Dynamic modeling of orographically induced precipitation

Abstract

Local orography governs the triggering of cloud formation and the enhancement of processes such as condensation and hydrometeor nucleation and growth in mountainous regions. Intense, lengthy precipitation events are typical upwind of the topographic divide, with sharply decreasing magnitude and duration on the lee side. Differences in mean annual precipitation of several hundred percent between windward slopes of orographic barriers and adjacent valleys or lee side slopes are not unusual. Because much of the streamflow in areas such as the western United States is derived from mountainous areas that are remote and often poorly instrumented, modeling of orographic precipitation has important implications for water resources management. Models of orographically induced precipitation differ by their treatment of atmospheric dynamics and by the extent to which they rely on bulk parameterization of cloud and precipitation physics. Adiabatic ascent and a direct proportionality between precipitation efficiency and orographically magnified updrafts are the most frequent assumptions in orographic precipitation modeling. Space-time discretization (i.e., resolution) is a major issue because of the high spatial variability of orographic precipitation. For a specific storm, relative errors as large as 50 to 100% are common in the forecast/hindcast of precipitation intensity and can be even larger in the case of catastrophic storms. When monthly or seasonal timescales are used to evaluate model performance, the magnitude of such errors decreases dramatically, reaching values as low as 10 to 15%. Current research is focusing on the development of data assimilation techniques to incorporate radar and satellite observations, and on the development of aggregation and disaggregation methodologies to address the implications of modeling a multiscale problem at restricted spatial and temporal resolutions.Under the same sky in which the stars pursue their orbits, as symbols of the unchangeable fullness of Nature, we see the clouds towering, the rain pouring, and the winds changing, as symbols--as it were--ofjust the opposite extreme, the most capricious of all processes in Nature, impossible to bring inside the fence of its Laws. H. von Helmholtz 1875 (as cited by Bergeron [1960a]) INTRODUCTIONMountains are a complex source of perturbations and deformation of atmospheric flows, the signature of which is detected over a wide range of time-space frequencies extending from the scale of boundary layer motions up to that of planetary waves [Namias, 1960;Smith, 1979;Kasahara, 1980;Wallace, 1987]. The influence of orography on the long-term spatial distribution of precipitation is readily observable in mountainous areas. For instance, Figures la and 1 b show the distribution of annual precipitation along east-west transects at the Canadian/U.S. border and through the Coast Range and Sierra Nevada Moun-•Now at Department of Civil and Environmental Engineering, Pennsylvania State University, University Park. tains of California. Orography expl...

The properties of convective storms in central Mexico: A radar and lightning approach

“…3a the relatively higher proportion of storms over the western and southern borders of Mexico City (SSE from the radar), as well as over the western slopes of the volcano Nevado de Toluca (SW from radar) can also be seen. This has been reported before (Barros, 1994;Magaña et al, 2003;Mendez et al, 2009) and it is attributed to orographic forcing.…”

Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica

“…The dependence of precipitation quantity upon elevation in such areas of high relief is a well established fact (see, for example Barros &Lettenmaier, 1994, andlocally Fernandez et al, (1996) however it would appear that the association of rainfall to ENSO is also controlled in a similar fashion. regard to mountainous basins, such as the Magdalena used by Hastenrath (1990), they appear to encompass mixed responses to ENSO, thereby eliciting apparently weaker regional associations than may actually exist at various points along their lengths.…”

Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

“…This potentially has a strong effect on the microphysics of orographic precipitation in this region by modulating the cloud condensation nuclei (CCN) spectrum (the so called indirect effect). Since both riming and collision of water droplets following coalescence is prerequisite for efficient precipitation (White et al, 2003;Lascaux et al, 2006;Rotunno and Houze, 2007), inhibition of this cloud microphysical process due to an increase in CCN concentration influences the precipitation efficiency of the system (Barros and Lettenmaier, 1994;Rosenfeld and Lensky, 1998;Khain et al, 2000;Ramanathan et al, 2001;Andreae et al, 2004;Tao et al, 2007;Lau et al, 2008;and many others). In order to study the aerosol indirect effect, a direct measurement of the CCN spectrum is required or, alternatively, it can be estimated from observed aerosol size distribution and chemical composition.…”