Abstract
Abstract. To probe the potential value of a radar-driven field campaign to constrain simulation of isolated convection subject to a strong aerosol perturbation, convective cells observed by the operational KHGX weather radar in the vicinity of Houston, Texas, are examined individually and statistically. Cells observed in a single case study of onshore flow conditions during July 2013 are first examined and compared with cells in a regional model simulation. Observed and simulated cells are objectively identified and tracked from observed or calculated positive specific differential phase (KDP) above the melting level, which is related to the presence of supercooled liquid water. Several observed and simulated cells are subjectively selected for further examination. Below the melting level, we compare sequential cross sections of retrieved and simulated raindrop size distribution parameters. Above the melting level, we examine time series of KDP and radar differential reflectivity (ZDR) statistics from observations and calculated from simulated supercooled rain properties, alongside simulated vertical wind and supercooled rain mixing ratio statistics. Results indicate that the operational weather radar measurements offer multiple constraints on the properties of simulated convective cells, with substantial value added from derived KDP and retrieved rain properties. The value of collocated three-dimensional lightning mapping array measurements, which are relatively rare in the continental US, supports the choice of Houston as a suitable location for future field studies to improve the simulation and understanding of convective updraft physics. However, rapid evolution of cells between routine volume scans motivates consideration of adaptive scan strategies or radar imaging technologies to amend operational weather radar capabilities. A 3-year climatology of isolated cell tracks, prepared using a more efficient algorithm, yields additional relevant information. Isolated cells are found within the KHGX domain on roughly 40 % of days year-round, with greatest concentration in the northwest quadrant, but roughly 5-fold more cells occur during June through September. During this enhanced occurrence period, the cells initiate following a strong diurnal cycle that peaks in the early afternoon, typically follow a south-to-north flow, and dissipate within 1 h, consistent with the case study examples. Statistics indicate that ∼ 150 isolated cells initiate and dissipate within 70 km of the KHGX radar during the enhanced occurrence period annually, and roughly 10 times as many within 200 km, suitable for multi-instrument Lagrangian observation strategies. In addition to ancillary meteorological and aerosol measurements, robust vertical wind speed retrievals would add substantial value to a radar-driven field campaign.
Highlights
Since the Intergovernmental Panel on Climate Change’s first scientific assessment report (IPCC, 1990), the conclusion has been generally strengthened that aerosol pollution from anthropogenic activities is likely to have commonly offset regional and global radiative forcing of the Earth’s climate by anthropogenic greenhouse gases to date, but uncertainty, especially in aerosol effects on cloud-related forcing, still remains high (IPCC, 2013)
The comparison of tracked cells from Houston Next-Generation Radar (NEXRAD) observations and a NASA Unified Weather Research and Forecasting (NU-WRF) simulation demonstrates the potential value of polarimetric weather radar observations for systematically observing and improving the understanding and simulation of convective cell physics
Future simulations could employ bin microphysics or other approaches to avoid errors associated with sedimentation or hydrometeor size distribution shape, as well as mixed-phase particle representation to improve forward simulation of polarimetric signatures (e.g., Ryzhkov et al, 2011; Kumjian et al, 2014a; Snyder et al, 2017a; Matsui et al, 2018b)
Summary
Since the Intergovernmental Panel on Climate Change’s first scientific assessment report (IPCC, 1990), the conclusion has been generally strengthened that aerosol pollution from anthropogenic activities is likely to have commonly offset regional and global radiative forcing of the Earth’s climate by anthropogenic greenhouse gases to date, but uncertainty, especially in aerosol effects on cloud-related forcing, still remains high (IPCC, 2013) Such anthropogenic aerosol radiative forcing will be diminutive relative to that from buildup of anthropogenic greenhouse gases on century timescales under most scenarios, the variable degree to which anthropogenic aerosols offset greenhouse gas warming in simulations that reproduce the observational record of surface temperature change since preindustrial times continues to be a leading factor limiting simulation constraints on Earth’s climate sensitivity (e.g., Kiehl, 2007). When coordinated with detailed high-resolution modeling, we argue that measurements optimized to observe convective cell evolution would be uniquely valuable for advancing understanding and accurate simulation of cloud processes such as entrainment and glaciation, thereby further addressing understanding of fundamental cloud processes relevant to climate sensitivity
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