High-resolution analysis of the initiation of deep convection forced by boundary-layer processes

The initiation of atmospheric convection is investigated using the synergy of different instruments. The impact of increased spatial data resolution on the detection of the initiation of deep convection is analyzed, and a methodology is developed and applied to determine the likelihood of deep convection over flat and complex terrains. With this purpose, Intensive Observation Periods (IOPs) are used from the Convective Storm Initiation Project (CSIP), and from the Convective and Orographically-Induced Precipitation Study (COPS).
Events with nonexistent or weak synoptic scale forcing were chosen to better understand the Planetary Boundary Layer (PBL) processes which play a critical role in the initiation of convective storms. To describe the likelihood of convection, convection-related indices are used representing atmospheric stability, convective available energy, inhibition and triggering mechanisms. The sensitivity of convection indices to the temperature and moisture variability observed in the PBL is investigated using high-resolution aircraft measurements. These measurements demonstrated that despite the dense radiosonde network deployed, the high spatial variability of the temperature and especially of the water vapour in the boundary-layer cannot be resolved only with radiosonde data. ... mehrAn increase of the spatial data resolution was obtained using the synergetic effect of data from the networks of radiosondes, AWS, SYNOP, and especially GPS stations. A new algorithm, 'adjustment method', is introduced to combine the IWV data from the GPS with the radiosonde profiles. This combined data set is used to calculate the spatio-temporal evolution of convection-related indices. The gained spatial resolution and the combination of convection-related indices presented successfully represented the areas where the likelihood of deep convection was higher.
Special emphasis is put on the investigation of the spatio-temporal evolution of low-level and mid-tropospheric stable layers or lids and their role in the inhibition of convection. The altitude, strength and persistence of these stable layers were observed to be important for the initiation of convection. High-resolution near-surface observations allowed a detailed localisation of convergence zones, whose role in deep convection triggering is seen to be decisive in all the investigated cases, even in cases where low atmospheric inhibition was observed. Location and timing of the initiation of convection were critically influenced by the structure of the humidity field in the boundary layer. Thus, it is demonstrated that PBL conditions exert a great impact on the initiation of convection because directly determine the convective indices, and surface and PBL processes often initiate convection. Moreover, it is seen that convergence and low CIN influenced the triggering of stor ms more than special high values of CAPE. However, the storm intensification was dominated by high CAPE and high mid-tropospheric moisture. From the joint evaluation of all CSIP IOPs investigated, some thresholds were established for the CSIP region. The thresholds confirm that convective instability and capping inversions in that region are often quite weak on convective occasions and triggering is controlled by boundary layer convergence lines. Furthermore, a 2-D diagram representing the atmospheric conditions required for the initiation of convection is presented for the prediction of deep convection.