Winter Precipitations of Northern Part of Farming-Pastoral Zone and Hulunbuir Grassland

In January 2018, a high record of monthly total precipitation in northern China drew our attention. The precipitation amount is 4 times more than that in normal winters over the past 30 years. Up to now, many researches over the world focus on the intensity of precipitations or the number of intense precipitations, however, no sufficient study is investigated in the continuous moderate precipitations. The abnormal precipitation in January 2018 in northern China is a typical case of the continuous moderate precipitation. The research region is composed by the northern part of the farming-pastoral zone and the Hulunbuir Grassland. The main method in this study consists of analyzing the abnormal precipitation month by identifying the precipitation types and locations, and exploring the atmospheric circulations at the moment of typical precipitations. The research found that the continuity is the key point for high precipitation amount in January 2018 in Northern China. Besides, this continuity is caused by frequent appearance of cyclones in the vicinity.


Introduction
Northern China is always considered as a dry region comparing to the southern China. Especially in winter, a dry lasting cold weather dominates in the north. However, the northern China turned to be rich in precipitation in January 2018. The research region, called Region hereafter, is defined by the rectangle limitation comprising northern part of the farming-pastoral ecotone and Hulunbuir Grass-intense precipitations, which are obvious and easy to draw out attention. Rare research is investigated in finding the hint causes of continuous moderate precipitations. Thus, we target our research at exploring precipitation phenomena in January 2018 in the Region, a typical case of continuous moderate precipitation, and using dynamical analyses to find out the causes of such precipitations. First, we explore the trend of the Region-averaged monthly total precipitations over the recent 30 years without that of January 2018. Second, we study the location of the monthly total precipitations of Januaries in the recent 7 winters. Third, the hourly precipitations within the month are explored. The precipitation rate that exceeds 2 mm/h is defined as "heavy precipitation events" for the Region. Lastly, we focus on typical individual heavy precipitation event in January 2018, and use dynamical analyses to investigate in the atmospheric circulations during those precipitations.

Definition of the Research Region
The Region comprises the north part of the farming-pastoral ecotone of China and Hulunbuir Grassland. It is a rectangle area limited by the union of the longitudes and latitudes of the ecotone and the grassland. This rectangle area helps us to understand better conditions of the precipitations and atmospheric circulations, than limiting our sights only on the banding field of ecotone and the square of grassland. The Northern part of Chinese farming-pastoral zone covers 36˚30'N -46˚42'N and 106˚16'E -124˚51'E [21]. The boundary of the Hulunbuir Grassland is limited by towns of Xinbaerhuyouqi, Xinbaerhuzuoqi, Chenbaerhuqi, Ewenkeqi, Hailarqu, Manzhouli, Yakeshishi, Eergunashi. Those towns encircle an area with latitude 48˚N -51˚N and longitude 116.5˚E -121˚E. Hence, the Region is defined by 36˚N -51˚N and 106˚E -125˚E.

Data Source and Methods
All data used in this research are publicly available. The data of Monthly Total precipitations (SURF_CLI_CHN_PRE_MON_GRID_0.5) have 0.5˚ * 0.5˚ horizontal resolution, and they are taken from National Atmospheric Science Data Center of China.
From European Centre for Medium Range Weather Forecasts (ECMWF), ERA5 data is used with temporal resolution of 6 hours, and the horizontal resolution is 1˚ * 1˚ for hourly precipitation, 0.1˚ * 0.1˚ for geopotential, U and V components of wind and specific humidity at 200 hPa, 500 hPa 700 hPa and 850 hPa.
The hourly precipitation rate is taken as the maximum precipitation amount gleaned in the Region from January 1st 0:00 to 31th 18:00 with time resolution of 6 hours. Then "heavy" precipitation hourly rate is defined by 2 mm/h because this value can separate clearly continuous precipitation fold line into individual precipitation events, to give more convenience to our analyses. The dynamical analyses include relative humidity, wind field, geopotential height to explore atmospheric circulation above the Region during each heavy precipitation event.

Precipitation Trends
The Region locates in the north part of China, and it is renowned for cold lasting winter days and small amount of precipitations. In our research we only consider the months with the lowest temperature of a year, that is we take December, January and February as winter months. For the recent 30 winters from December 1988 to February 2018, the Regional maximum monthly total precipitation is drawn in Figure 1. The maximum precipitation values oscillate between 0 mm and 100 mm with a stable behavior, except the instance of January 2018 which exceeds 400 mm. For the Region-average precipitation, the value of January 2018 is about three times higher than that of normal months.
To see where the value of the maximum precipitation in January 2018 situates through the 30 winters, we draw in Figure 2 Region-averaged and maximum monthly total precipitation using GEV distribution. The Region-averaged monthly total precipitation in dark blue line is centered at 4 mm, and the probability falls to 0 at 15 mm. For the maximum monthly total precipitation in cyan color, the maximum probability locates at 25 mm, and it equals to 0 for precipitation value larger than 150 mm. This means the precipitation value of January 2018 is unprecedented in history.
To understand precipitation trend of the Region, the Region-averaged monthly total precipitation for the 30 winters is drawn in Figure 3. Only in December 1990 the Region-averaged precipitation value is more than 10 mm, while The horizontal axis is time line and the vertical axis is precipitation amount with units of mm. The green solid line is Regional maximum monthly total precipitation value, and the gray dashed line is the Region-averaged monthly total precipitation. The red spot is the Regional maximum monthly total precipitation value of January 2018.   for other months, this value never exceeds 8 mm. The slope of the trend line is positive, which means the precipitation amount is increasing in the Region. The small value of the slope expresses the tiny amount of growth. The precipitations of the Region are estimated as in a stable state.

Precipitation Locations
For the recent 30 years, the locations of the maximum monthly total precipitation of the Region are mainly separated to five sites. These precipitations are named after those sites: East border precipitation, Northeast precipitation, Southwest precipitation, Coastal precipitation and Inland precipitation. The precipitations of January 1997 shown in Figure 4 is a typical case for East  border precipitation because its maximum monthly total precipitation locates at the east border of the Region. More precisely the precipitation is in Liaoning province, the south part of Changbai Mountain. The maximum precipitation value is around 20 mm. In Figure 5, the precipitations of January 2002 stand for Northeast precipitation since its highest monthly total precipitation is in the northeast corner of the Region. It locates within 47˚N -51˚N, 116˚E -125˚E. This area is within Inner Mongolia, in the north part of Daxinganling Mountain. The precipitation amount ranges from 20 mm to 40 mm. Figure 6 shows the precipitations of January 2008 which illustrates Southwest precipitation. The precipitation covers the area of 36˚N -40˚N, 106˚E -112˚E, which is mainly in the north part of the farming-pastoral ecotone without reaching up to Hebei, Beijing or Tianjin. This is in the Liupanshan Mountain.
The precipitations in January 2017 in Figure 7 illustrate Coastal precipitation because the maximum precipitation location is within the east part of Shandong province with 36˚N -38˚N, 118˚E -123˚E. This is the only part that thrusts into the ocean, with topography of plain.
The precipitations in Figure 8 stand for Inland precipitations, and they occurred in January 2018. Those precipitations are typical because they cover a band area from 112˚E 36˚N to 122˚E 51˚N with a linear line shape. The dark green color in the figure shows that the precipitation amount is largely saturated by the limit of 100 mm as shown on the color bar to the right of the figure.

Precipitation Rate
Since we find that from 1988 to 2018 the winter precipitations are stable, we focus only on precipitations in Januaries from 2012 to 2018. We need to know individual heavy precipitation event in the month to get more details about the phenomena, thus precipitation per hour is drawn for Januaries from 2012 to 2018 in Figure 9 as (a) to (g). The precipitations which have rates exceeding 2 mm/h are chosen as "heavy" precipitations of the Region, because this rate value can clearly separate individual precipitation event within one month, in order to extract relative heavy precipitations. It should be noted that for one precipitation event, the time interval of non-precipitating should be less than 24 hours. There  Figure 10 illustrates the sum of the heavy precipitation duration for each January from 2012 to 2018. The January 2018 has the longest duration, which corresponds to 260 hours. The January 2013 has the minimum duration, which is 90 hours. The growth of the heavy precipitation hours is obvious from 2013 to 2018. However, the months with the peak precipitations such as January 2013 and January 2017 do not have the largest precipitation duration hours. Thus, for January 2018 it is the precipitation frequency which drastically increased comparing to other months, which makes it the longest precipitation month.

Precipitation Analyses of the Region
The heavy precipitation events in January 2018 of the Region are analyzed in details, concluding the precipitation locations and the atmospheric circulations.

Precipitation Locations
According to the locations of the precipitations, there are five types of events:     For the Northeast precipitation, the highest precipitation is in the northeast corner of the Region. The wind steps down from the north, and a trough is to the southeast of Japan, on the Pacific. The Southwest precipitation has relative high geopotential height comparing to the northeast corner in the Region. The West to east precipitation has many precipitation spots dispersing over the whole Region. The geopotential height is also higher in the southwest corner and lower in the northeast corner of the Region. For the East border precipitation, the precipitations in the Region are centralized at the east border to the Region. A low pressure trough is above the Sea of Japan. Finally, the Inland precipitation has a trough with its center at 118˚E 46˚N. The geopotential height is high to the south and low to the north.

Atmospheric Circulation Patterns
The atmospheric circulation for the Northeast precipitation is shown in Figure 12.
At 200 hPa the Region is dominated by west wind. At 500 hPa two cyclones are created, the first one is at 88˚E 48˚N, and the second is at 131˚E 45˚N. The northeast corner of the Region is mainly dominated by north wind induced by the second cyclone. Despite at 700 hPa the two cyclones are subdued, the northeast corner of the Region is still influenced by the north wind. At 850 hPa the first cyclone loses its pattern, while the second cyclone persists. Through the whole precipitation event, at 500 and 700 hPa a cyclone centered at 122˚E 48˚N is moving towards the second cyclone, and merges with it. At 850 hPa this moving cyclone does not appear. The second cyclone occludes throughout the precipitation event above the Sea of Japan. The Northeast precipitation of the Region is caused by the occluding cyclone above the Sea of Japan.
The Southwest precipitation is shown in Figure 13.   Figure 14 illustrates the West to east precipitation. At 200 hPa, the Region is dominated by west wind. At 500 hPa a convergence appears at 110˚E 44˚N in the central part of the Region. Stepping downward to 700 and 850 hPa, the convergence is gradually weakening until losing its patterns. This convergence is in forms of a cyclone at 500 and 700 hPa at the beginning hours of the precipitation. As it moves eastward, it losses cyclone patterns progressively. This time evolution process is not presented here because of its huge size in figures. At 850 hPa, no significant cyclone features are observed. This West to east precipitation is due to convergence caused by pressure pattern in the atmosphere. The East border precipitations are represented in Figure 15. At 200 hPa, northwest wind is prominent in the Region. At 500 and 700 hPa, north wind dominates, and a cyclone without clear patterns is formed at 133˚E 45˚N, near the Sea of Japan. At 850 hPa, that cyclone enforced its patterns, and it locates just above the Sea of Japan. During the precipitations, the cyclone above the Sea of Japan only appears at ground level. Consequently, the East border precipitation is caused by the cyclone above the Sea of Japan.
The Inland precipitation is shown in Figure 16. From 200 to 700 hPa the west wind dominates in the Region. A slight curvature of the geopotential line appears at 500 hPa at 115˚E 46˚N and enhances downward until 850 hPa to form a small cyclone. During the precipitation, at 200 and 500 hPa the Region is dominated by west wind. At 700 hPa, strong southwest wind invades the Region, interacting with west wind. This interaction leads to a small convergence in the wind field. At 850 hPa, a small inland cyclone is formed due to the active interactions of the two winds. That is, the Inland precipitation is caused by a ground breeze induced small cyclone.

Discussion
The five precipitation types in January 2018 are Northeast precipitation, Southwest precipitation, West to East precipitation, East border precipitation and Inland precipitation. Among the precipitations, the Northeast precipitation is caused by occluding cyclones above the Sea of Japan, while the East border precipitation is induced by the same cyclone but formed at sea level. The Southwest precipitation is wrought by the frontal effects between southwest moist wind and the northwest dry wind. The West to east precipitation is due to the change of pressure pattern inside the Region, and the Inland precipitation is induced by ground breezes.
Comparing to other typical precipitations through the 30 winters, the Coastal precipitation in January 2017 is not seen, while the West to east precipitation and Inland precipitation are additional types in January 2018. The former can also be contributed to existence of cyclones above the ocean surface because of its location, and the latter is mainly due to the pressure change and local breezes of the Region. The high amount of monthly total precipitation in January 2018 resulted from a relatively high precipitation frequency, rather than an augmentation of precipitation intensities. Thus, the high precipitation amount in Janu-

Conclusions
For the Region in January 2018, the maximum of the monthly total precipitation is 4 times more than that in other winter months for the recent 30 years. The highest value of precipitation mainly concentrates along the mountainous areas.