Analysis of the atmospheric water vapor transport and the hydrologic cycle simulated in a global circulation model Public Deposited

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  • In order to understand the atmospheric branch of the earth's hydrologic cycle on the global scale, an atmospheric moisture balance is diagnostically analyzed from the January and July data of the OSU atmospheric general circulation model, which has been integrated for thirty-nine months of simulation with seasonally-varying sea-surface temperature and solar insolation. The model hydrologic processes analyzed for the balance include the surface evaporation, the precipitation by large-scale and cumulus condensation, the vertical transport by large-scale and cumulus mass fluxes, and the horizontal transport of water vapor. The large-scale transports include the contributions from the standing and transient components of motion. Also analyzed are the potential and stream functions of horizontal transport, and the statistics of seasonal and interannual variabilities of the global and hemispheric effects of the hydrologic processes. As a result of these analyses, the hydrologic cycle is constructed and understood for both January and July of the model. Large-scale vertical transport moistens the upper layer; the standing and transient motions contribute mostly in the tropics and higher latitudes, respectively. Large-scale horizontal transport moistens the continental atmosphere except for the relatively small transport from the continents to the oceans by the standing motion in the upper layer; the runoff occurs in the model to balance the marine transport but seasonal trends exist such that snow assumulates during January and melts during July on the global average. Cumulus convection drys not only the lower layer but also the upper layer of the model, and the penetrating cumuli are a major mechanism of maritime precipitation, whereas the large-scale condensation and penetrating cumuli have the dominating effect on the continental precipitation during January and July, respectively. The seasonal precipitation over the Northern Hemisphere continents concurs with strong surface evaporation in summer and also with strong cyclonic activity in winter. Comparison with other models and observational data indicates that the model reproduced some basic features of the atmospheric branch of the hydrologic cycle and its seasonal variation. The intense evaporation (≥ 5 mm day⁻¹) over the Pacific and Atlantic oceans and the rain belts in the tropics are well simulated for both January and July. The poleward transport in the northern middle and high latitudes is in good agreement with observations. The maximum toward-thermal-equator transport in the tropics occurs, however, at the geographic equator for both January and July, indicating that these maxima are about 5 degrees of latitude closer to the seasonal thermal equator than the observed maxima. Nevertheless the global statistics of the model atmosphere are not significantly different from that of the real atmosphere. Among others, we mention the following common features of the January and July moisture balances in the present model. Most precipitation of penetrating convection occurs in regions of strong surface evaporation even though some occurs in the moisture convergence zones where most of heavy mid-level convection is located. In the regions of intense penetrating convection, however, the standing part of surface evaporation is much larger in magnitude than the negative transient part which is essentially due to the positive correlation between the turbulence intensity and surface humidity over wet surfaces. Moreover, the horizontal structure of the standing part conforms to that of the standing vapor pressure difference between the air and the underlying surface. A strategy for further studies is recommended on the basis of our understanding of these features.
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