HRRRv4 upgrade June 2020 (also RAP to v5)

Jeff Duda

Resident meteorological expert
Staff member
Oct 7, 2008
Broomfield, CO
Quick brief:

Various NWS regional centers and science offices approved the proposed upgrades to the HRRR model at the science level in December 2019. Therefore, the upgrade of HRRR from its current version (v3, since June 2018) to version 4 is on-time and currently scheduled to become operational on 2 June 2020 (yes, towards the end of chase season). The remaining obstacles are a combination of procedural/bureaucratic & computational in nature, and thus are beyond the level of anything scientific or of forecast performance.

Some of the bigger system enhancements for this upgrade include:
-More accurate initial conditions from using a storm-scale ensemble component (HRRRDAS) to initialize each forecast (actually, the ensemble mean will be used); previous versions initialized off of the RAP
-Improved data assimilation through the use of storm-scale (HRRRDAS) background error covariances in the hybrid 3DVar-EnKF formulation, which should make better use of observations and further improve ICs; previous versions used coarser ensemble error covariances
-Increased forecast lengths (forecasts every 6th hourly cycle will go out to 48 hours); the other cycles may go out to 21 or 24 hours, but I can't remember that for sure.

As far as storm chasers are concerned, there are a limited number of changes that you will probably notice:
-Previous versions of HRRR have been too aggressive with mixing out the PBL during the day, thus usually resulting in higher afternoon temps/lower afternoon dewpoints, higher LCLs, and greater propensity to falsely initiate storms. Through the improvement related to handling sub-grid clouds in the PBL scheme, this tendency has been completely eliminated, and in fact has been slightly overcorrected. HRRRv4 will likely exhibit a slight cool and moist bias during the afternoon, and has shown a slight tendency to have delayed CI near the dryline on weakly-forced convective days. For synoptically forced events, it should be pretty damn good on timing.
-A new bug was introduced into the reflectivity diagnostic, which may cause larger-scale precipitating bodies (like squall lines, MCSs, MCCs etc.) to have anomalously high reflectivity in the stratiform regions. A fix has currently been suggested, but I do not know at this time if it has been implemented during testing. It is tentatively scheduled to make it into the final operational version, though.

Please ask any other questions if you have them. I bet a TIN will be released sometime in the early spring (Mar-Apr) detailing this in a complete manner.

As always: current real-time parallel runs of the experimental version (which is currently identical to the proposed operational version that has passed evaluations) can be found on GSD's website here: HRRR Model Fields - Experimental