Are long-term measurements at the ARM Southern Great Plains site adequate for detecting aerosol invigoration of deep convection?

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Description

Many studies have successfully used models to show that environmental aerosol properties may have a complex and wide array of impacts on properties of deep convective systems. One of the most commonly cited impacts is that of convective updraft invigoration by increases in cloud condensation nuclei (CCN), which operates through increased lofting and freezing of condensate. However, high-resolution simulations using a range of complex microphysics schemes are commonly biased with respect to convective cloud observations regardless of aerosol representation, and thus, observational support of models is needed. While a number of important studies using ground-based or satellite measurements have claimed support for the convective invigoration by aerosols hypothesis, some of them overstate the significance of their results for a number of reasons that will be highlighted. Results showing the complications of detecting aerosol invigoration of deep convection at the ARM Southern Great Plains (SGP) site will be presented using aerosol, cloud, and atmospheric state measurements over a 14-year period. They suggest that many more measurements are likely needed to adequately address this question and that both satellite and ground-based observations are necessary. If current measurements are indeed inadequate, then new questions emerge. What measurements are required? What accuracy and how many independent samples are required? Where are the best locations to make these measurements? In addition to current observational datasets, high-resolution simulations with bin microphysics may help to answer these questions. Results from idealized 2D WRF simulations will be presented that show that the impact of large increases in CCN on a continental squall line are comparable to impacts created by a 5% or lesser change in low-mid tropospheric water vapor mixing ratios and a 1 m s-1 km-1 change in vertical wind shear. Because the impact of CCN on convective system properties varies as a function of model setup, environment, and convective organizational mode, further tests need to be performed. However, these preliminary results suggest that the environmental properties surrounding convection may need to be known with fairly high accuracy to isolate an aerosol effect, and that the SGP site may not be an ideal location for observing aerosol impacts on convective systems because of large atmospheric and convective mode variability as well as consistently dirty conditions.