentle air, so there is no more obvious cold air to destroy the stability of the atmosphere, and it is conducive to the maintenance of fog.
The second category is the C series fog.Because during the fog atmosphere is a static structure, in the C-type the airport is not inside the cyclone, but in the relatively low pressure zone near the cyclone, that is, high and low pressure transition zone.This type of fog usually manifests as the Mongolian high pressure center east moved, high pressure west extension.Southern Xingjiang is controlled by low pressure, and the airport is in the north-high-south-low transition zone.
The first is the C-type fog circulation situation (Figure 6(a)).This situation has a total of five times.The whole process of fog is C-type circulation.Mongolian high pressure controls most of the northern Xinjiang.However, the northern Xinjiang is at periphery of high pressure, and the southern Xinjiang is controlled by low pressure, which is in a low pressure zone with the West Siberian

data from 1
January 1948 to 31 December 2004 are used to derive daily six circulation indices and corresponding 27 circulation types for 16 sub-regions in China.The frequency of circulation types over different sub-regions and seasons is computed and described.Results show that Lamb-Jenkins on classification system is applicable for most of China, although in the west of China where might exist some errors in sea-level pressure due to high elevation.It automati-cally divides the circulation types to obtain the objective digital description for the local circulation, which overcomes the shortcomings of subjectivity, and has the advantages of easy calculation, objective result and clear meaning in synoptic meteorology.Its advantages are gradually recognized, so it is widely used in studies of climate change.From the perspective of synoptic climatology to study climate change, there is a potential value in climate prediction.

Figure 1 .
Figure 1.The 16 grid points in constructing the circulation climatology.

3. 2 . 1 .
Frequency of Circulation TypeBy using the method shown in Section 1.2 to divide the circulation types from January 1, 1985 to December 31, 2014, Figure3shows the frequencies of the circulation types of the Urumqi Airport in the above time range.In addition, five circulation types, E, SE, W, A, and C, in the proportion of each month are not the same.According to Table2, the SE-type fog appeared more in December and January (two months for the most fogs), followed by February and November, less in March (month for the least fogs), that is, SE-type fog's inter-monthly distribution and whole fog accumulated time distribution are the same.

Figure 2 .
Figure 2. Trend of appearance time for fog in the Urumqi Airport during 1985 to 2014.

Figure 3 .
Figure 3. Frequency distribution of the circulation types during 1985 and 2014.
tion.The situation has a total of 11 times.The whole process of fog is type A circulation (Figure 5(a)).In this circulation conditions, most of the northern Xinjiang and even whole Xinjiang have been controlled by high-pressure, and the airport is in the anti-cyclone center or near to it.Followed by the A-SE type fog circulation situation (Figure 5(b)).This situation

Figure 4 .
Figure 4. Daily variation regularity of circulation types.

Table 1
).We define airflow type when |ξ| ≤ V, which includes north (N), northeast (NE), east (E), southeast (SE), south (S), southwest (SW), west (W) and northwest (NW); When |ξ| ≥ 2V, it is for the rotation of the circulation, including anti-cyclone (A) and cyclone (C); There is also a combination between the above two, that is, V < |ξ| < 2V, which is called the mixed circulation, that is, the combination of flat circulation and rotating circulation; If |ξ| < 6 and V < 6, the circulation has not a exact characteristic, known as undefined, expressed as UD.This can define 27 different types of circulation.N is North, NE is Northeast, E is East, SE is Southeast, S is South , SW is Southwest, W is West, NW is Northwest; C is Cyclone, A is Anti-cyclone; CN isCyclone North, CNE is Cyclone Northeast, CE is Cyclone East, CSE is Cyclone

Table 1 .
Lamb-Jenkinson circulation classification.CS is Cyclone South, CSW is Cyclone Southwest, CW is Cyclone West, CNW is Cyclone Northwest; AN is Anti-cyclone North, ANE is Anti-cyclone Northeast, AE is Anti-cyclone East, ASE is Anti-cyclone Southeast, AS is Anti-cyclone South, ASW is Anti-cyclone Southwest, AW is Anti-cyclone West, ANW is Anti-cyclone Northwest.

Table 2 .
Monthly frequencies (%) of the circulation types between 1985 and 2014.