An Intercomparison Study of MODIS-Derived and WRF-Simulated Cloud Data

As a first step in evaluating the realism of the model-simulated cloud fields, the observed and simulated 11 μm MODIS brightness temperatures are shown above. A detailed comparison indicates that the PBL and cloud microphysics schemes both exert a strong influence on the spatial distribution of the simulated clouds. For instance, compared to the WSM6 and PLIN simulations, the upper-level cloud shield covers a larger area when the SEIF and THOM microphysics schemes are used. Likewise, for a given microphysics scheme, slightly cooler brightness temperatures occur within the cloud shield when the YSU PBL scheme is employed. The model simulations also exhibit substantial variability within the stratocumulus cloud region to the northwest of the cyclone. The MODIS brightness temperatures indicate that these clouds have a relatively uniform appearance over a large geographical region. The WSM6 and PLIN simulations, however, failed to reproduce these features and are instead characterized by a heterogeneous cloud deck containing large clear areas between the cumulus cells. By comparison, the THOM and SEIF simulated brightness temperatures have a more uniform appearance that better matches the observations. Simulations employing the YSU PBL scheme generally outperformed the MYJ scheme within this region, particularly for the SEIF and THOM simulations. The simulated and observed COT – CTP and CWP – CTP probability distributions are shown above. All of the model simulations realistically capture the height of the upper-level clouds, though it is evident that the heights are too low for some of the optically thicker clouds (COT > 30). The downward height bias is consistent with the slightly warmer brightness temperatures within the deep convection to the north and east of the cyclone center. Simulations employing the YSU PBL scheme generally contain more upper level clouds than those using the MYJ scheme, which demonstrates that vertical fluxes generated by the PBL schemes effect all of the clouds in the column, not just those in the boundary layer. The mid-level maximum associated with the stratocumulus cloud deck is poorly predicted by all of the simulations except for the SEIF-YSU simulation. The warmer brightness temperatures to the northwest of the cyclone indicate that the simulated cloud top heights are too low relative to the observations. The lack of mid-level clouds suggests that the PBL schemes generate insufficient vertical mixing, which prevents the cloud-topped boundary layer from reaching its proper depth within this region. In the lower troposphere, the model simulations contain fewer (more) optically thin (thick) clouds than the MODIS dataset. Although simulations employing the MYJ PBL scheme tend to overestimate the optical thickness of the low-level clouds, it is clear that these simulations contain a more realistic depiction of the low-level clouds than those using the YSU PBL scheme. The simulated CWP data more closely resembles the MODIS observations, particularly in the lower troposphere. A detailed inspection of the MODIS cloud data revealed a tendency for relatively large effective radii to occur along the edges of optically thin cumulus clouds in the western portion of the domain. Since the MODIS CWP is proportional to the product of the COT and effective radius retrievals, it is likely that the apparent biases partially offset each other and therefore result in a higher CWP in these regions.