Research progress on horizontal convective rolls：A review

Horizontal Convective Rolls (HCRs) are defined as horizontal vortices that rotate in the opposite direction within the convective boundary layer. They are one of the common forms of shallow convection in the atmosphere. HCRs can cause strong turbulence and water vapor mixing in the boundary layer as well as exchanges of mass, momentum, and heat flux between the boundary layer and the free atmosphere. Meteorologists have conducted systematic research on structural characteristics, formation mechanisms, and impacts on the boundary layer of HCRs through field observation experiments, theoretical derivation, flume experiments, and numerical simulations. The results indicate that inflection-point instability and thermal instability are the main mechanisms for the formation of HCRs. The organized turbulent transport associated with HCRs can cause non-uniform distribution of flux in the horizontal direction of the boundary layer. The vertical motion, high specific humidity, and positive temperature anomalies in the ascending branch of the HCRs provide favorable conditions for cold-flow snowstorm and deep convection. At present, large eddy simulation is the main numerical method for studying HCRs. However, the mechanisms by which HCRs trigger heavy snowfall and independently trigger deep convection are still unclear.It is recommended that greater use be made of new remote sensing data in the construction of HCRs' three-dimensional structural models and in the analysis of shallow convective cloud morphology. Cloud penetration experiments on cold-flow snow processes should be conducted to explore the effects of HCR-related aerosols and flux transport on ice microphysical processes. Based on a clear understanding of the structural characteristics of HCRs and the environmental conditions favorable for deep convection initiation, a nowcasting method for HCR-triggered deep convection should be developed using key precursor factors to improve the operational forecasting of HCR-induced hazardous weather.