From late December 2022 to mid-January 2023, California experienced a series of nine consecutive atmospheric rivers (ARs), resulting in significant impacts. Despite the devastating consequences of these events, there has been limited research on the spatiotemporal compounding of ARs, a type of low-likelihood high-impact (LLHI) events, and their effects. In our study, we introduce an unsupervised machine-learning technique to define an AR cluster as a short period characterized by consecutive ARs. We demonstrate that cluster density, defined as the fraction of AR conditions within a cluster, significantly influences their characteristics and impacts. Focusing on the ARs making landfall along the U.S. West Coast, we discovered that clusters with high density feature higher AR categories and a greater likelihood of extreme precipitation and severe land surface responses. We identified that the primary circulation patterns driving AR clusters are linked to subseasonal variability. Our findings also suggest that AR clusters of higher density and category are likely to become more frequent in a warming climate. Beyond the connection between large-scale circulation and AR cluster activity, the response of the land surface, particularly runoff, to ARs remains an area for further exploration. We hypothesize that runoff's response to ARs varies by AR category, the spacing between AR events (i.e., clustering), and land preconditions such as soil moisture and snow. Our results show that higher AR categories lead to higher runoff peaks and longer recovery times. Back-to-back ARs with short spacing significantly increase the portion of active snowpack, and lead to a 100% increase in runoff response. Our study underscores the critical role of AR clusters in climate adaptation and resilience planning. The runoff response to ARs has significant implications for estimating local responses to consecutive ARs and for flood risk assessment in AR predictions. Future investigation includes understanding how AR clusters connect to other LLHI events and how land surface responses to AR change with the warming climates.