Spatiotemporal Distribution of Precipitation Recycling across the Arid Regions of Asia and Africa

The degree of water vapour recycling in terrestrial precipitation is still a debatable topic, particularly in arid regions. Here, the spatiotemporal evolution of evaporation, calculated via the water balance method [1], and the precipitation recycling ratio, calculated using a water recycling model [2], are investigated across Asia and Africa for the 1984-2013 time period. The results show that the precipitation recycling ratio in North Africa and China-Mongolia is stronger in summer but weaker in winter. However, it is stronger in winter and spring in West Asia but weaker in summer. Evaporation accounts for a small proportion of the precipitation uptake in arid regions, with external water vapour transportation exerting the primary influence on precipitation recycling. Increasing global temperatures and evaporation potentials over the past 30 years have driven the actual evaporation and precipitation recycling ratio increases in North Africa and West Asia and corresponding decreases in China-Mongolia.


Introduction
The precipitation forming mechanism in arid regions continues to be a debated topic.There are two key water vapour components that lead to precipitation, local evaporation and external water vapour transportation [3].The total oceanic water vapour evaporationis 4.2 × 10 5 km 3 •a −1 , but only 3.9 × 10 4 km 3 •a −1 is transported to land, whereas terrestrial evaporation accounts for 8.1 × 10 4 km 3 •a −1 , the same as in Thomas [14], and are identified in Figure 1.Drought, one of the major calamitous climate disasters, is generally driven by one of two primary aspects in arid regions, the sinking of tropical Hadley circulation or topography [1].The three studied arid regions are representative of these two typical types of arid regions.North Africa and West Asia appear to be primarily influenced by the sinking movement of the radial Hadley circulation in subtropical areas, whereas the topography of China-Mongolia makes it difficult to transport large amounts of water vapour due to the long distance from the ocean and the large orography of the region.

Methods
Here evaporation is computed from the water mass balance with atmospheric moisture changes neglected [1].For the multi-year average water balance, its factors use the average value for many years.During a long period of time, the change in water vapor in the atmosphere tends to be balanced.And the water balance equation can be simplified [16] [17]: is the divergence term of the moisture flux and Q (kg•m −1 •s −1 ) is the precipitable water, which is the vertically integrated water vapour flux.This method is one of the simplest methods to calculate evaporation, and many previous studies have shown that the monthly estimates from this scheme are reliable over long timescales [18].
The water vapour source in the air above an area consists of two components, local evaporation and water vapour that has been transported into the area from the land and ocean surfaces.The precipitation recycling in a region is the ratio of precipitation with an evaporation source to total precipitation, which indicates the proportion of local water circulation in total precipitation.Here the DRM [2] where x and y are the horizontal Cartesian coordinates, and t is the time.The evaporation E, precipitation recycling ratio ρ (mm•mm −1 ) and precipitable water Q are then converted to the Lagrangian coordinate system as follows: where ε (mm•d −1 ), R (mm•mm −1 ) and ω (kg•m −1 •s −1 ) are the evaporation, precipitation recycling ratio and precipitable water in the Lagrangian coordinate system, respectively.
The assumption of a well-mixed atmosphere allows the precipitation recycling ratio to be rewritten as

Result and Analysis
Evaporation rates of less than 3 mm•d −1 are observed at high latitudes (45˚ -75˚N), with rates of higher than 4 mm•d −1 observed at mid-low latitudes (0˚ -45˚N) (Figure 2 Precipitation recycling is a robust component of precipitation formation, particularly in terrestrial precipitation, which has strong regional characteristics The precipitation recycling rate calculated by Li [19] shows that the arid regions are not the minimum precipitation recycling ratio centres since they possess much larger values than other regions.However, the results of Trenberth [5] and Ent, Savenije [20] show that the annual precipitation recycling ratio in North Africa, West Asia and China-Mongolia is less than 0.03, which is similar to our results.Differences arise at the high latitudes, though, with a much stronger precipitation recycling ratio estimated here.Dirmeyer et al. [21] showed that the precipitation recycling ratio is higher at high latitudes, with different evaporation patterns being the primary reason.The evaporation limited by the water supply is much smaller than that by Penman evaporation, such that the proportion of precipitation due to evaporation relative to the total precipitation would be small. The precipitation recycling ratio in the three arid regions has similar characteristics.Compared with it in the surrounding areas, arid regions are always the small value regions.Evaporation has little effect on precipitation in the three arid regions.The minimum precipitation recycling ratio is observed in the arid regions, and the evaporation vapour contribution to precipitation is smaller in the arid regions than in the other regions.This means that precipitation in arid regions is heavily dependent on water vapour transportation, with evaporation contributing less to precipitation.Water vapour transportation is an important part of the global water cycle and plays an important role in regional water balances.It is therefore necessary to understand the water circulation transport function to fully explain how water recycling can be sustained.Figure 4 shows the average water vapour transport flux and its divergence from 1984 to 2013.Water vapour transportation via the west wind belt plays an important role in the annual moisture supply to the three arid regions (Figure 4(a)).The moisture entering North Africa is mainly supplied via the West Atlantic inflow, and the main sources of water vapour in West Asia are the Red Sea and Mediterranean Sea.However, the main water vapour sources for China-Mongolia are land based.The corresponding water vapour flux divergence field shows that the water vapour transportation over China-Mongolia and West Asia is mainly manifested as water vapour convergence.However, convergence and divergence zones lie to the west and east of North Africa, respectively.Water vapour flux is stronger in China-Mongolia in summer (Figure 4(c)) and weaker in winter (Figure 4(e)), whereas it is stronger in West Asia from winter to spring and weaker in summer.The summer moisture in North Africa is sourced from the northeast (Figure 4(c)).The water vapor sources of precipitation in arid regions are mainly upwind regions.However, something different with the sources for arid regions, the area of water vapor that affects North Africa and West Asia is the surface of the ocean, but the area that affects China-Mongolia is the land.This can be indirectly explained, the formation reason of the three arid regions is different.
There has been a significant increase in global temperatures over the past 30 years, particularly in the arid regions of the Northern Hemisphere [22].However, the effect of this increase in temperature on evaporation and the precipitation recycling ratio in arid regions is still debated.Figure 5 indicates that the arid regions have undergone minimal changes in evaporation and the precipitation recycling ratio during the 1984-2013 time period.Li [19] results revealed that Penman evaporation increases in these arid regions with increasing temperature.However, the actual evaporation decreases in North Africa and West Asia, and the annual evaporation gradient is 0.11 and 2.23 mm•a −1 , respectively.The evaporation rate ranges from 0.2 to 0.4 mm•d −1 in North Africa and from 0.2 to 0.7 mm•d −1 in West Asia.The trend is weakly increasing in China-Mongolia.The evaporation contribution to precipitation decreases in North Africa and West Asia but increases in China-Mongolia, which differs from the results in Li [19] and Dominguez and Kumar [23].The annual average precipitation recycling ratio ranges from 0.02 to 0.05, 0.01 to 0.02 and 0.01 to 0.05 in China-Mongolia, North Africa and West Asia, respectively.

Discussion and Conclusions
This study investigated the evaporation characteristics in the arid regions of Asia and Africa using the precipitation recycling ratio to compare the key features of these arid regions.The main results are summarized as follows.The spatial distribution of precipitation recycling is obvious, with an observed minimum in arid regions that increases significantly in the high-latitude arid regions.There are also significant seasonal changes.The precipitation recycling ratio in North Africa and China-Mongolia is higher in summer but lower in winter.However, precipitation recycling in North Africa is much stronger in winter and spring but weaker in summer.
Water vapour transport is the main precipitation source in arid regions.The main source of water vapour for North Africa is the Atlantic Ocean, which is to the west of this arid region.Water vapour is transported from the Red Sea and Mediterranean to the West Asia arid region.China-Mongolia is affected by terrestrial water vapour transmission from the west.Water vapour transportation over the arid regions is primarily manifested as water vapour convergence.
The precipitation recycling ratio is mainly dependent on evaporation.Precipitation recycling in arid regions is recalculated on the basis of evaporation via the water mass balance method.The results show that precipitation recycling is low in arid regions due to the low water supply via evaporation, and the contri-

Figure 1 .
Figure 1.Annual precipitation (mm) distribution across Asia and Africa, with the main arid regions (red boxes, the left box for North Africa, the middle box for West Asia, and the right box for China-Mongolia) calculated for the 1984-2013 time period.
(a)).The minimum evaporation across the three studied arid regions studied is less than 0.5 mm•d −1 .Seasonal variations in the evaporation rate are much stronger in summer and weaker in winter across much of the Northern Hemisphere.For example, the evaporation rate in China-Mongolia and North Africa is around 0.8 mm•d −1 in summer, decreasing to less than 0.5 mm•d −1 in the winter.However, the evaporation rate in West Asia is stronger in winter, at around 1.2 mm•d −1 , and weaker than 0.4 mm•d −1 in summer.West Asia is affected by the Mediterranean climate, with more precipitation in winter and spring, whereas North Africa and China-Mongolia are affected by the monsoon climate, with more precipitation in summer.Evaporation in arid regions is always the minimum in the North Hemisphere and the seasonal evaporation characteristics yield trends that are similar to the precipitation patterns in arid regions.This highlights that evaporation in arid regions is primarily limited by the lack of water vapour, with the arid regions possessing the minimum evaporation values throughout the year.