A ubiquitous ice size bias in simulations of tropical deep convection

Mckenna Stanford , Adam Varble , Ed Zipser , John Walter Strapp (1) , Delphine Leroy (2) , Alfons Schwarzenboeck (3) , Rodney Potts (4) , Alain Protat (4)

1 Met Analytics Inc. [Toronto]
2 UMR CNRS 8179
3 LaMP - Laboratoire de Météorologie Physique
4 BoM - Australian Bureau of Meteorology [Melbourne]

The High Altitude Ice Crystals-High Ice Water Content (HAIC-HIWC) joint field campaign produced aircraft retrievals of total condensed water content (TWC), hydrometeor particle size distributions (PSDs), and vertical velocity (w) in high ice water content regions of mature and decaying tropical mesoscale convective systems (MCSs). The resulting dataset is used here to explore causes of the commonly documented high bias in radar reflectivity within cloud-resolving simulations of deep convection. This bias has been linked to overly strong simulated convec-tive updrafts lofting excessive condensate mass but is also modulated by parameterizations of hydrometeor size distributions , single particle properties, species separation, and microphysical processes. Observations are compared with three Weather Research and Forecasting model simulations of an observed MCS using different microphysics param-eterizations while controlling for w, TWC, and temperature. Two popular bulk microphysics schemes (Thompson and Morrison) and one bin microphysics scheme (fast spectral bin microphysics) are compared. For temperatures between −10 and −40 • C and TWC > 1 g m −3 , all micro-physics schemes produce median mass diameters (MMDs) that are generally larger than observed, and the precipitating ice species that controls this size bias varies by scheme, temperature , and w. Despite a much greater number of samples, all simulations fail to reproduce observed high-TWC conditions (> 2 g m −3) between −20 and −40 • C in which only a small fraction of condensate mass is found in relatively large particle sizes greater than 1 mm in diameter. Although more mass is distributed to large particle sizes relative to those observed across all schemes when controlling for temperature, w, and TWC, differences with observations are significantly variable between the schemes tested. As a result, this bias is hypothesized to partly result from errors in parameterized hydrometeor PSD and single particle properties, but because it is present in all schemes, it may also partly result from errors in parameterized microphysical processes present in all schemes. Because of these ubiquitous ice size biases, the frequently used microphysical parameterizations evaluated in this study inherently produce a high bias in convective re-flectivity for a wide range of temperatures, vertical velocities, and TWCs.

Domaines: Océan, Atmosphère

Mckenna Stanford, Adam Varble, Ed Zipser, John Walter Strapp, Delphine Leroy, et al.. A ubiquitous ice size bias in simulations of tropical deep convection. Atmospheric Chemistry and Physics, 2017, 17 (15), pp.9599-9621. ⟨10.5194/acp-17-9599-2017⟩. ⟨hal-01980956⟩