Tropical Convection RP Report - December 2017

Nidhi Nishant has been working toward conclusion of her Ph.D. (anticipated at the end of 2017) on aerosol effects on tropical convection. Her paper "A Cloud-Resolving Model Study of Aerosol-Cloud Correlation in a Pristine Maritime Environment” came out in Geophysical Research Letters in June of this year, examining a highly publicised case of apparent aerosol-limited convective growth in the south Pacific. She found that cloud variations that had been ascribed to aerosol could actually be explained meteorologically using the WRF model, and that wind variations were simultaneously increasing aerosol and cloud amounts, producing a misleading correlation. Other work of hers has discovered a new indirect aerosol radiative forcing mechanism involving deep-convective adjustment; a paper on this is just being submitted.

Another UNSW student Maxime Colin has been using a cloud-resolving model to explore the inertia of convection associated with small-scale structures unresolved by global atmosphere models. He finds that there is modest inertia when convection is disorganised, associated with cold pools, but much more inertia when it is organised. This inertia is not considered in most convective schemes, which diagnose convection from the instantaneous resolved state. Maxime presented this work recently at the WCRP workshop on the Future of Cumulus Parameterisation in the Netherlands, and visited CoE partner institution IPSL in Paris where he plans to receive a joint Ph.D.

Former University of Melbourne honours student Andrew Brown has recently had his paper on satellite estimates of the sea breeze near Darwin accepted in the Quarterly Journal of the Royal Meteorological Society (QJRMS). As has been demonstrated by a number of prior ARCCSS studies the tropical sea and land breezes play a critically important role in the diurnal cycle of rainfall in the maritime continent, especially offshore. Measurements of the sea / land breeze system (SLBS) offshore are difficult to obtain, and Andrew used satellite derived scatterometer wind measurements to examine the composite structure of the SLBS off the coast of Darwin. He also examined how the SLBS varies with phase of the monsoon and compared the observations with high-resolution simulations from WRF and ACCESS. He found that the model predictions of the SLBS compared well to observations, implying that the difficulties models have reproducing the diurnal cycle are likely not related to the representation of the SLBS.

Another former Melbourne student Dr Rachel Badlan (now a postdoc at ADFA/UNSW Canberra) has recently had her paper on convective momentum transport (CMT) accepted in QJRMS. In this study she used idealised simulations of organised convection to examine the fundamental processes governing CMT and the assumptions underlying its parameterisation. Among other things she demonstrated that current parameterisations are unable to represent CMT by organised systems realistically because many of the underlying assumptions are only valid for disorganised systems. 

In terms of convection parameterisation development, Benjamin Moebis finished coupling a stochastic multi cloud model (SMCM) to the new Icosahedral non-Hydrostatic General Circulation Model (ICON), which is jointly developed by the German Weather Service (DWD) and the Max Planck Institute for Meteorology. The new stochastic modeling approach represents multiple cloud types and updrafts per climate model grid cell and timestep.

Also, Martin Bergemann developed a modeling framework to represent the occurrence of coastal clouds and rainfall in the tropics within numerical weather prediction and global climate models. The new approach first develops a decision algorithm, or trigger function, for the existence of coastal convection. The function is then applied in the above-mentioned SMCM to increase the occurrence probability of deep convection when land-sea interactions are diagnosed to be important. Martin's work has recently been submitted to the Journal of Advances in Modelling Earth Systems.

Two new research fellows, Bethan White and Malcolm King (former University of Melbourne PhD student), have recently joined the group at Monash University. Bethan is working on identifying and quantifying relationships between convective organisation and precipitation extremes in Australia. Malcolm is looking at changes in precipitation behavior in the western Indian Ocean.

Finally, there was a good showing of current and former ARCCSS staff and visitors at the recent American Meteorological Society 17th biennial conference on Mesoscale Processes held in San Diego, CA USA.

Abhnil Prasad presented his work on constraining WRF simulations of convective environments using observations. Todd Lane gave an invited keynote on turbulence caused by organised convection. Recent ARCCSS student visitors Thibaut Dauhut (CNRS, France) and Leah Grant (Colorado State University, USA) presented their research (conducted while visiting ARCCSS) on simulating tropical convection. Former ARCCSS postdoc Karsten Peters (now at MPI Germany) and AI Mitch Moncrieff (NCAR) were also at the meeting, which gave us the opportunity to reconnect and have many fruitful discussions.

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