Search

When precipitation evaporates in a sub-saturated boundary layer, it cools the air and produces dense downdrafts, which flow towards the surface and can spread horizontally as a gust front. These spreading “cold pools” (CPs) can trigger convection and thus new precipitation events due to dynamical and thermodynamical lifting mechanisms. Due to their role in the local organization of convection, CP properties are currently being studied with the use of high-resolution numerical simulations. Measurement campaigns have been conducted over the ocean to validate the models. However, fewer studies have specifically targeted cold pools over land.We use the observational network of the Netherlands (meteorological stations and radar) to study CPs developing from summer convection and their role in triggering new convective events over land. Detailed information about CP gust fronts in terms of temperature, wind speed, heat fluxes, moisture and pressure at high vertical resolution is obtained from time series, measured at the 213-meter Cabauw tower. We aim to create an algorithm that detects the passage of a CP from the tower time series to automatize the finding of CPs from a point measurement. To confirm the results, we have access to temperature time series from a spatially dense crowdsourcing weather station network (WOW-NL).The properties of the detected CPs are further studied with imagery from the Herwijnen Doppler radar, situated in proximity to the Cabauw tower. We can see clear signatures of spreading CPs in reflectivity plots, probably caused by the upwelling of dust and insects in the gust front. We currently explore how this can serve as a direct way of visualizing the dynamics of CPs and their collisions.With enough observations of CPs, we expect to learn more about the CP spreading velocity and lifetime in dependence of precipitation intensity of the generating precipitation cell and eventually triggered cell. This link will help gain more insight into the role of CPs in organizing convection over land.

We provide a detailed analysis of convectively generated cold pools (CPs) over flat midlatitude land, combining ten-year high-frequency time series of measurements at several heights available from the 213-m tower observatory at Cabauw, the Netherlands, with a collocated 2D radar rainfall dataset. This combination of data allows us to relate observations of the CP's temporal and vertical structure to the properties of each CP's parent rain cell, which we identify by rain-cell tracking. Using a new detection method, based on the anomalies of both the vertically averaged wind and the temperature, we monitor the arrival and passing of 189 CPs during ten summers (2010-2019). The time series show a clear signature of vortex-like motion along the leading CP edge in the vertical and horizontal wind measurements. The arrival of CP gust fronts is characterized by a steep decrease in both temperature and moisture, with a recovery time of approximately two hours. We see no evidence of moisture rings on the gust front edge, and therefore no indications for thermodynamic convective triggering. From the tower data, we obtain a median CP temperature drop of Tdrop approximate to-2.9 K and a height-averaged horizontal wind anomaly of Delta umax approximate to 4.4 m center dot s-1. Relating the individual CP's horizontal wind anomalies and temperature drops, we confirm the validity of the theoretical density current relationship, Delta umax proportional to Tdrop1/2. We further propose a simple statistical model to relate the CP strength defined by Tdrop to the environmental properties influencing the CP: rain intensity and lower boundary-layer saturation. A multivariate linear regression suggests a 1 K colder CP for a 4 mm center dot hr-1 more intense rain cell (instantaneous area-averaged rain intensity) or for a 2.5 K larger pre-CP dew-point depression.