The Land-Surface-Cloud InteractionPrimary Author: Betts, Alan
The Land-Surface-Cloud Interaction
Alan K. Betts
On timescales of a day and space scales of order 800km, the climate over land is a complex balance of many highly coupled processes. In the atmosphere, water vapor convergence is linked to precipitation and clouds, which in turn modify the radiation field. The surface energy budget is strongly influenced by the cloud field, and the availability of water for evaporation. Cloud feedbacks in models have long been regarded as a major source of uncertainty in climate modeling. We propose that the effective cloud albedo at the surface is a missing and observable link, which connects the cloud fields to both surface and large-scale processes. We have developed a methodology using daily averaged data for understanding the coupling between physical processes in models, so that different models can be compared with each other and with data. We illustrate this first using model data from ERA 40 for the Madeira River, a south-western basin of the Amazon, which has a large seasonal cycle with a dry season in the austral winter. Daily-mean land-surface fluxes and state variables can be used to map the transitions of the surface ⿾climate' of a model; and to quantify the links between the soil moisture, the mean cloud-base and cloud field, the short-wave and long-wave radiation fields at the surface, the vertical motion field, the atmospheric precipitable water and the surface precipitation. We then use long time-series of daily mean data from the three BERMS flux sites in central Saskatchewan to explore biases in ERA-40 on the grid-point scale, and to study the relationships between surface variables and fluxes and cloud cover in the observed and model data sets. On the seasonal timescale the biases in ERA-40 of temperature and humidity are small, but the model has a high bias of evaporative fraction in the warm season, and except in mid summer a low bias of reflective cloud, which peaks in mid winter. Reanalysis data for sub-basins of the Mississippi are then used to explore the links between these processes on river basin scales, using for evaluation observed surface precipitation and surface shortwave fluxes derived by the International Satellite Cloud Climatology Project. The satellite data show that ERA-40 has the same seasonal bias in cloud albedo [reaching -10% in winter) over the Mississippi as was seen in central Saskatchewan from comparisons with the BERMS flux-tower measurements. Other critical model relationships are explored. Moisture convergence leads to clouds and precipitation, but the relation of the diabatic precipitation heating and the surface cloud radiative forcing, while a function of cloud albedo, is largely independent of moisture convergence, although it does depend on cloud base. The surface cloud radiative forcing determines the surface net radiation, while evaporative fraction is primarily determined in the model by temperature and soil water. Near-surface relative humidity, the lifting condensation level, soil moisture and precipitation are also closely linked.
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