Assessment of Convective-scale Attributes of the FV3 Dycore using Idealized Simulations

Over the past several years the Rapid Refresh Forecast System (RRFS) component of the Unified Forecast System (UFS) has been continuously under development and evaluation by a number of NOAA units (i.e., the Global Systems Laboratory (GSL), the Environmental Modeling Center (EMC), and the National Severe Storms Laboratory (NSSL)). The RRFS is configured to replace the current High Resolution Rapid Refresh system (HRRR) sometime during late 2024. RRFS development via the NOAA units have resulted in considerable advancement and improvements in its forecasting performance. During the warm season, however, the depiction of individual convective storms and their characteristics have shown some noticeable differences from our current operational convective allowing models (CAMs) such as the HRRR.

To understand the origin of these issues, a new set of idealized convective storm simulations for CAMs have been created to benchmark the expected shapes, sizes, and intensities for a variety of convective storms and systems. While the original FV3 system was tested using a single splitting supercell case where updraft maximum and precipitation were examined (Ullrich et al. 2017), a broader range of environments is needed to provide a wider set of statistics to help assess how a prediction system represents convection. Whenever possible, comparisons with real data are critical. However, due to the lack of in-situ observations from within individual storms (e.g., vertical velocity, temperature, hydrometeor contents) some comparison with other convective scale models is inevitable, particularly when those models produce forecasts of convection that have rather good fidelity with the available observations (e.g., reflectivity, storm-type, storm rotation, and precipitation).

This work summarizes NSSL’s evaluation of the FV3 dycore¹ for the prediction of individual convective storms. Over the past two years a set of analyses using idealized simulations, have compared FV3 dycore to current operational/research models. It is found that idealized simulations can replicate some of the remaining attributes and biases seen in the current Rapid Refresh Forecast System candidate scheduled for operation implementation in late 2024. An example of real data forecast biases for precipitation is shown in Fig. 1. The precipitation frequencies from the RRFS are excessively larger than the HRRRv4 model for amounts greater than 1 inch. The biases arise from very intense “popcorn” convection in the southeast U.S. summer time regime. Figure 2 shows total accumulated precipitation from 6 hour long idealized squall line simulations using the FV3, WRF, and CM1 models. All three simulations are configured as similarly as possible, and use Kessler microphysics to further help make the comparisons as close as possible. The FV3 solutions consistently produce the largest precipitation accumulation for all four CAPE values, but most especially for the lowest CAPE case, which may be analogous to the southeast summertime environment (weak shear, moderate CAPE). This behavior appears to replicate the biases from the real data biases seen in Fig. 1 and is also present in environments having somewhat larger vertical wind shear in the squall line regime.

The presentation will discuss attributes associated with precipitation, updraft intensity, as well as a measure of the model’s effective resolution of the individual updrafts. Importantly, a new analysis will show that the vertical velocity field in the FV3 dycore is only an approximate, representation of the true vertical velocity associated with the vertical coordinate’s Lagrangian motion. This approximation has implications for UFS research as well as for data assimilation and off-line aerosol transport, especially in the stratosphere where the differences are largest.

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¹We adopt the GFDL convention that there is a distinction between a dynamical core such as FV3 and its transformation into a full NWP prediction model (e.g., the UFS GFS or RRFS) which also includes a large number of physical packages, a data assimilation system, etc. A simplified version of the FV3 dycore, called “Solo”(Harris et al 2020) is used for the idealized simulations with minimal physics.

References

Harris, L., and Coauthors, 2020b: GFDL SHiELD: A Unified System for Weather-to-Seasonal Prediction. Journal of Advances in Modeling Earth Systems, 12 (10).

Ullrich, P. A., and Coauthors, 2017: DCMIP2016: a review of non-hydrostatic dynamical core design and intercomparison of participating models. Geoscientific Model Development, 10, 4477–4509.