Validation of clouds in the ECHAM4 model using a dynamical adjustment technique
Clouds are an important regulator of the Earth's radiation budget and represent a major link between radiation and the hydrological cycle. In contrast to the "standard" method of compar- ing the model results with long-term climatologies, a different approach to validate clouds and cl...
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| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
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University of Hamburg
2000
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| Online Access: | http://hdl.handle.net/21.11116/0000-0009-E317-D http://hdl.handle.net/21.11116/0000-0009-E319-B |
| Summary: | Clouds are an important regulator of the Earth's radiation budget and represent a major link between radiation and the hydrological cycle. In contrast to the "standard" method of compar- ing the model results with long-term climatologies, a different approach to validate clouds and cloud systems in ECHAM4 T106 is used by investigating the representation of synoptic-scale cloud systems. The aim is to explore whether the realistically reproduced mean cloud amounts are the result of a realistic representation of clouds in a higher temporal resolution. To enable a validation against observations, a dynamical adjustment approach based on the so-called New- tonian relaxation technique (nudging) is used, which relaxes the model state towards reanalysis data by adding a non-physical relaxation term to the model equations. In the simulations vor- ticity, divergence, temperature and the logarithm of surface pressure are adjusted to ECMWF reanalysis fields. The strength of the forcing is controlled by an adjustment time scale which is different for each adjusted variable. The development of an extraordinary strong cyclone along the East Coast of the U.S.A. and a North Atlantic blocking situation are chosen as case studies. The synoptic systems have been selected to cover a large range of typical phenomena. A third situation, namely tropical convection in the'r¡¡estern Pacific warm pool region, has been excluded from the cloud validation, since the Newtonian relaxation cannot force the model to the observed state for single convective events. Compared to observations, the synoptic-scale features are well reproduced by the model. This is true even for variables which are not adjusted to the observed state. The general features of the horizontal and vertical cloud distribution are well reproduced for both synoptic systems. Nevertheless, systematic differences occur. The model underestimates clouds in low and middle levels of the troposphere and therefore total cloud amounts. Low-level clouds are most obviously underestimated ... |
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