Land Interaction Processes

MERRA Production has been moving along steadily, still on track for a continuous time teries (1979-2006) to be available in early August. Data (missing 1988, 1997, 2005) is presently available for download, see the MERRA home page for access information.

With much of the record available, comparisons to previous studies of reanalyses are possible. For example, there has been a lot of work on the land interaction processes in NCEP and ERA40. Betts and Viterbo (2005) has defined cloud albedo as an observable diagnostic of the all-sky radiative forcing of the surface (Acld = -{SWDNsfc-SWDNsfcclr}/SWDNsfcclr). Below is an example of the cloud albedo comparison for ISCCP and ERA 40 over the Madeira basin, followed by a similar figure for MERRA. ERA40 tends to overestimate the cloud albedo, while MERRA underestimates it. This indicates that for the Madeira, the shortwave at the surface is

too close to the clear sky values. This bias is especially pronounced during the austral winter and less so in summer.

In evaluating the coupling processes, the links between the surface, surface layer and boundary layer relate to precipitation. Below, Betts (2009) shows the relationships of boundary layer (through LCL height in pressure thickness), surface layer (EF, evaporative fraction) and soil wetness (SMI). While lower wetness is often and indicator of higher LCL and lower precipitation, there is a range in the characteristic values. EF results are similar. The LCL - wetness connection in MERRA appears stronger and more linear than that of ERA40, where the MERA LCL height is almost double that determined from ERA40. (Note that the soil moisture index in Betts 2009 is not the same as the MERRA surface soild wetness, the SMI cannot be easily reproduced in MERRA data). The MERRA relationship between LCL and EF is much tighter than that shown for ERA40.

In the figures above, the colors indicate the amount of daily preciitation that occurs relative to the LCL and Wetness/EF values. For the MERRA figures, the dots are each daily mean showing the range in the data, and for ERA40, the range it demonstrated by the error bars.

Betts, A. K., 2009: Land surface coupling in observations and models. J. Adv. Model. Earth Sys. Articles in Press.

Betts, A. K. and P. Viterbo, 2005: Land-surface, boundary layer, and cloud-field coupling over the southwestern Amazon in ERA-40, J. Geophys. Res., 110, D14108, doi:10.1029/2004JD005702