Developments in the Met Office hourly 4D-Var system
In July 2017, the operational Met Office UKV model moved from 3-hourly 3D-Var to hourly cycling 4D-Var. To accommodate the more rapid cycle updates within our operational schedule we initially introduced a 45 minute observation cutoff on all cycles which led to the unfortunate loss of some pivotal observations, particularly lower tropospheric radiosonde data. We have subsequently increased the cutoff on 2 key cycles, 11Z and 23Z, which allows the recovery of the majority of the previously lost sonde data. We have also recently started to assimilate a limited number of visibility observations from roadside sites.
Upcoming changes include the re-introduction of a modified Moisture Incrementing Operator, which is less susceptible to precipitation spin-up issues than previous implementations, and also the assimilation of Mode-S aircraft wind data which leads to improved short range vertical wind profile forecasts as well as increased precipitation skill.
Slightly further ahead we anticipate the introduction of soil moisture data assimilation using a simplified Extended Kalman Filter to derive soil moisture increments from screen temperature and humidity observations, as well as ASCAT soil moisture estimates.
Future plans, which include radar reflectivity and hybrid DA, will also be briefly addressed.
Mr. Gareth Dow Met Office