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≈−2.9 K and a height‐averaged horizontal wind anomaly of Δumax≈ 4.4 m·s−1. Relating the individual CP's horizontal wind anomalies and temperature drops, we confirm the validity of the theoretical density current relationship, Δumax∝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·hr−1 more intense rain cell (instantaneous area‐averaged rain intensity) or for a 2.5 K larger pre‐CP dew‐point depression.

<p>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.</p><p>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).</p><p>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.</p><p>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.</p>

<p>Within the atmospheric modelling community, a large focus in recent years has been on the concept of Convective Self-Aggregation (CSA): In an environment of radiative convective equilibrium, with homogeneous initial conditions and a constant-temperature tropical sea surface, convection can spontaneously aggregate into domain-wide patterns of persistent dry areas and constrained rainy areas over a temporal timescale of weeks to months. CSA, albeit still a modeling paradigm, could reveal the mechanisms behind some of the convective organization observed in the tropics.</p> <p>This process of forming domain-wide structure can be accelerated to the order of days by imposing oscillating surface temperatures with a large enough amplitude [1]. The ‘diurnally aggregated’ cloud field is similar to CSA as it also constrains the surface rain field to certain parts of the domain. Further, pattern formation was found to initiate first as persistent dry patches in the uppermost layers of the simulated atmosphere. The dry patches subsequently penetrate through to the subcloud layer [2].</p> <p>In this work we investigate how diurnal surface temperature amplitudes, typical of tropical land, affect the formation of persistent dry patches and the spatio-temporal extent of the emergent mesoscale convective systems. We run a set of cloud resolving simulations initialized with typical profiles of temperature and humidity. We impose a large-amplitude diurnally oscillating surface temperature, which we then set to constant at different times, to see the effect on the diurnally aggregated cloud field. We present the results of this study, which show a strong dependence on the degree of aggregation over ‘land’, in determining the aggregation over ‘sea’, and a form of hysteresis arises.</p> <p> </p> <p>1. Haerter, Jan O., Bettina Meyer, and Silas Boye Nissen. ‘Diurnal Self-Aggregation’. <em>Npj Climate and Atmospheric Science</em> 3, no. 1 (30 July 2020): 1–11. https://doi.org/10.1038/s41612-020-00132-z.<br />2. Jensen, Gorm G., Romain Fiévet, and Jan O. Haerter. ‘The Diurnal Path to Persistent Convective Self-Aggregation’. <em>Journal of Advances in Modeling Earth Systems</em> 14, no. 5 (2022): e2021MS002923. https://doi.org/10.1029/2021MS002923.</p> <p> </p>

<p>Mesoscale convective systems (MCSs), long-lived convective clusters spanning more than 100 km horizontally, are known to be the dominant source of rainfall in the tropics, and the longest-lived MCSs are shown to be largely responsible for tropical extreme precipitation [Roca and Fiolleau, 2020]. Globally, the most extreme storms tend to be located over land, and the most intense storms over oceans tend to be adjacent to land, where motion is favored from land to ocean, e.g. tropical West Africa and the adjacent Eastern Atlantic Ocean [Zipser et al., 2006]. These systems are organized and maintained by the atmospheric characteristics needed for deep convection (moisture, instability, and lift), and the presence of vertical wind shear [Schumacher and Rasmussen, 2020]. Dry soils seem to have a large influence on strengthening organized convection [Klein and Taylor, 2020]. However, the mechanisms behind the intensification or dissipation of MCSs advected from land to sea are not well established yet.<br>To address this shortcoming, we investigate the evolution of MCSs emerging from satellite data over tropical Africa and the Eastern Atlantic Ocean. We use a database of tracked MCSs from infrared satellite data, TOOCAN [Fioellau and Roca, 2013]. Using these data we built a lagrangian tracker by which groups of MCSs - occurring in spatial proximity of each other with a 15 deg x 15 deg patch - are followed. We study the evolution of the cloud field within the patch, initiated at the time and latitude of maximum convective activity in the season. We superimpose a collocated satellite precipitation dataset, IMERG, to gain insight into the precipitation field related to the tracked MCSs, and study the environmental properties (temperature, wind profiles) using ERA5 reanalysis datasets. Over land, we find (i) that the MCS cover exhibits a clear diurnal cycle with peaks in the late afternoon and (ii) the lagrangian patch moves with a near constant velocity. Over ocean, we find a (i) decrease of the MCS cover which does not correspond to a decrease in precipitation and that (ii) at times the MCS evolution becomes stationary, corresponding to near-zero wind profiles. By generalizing these results to five years of tracked MCSs, we aim to gain insight into what environmental conditions are necessary for the development of strongly organized MCS fields over the coastal regions and the ocean, which, if persistent in time, could eventually evolve into tropical cyclones.</p>