A validation of subtropical marine low clouds in the HARMONIE regional weather model

Abstract

One of the areas of major importance when it comes to weather prediction are clouds, due to their significant impact on the climate and because they are major source of the spread in climate sensitivity in climate models. This paper will focus on the main numerical weather prediction model, HARMONIE, used by the Royal Netherlands Meteorological Institute, with the objective to determine whether this model is able to accurately simulate the meso-scale organization and structures of subtropical low marine clouds.
This is done by comparing various HARMONIE simulation outputs with the observed variables obtained from the Barbados Cloud Observatory. To determine how the different processes within HARMONIE are affecting the simulation outputs, several experimental runs are analyzed. Based on the results it can be
seen that HARMONIE cycle 43 does produce a higher amount of low cloud cover compared to HARMONIE cycle 40. This is expected as HARMONIE cycle 43 does contain the additional height variance which was added in order for the model to produce more low clouds. However, it is overestimating the total low
cloud cover, in contrast to the underestimation by HARMONIE cycle 40. It can be seen that when the additional height variance is removed the simulation of the low cloud cover is significantly improved and is comparable to the observational low cloud cover. Furthermore, when the additional height variance and
shallow convection are both removed the amount of low cloud cover produced by HARMONIE cycle 43 also closely resembles the observed low cloud cover amount. When it comes to cloud fraction in the atmosphere, all of the experimental runs are not able to produce clouds that are present above 3000 m. This result was also obtained from simulation outputs of HARMONIE cycle 40. Besides this, also the specific humidity and potential temperature from the simulation outputs of HARMONIE cycle 43 are not accurately modelled.
All of the experiments seem to have a dry BIAS near the surface and a cold BIAS throughout the whole entirety of the atmosphere. Based on these findings it can be concluded that HARMONIE cycle 43 is able to produce meso-scale subtropical marine clouds, however it does not do this accurately but is overestimating
at all times. Moreover, it can be seen that the additional height variance has significant impact on the cloud production and it is recommended it be removed as the simulation outputs more accurately describe the observational data when it is not present. Furthermore there is no major improvement between cycle 40 and 43 in the cloud formation above 3000m. Both cycles are not able to produce any clouds in the simulation outputs. Based on this new understanding of what is the major source affecting the cloud production in HARMONIE, it is possible to further look into them with the aim to improve the overall weather prediction
of HARMONIE.