Simulation and analysis of the mechanism of a downburst process

Using conventional surface observations, Doppler radar imagery and products of the RMAPS model, a gust caused by a downburst event occurred on July 9, 2017, in Shunping of Hebei Province was studied. The results showed that: (1) The northwesterly flow from the convective cloud and below the cloud is affected by precipitation, which resulted in the downward momentum transport accompanied by surface divergent winds. The subsidence center near the surface is above the gust region. (2) Simulation showed the rain and graupel/hail with high mixing ratio. The moisture contribution to the growth of hail embryos was mainly from snow and cloud water. Rain drops increased with graupel/hail melting. (3) Compared to equivalent cooling temperature with the cloud and rain water, rainfall evaporation contributes the most to the gust, followed by the equal contribution from the melting mechanism of hail and the dragging effect, and the contribution of rainfall dragging is greater than that of hail melting below 700hPa. (4) The downdraft, together with surface divergence of cold pool flow led to an extreme gust with a new record in downstream of the surface divergence center. In addition, the surface wind speed was reduced downstream of the extreme winds due to the heating effects of latent heat release by cloud water condensation and hail embryo growth, which offset partly cooling effects of water, as well as weakened outflow at surface.