Regional Heavy Snowfall Events over the Northwestern Tibetan Plateau Dominated by Distinct Circulation Patterns

Abstract The northwestern Tibetan Plateau (TP) is recognized as one of the primary heavy snowfall regions in the TP. Mechanisms dominating the occurrence and characteristics of the regional heavy snowfall (RHS) in the northwestern TP are important for understanding local climate change and ecological conservation but remain less known. In this work, a method is first developed to project station-observed snowfall thresholds onto ERA5 reanalysis, and a dataset of the RHS events in the northwestern TP during 1979–2020 is built. Two dominant circulation patterns [self-organizing map (SOM) 1 and SOM2] responsible for the RHS in the northwestern TP are achieved by using the self-organizing map methodology. Both types (SOM1 and SOM2) are featured by an anomalous cyclonic circulation over the northwestern TP, corresponding to the Tibetan Plateau vortex. Additionally, an anomalous high (low) is located north of the TP and an anomalous low (high) is located east of the TP in SOM1 (SOM2). Compared to SOM1, the snowfall intensity is larger in SOM2, with the maxima positioned further north. More intense snowfall in SOM2 is attributed to the more abundant moisture and the less atmospheric stability in the study area (SA). Regarding the locations of snowfall areas, although the vertically integrated moisture convergence is centered in southern SA in both SOM1 and SOM2, intense snowfall is also observed in northern SA, particularly in SOM2. Further investigation demonstrates that the locations of snowfall areas are greatly determined by the distribution of hydrometeor responding to the environmental winds and the precipitation efficiency. Significance Statement The northwestern Tibetan Plateau (TP) is a critical nature reserve and primary snowfall region globally. Understanding heavy snowfall mechanisms there is essential for understanding changes in snowpack, water resource, and ecological risk in the TP, but these mechanisms remain largely unknown. This work identifies two dominant circulation patterns driving the regional heavy snowfall there, both featuring a TP vortex but differing in surrounding conditions. We reveal that distinct snowfall intensity arises from different moisture conditions and atmospheric stability, and spatial distribution differences stem from the response of cloud particles to the winds and the spatial heterogeneity of precipitation efficiency. The new findings in this work help understand climate change and ecological conservation in the TP.