HWRF HC35/HDTR Forecast Comparison

HC35/HDTR Forecast Comparison Executive Summary

• The DTC conducted an extensive testing and evaluation activity to compare the error characteristics of two sets of runs: a control configuration similar to the HWRF 2013 operational configuration (HC35), and a variant (HDTR) in which the RRTMG radiation parameterization (invoked every 4h) was used in lieu of the GFDL radiation parameterization (invoved hourly) and the Thompson microphysics was used instead of the Ferrier one. Both configurations used boundary layer diffusivity parameter gfs_alpha=0.7.

• For each of the two HWRF configurations, 517 retrospective forecasts for the 2012 season of the eastern North Pacific (EP) and North Atlantic (AL) basins were conducted in order to produce a large sample from which robust conclusions could be derived.

•The experimental configuration (HDTR) slightly improved track errors in the AL, but degrade them in the EP.

• The impact of HDTR on storm intensity was mixed. In the EP, it decreased intensity, especially after 24h, resulting in a negative bias of up to 8 kt at 102 h, while the control had near zero bias. In the AL, the control had positive bias after the 12-h forecast lead time, stabilizing at 5 kt after 72 h. The HDTR configuration exacerbated this bias up to 48 h but acted to reduce the bias to near zero later in the forecast. Mean absolute intensity errors were lower for HDTR than for the control in the AL up to the 48-h lead time, but were higher in both basins at all lead times.

• The original intent of this test was to call the RRTMG radiation parameterization four times as often (every 15 minutes) than the GFDL radiation is called in the operational HWRF (every 60 minutes). An error in configuration led it to be called four times less often (every 4 hours). A follow up investigation was performed by the DTC to quantify the impact of this mistake on the results. Twenty-five cycles of Hurricane Daniel, a storm whose error characteristics were representative of the overall EP results, were run with RRTMG called every 15 minutes and compared against the control (HC35). Results indicated that the error increase in HDTR was not alleviated with this change.

• Based on the track and intensity results, EMC decided not to consider the HDTR innovations for operational implementation. Instead, additional investigation and development were recommended. The DTC followed up on that suggestion and, in 2014, developed and tested a partial cloudiness parameterization that was implemented along with RRTMG in the 2015 operational HWRF (more information here). Additionally, in 2015 the DTC is continuing to investigate and test the Thompson microphysics parameterization.

• The precipitation verification for Hurricane Sandy indicated that both configurations overpredicted precipitation when the entire parent domain or the subdomain centered around the track were considered. However, in the subdomain limited to the location of the storm at any given time, both configurations overpredicted earlier in the forecast but underpredicted towards the end. Sizeable differences in precipitation bias between the two configurations were noted only near the center of the storm, with HDTR leading to higher accumulations.

• Comprehensive results of large scale evaluation were provided, along with diagnostics of the storm-scale environment. A few hypothesis were raised, such as a possible link between negative bias in upper level winds and excessive storm intensity in the AL. Those hypotheses require further investigation.

    • Model output files have been archived and are available to the community for future studies. Verification graphics, along with additional information are available in the DTC website.