Back-trajectory Analyses of Water Vapor in Northern Mongolia

Knowledge of precipitation sources is indispensable for prediction of extreme events as droughts and flood [Dirmeyer and Brubaker, 1999]. In this paper, the transport pathways of water vapor that precipitates in northern Mongolia were identified using back-trajectory analyses in order to find out factors causing such events in arid/semi-arid area. First, a back-trajectory model of atmospheric water vapor was developed. An air parcel is placed on an isentropic plane over the target site at each time of precipitation. Then, back trajectories was calculated with a kinematic method following the implicit technique [Merrill et al., 1986; Merrill, 1989]. Each of the air parcels was tagged with the precipitation time and the altitude, and then tracked back in time for 5 days on the isentropic surface. Japanese 25-year Reanalysis/JMA Climate Data Assimilation System (JRA-25/JCDAS) of Japan Meteorological Agency [Onogi et al., 2007] was used for 3D field of meteorological variables for the calculation. As a validation, the model was compared with two others, namely, Meteorological Data Explorer of the Center for Global Environmental Reserch (METEX/CGER) [Zeng et al., 2003], and the trajectory model of the National Institute of Polar Research (NIPR) [Tomikawa and Sato, 2005]. The comparison found that model results are fairly robust within 5 days from the computational start, i.e., the end of the trajectory, regardless of different datasets and different schemes employed in these models. Then, the back-trajectory model was applied to the observed precipitation at the target site, a surface station in northern Mongolia called Kherlenbayan-Ulaan(KBU), where highly accurate and temporarily dense precipitation measurements are available. Back trajectory lines were calculated for each of the observed precipitation during the warm season of the years 2003 to 2009, on the isentropic surfaces of 300K, 310K and 320K where the highest value of water vapor is observed. The results show that, in general, the back trajectory lines spread toward the north and the west region of the target site. This indicates that, the source regions of water vapor precipitated at KBU are located in the region which includes central Asia and Siberia, toward the Atlantic Ocean and the Arctic Ocean. This is consistent with the result of previous study [Sato et al., 2007], which used a regional climate model (RCM). It was found that the year 2003 had a significant difference from the other years in the following points: In this year 2003, 1) the total precipitation was anomalously large, 2) the number of trajectory lines traveling from the north was larger than in the other years, 3) the value of specific humidity above KBU tended to be higher with the northern trajectory lines than with the western trajectory lines, and finally, 4) the amount of precipitation associated with the northern trajectory lines was larger than that in other years. From these results, it can be hypothesized that with the more trajectory lines traveling from the north the more water vapor was brought and caused larger precipitation during the warm season at the northern Mongolia.