Formation Of Orographic Clouds Research Articles

Simulating the seeder–feeder impacts on cloud ice and precipitation over the Alps

Abstract. The ice phase impacts many cloud properties as well as cloud lifetime. Ice particles that sediment into a lower cloud from an upper cloud (external seeder–feeder process) or into the mixed-phase region of a deep cloud from cirrus levels (internal seeder–feeder process) can influence the ice phase of the lower cloud, amplify cloud glaciation and enhance surface precipitation. Recently, numerical weather prediction modeling studies have aimed at representing the ice crystal number concentration in mixed-phase clouds more accurately by including secondary ice formation processes. The increase in the ice crystal number concentration can impact the number of ice particles that sediment into the lower cloud and alter its composition and precipitation formation. In the Swiss Alps, the orography permits the formation of orographic clouds, making it ideal for studying the occurrence of multi-layered clouds and the seeder–feeder process. We present results from a case study on 18 May 2016, showing the occurrence frequency of multi-layered clouds and the seeder–feeder process. About half of all observed clouds were categorized as multi-layered, and the external seeder–feeder process occurred in 10 % of these clouds. Between cloud layers, ≈60 % of the ice particle mass was lost due to sublimation or melting. The external seeder–feeder process was found to be more important than the internal seeder–feeder process with regard to the impact on precipitation. In the case where the external seeder–feeder process was inhibited, the average surface precipitation and riming rate over the domain were both reduced by 8.5 % and 3.9 %, respectively. When ice–graupel collisions were allowed, further large reductions were seen in the liquid water fraction and riming rate. Inhibiting the internal seeder–feeder process enhanced the liquid water fraction by 6 % compared to a reduction of 5.8 % in the cloud condensate, therefore pointing towards the de-amplification in cloud glaciation and a reduction in surface precipitation. Adding to the observational evidence of frequent seeder–feeder situations, at least over Switzerland, our study highlights the extensive influence of sedimenting ice particles on the properties of feeder clouds as well as on precipitation formation.

Testing of a Cell-Integrated Semi-Lagrangian Semi-Implicit Nonhydrostatic Atmospheric Solver (CSLAM-NH) with Idealized Orography

Abstract A recently developed cell-integrated semi-Lagrangian (CISL) semi-implicit nonhydrostatic atmospheric solver that uses the conservative semi-Lagrangian multitracer (CSLAM) transport scheme is extended to include orographic influences. With the introduction of a new semi-implicit CISL discretization of the continuity equation, the nonhydrostatic solver, called CSLAM-NH, has been shown to ensure inherently conservative and numerically consistent transport of air mass and other scalar variables, such as moisture and passive tracers. The extended CSLAM-NH presented here includes two main modifications: transformation of the equation set to a terrain-following height coordinate to incorporate orography and an iterative centered-implicit time-stepping scheme to enhance the stability of the scheme associated with gravity wave propagation at large time steps. CSLAM-NH is tested for a suite of idealized 2D flows, including linear mountain waves (dry), a downslope windstorm (dry), and orographic cloud formation.

Measurements of Aerosol Chemistry during New Particle Formation Events at a Remote Rural Mountain Site

Determining the major sources of particles that act as cloud condensation nuclei (CCN) represents a critical step in the development of a more fundamental understanding of aerosol impacts on cloud formation and climate. Reported herein are direct measurements of the CCN activity of newly formed ambient particles, measured at a remote rural site in the Sierra Nevada Mountains of Northern California. Nucleation events in the winter of 2009 occurred during two pristine periods following precipitation, with higher gas-phase SO(2) concentrations during the second period, when faster particle growth occurred (7-8 nm/h). Amines, as opposed to ammonia, and sulfate were detected in the particle phase throughout new particle formation (NPF) events, increasing in number as the particles grew to larger sizes. Interestingly, long-range transport of SO(2) from Asia appeared to potentially play a role in NPF during faster particle growth. Understanding the propensity of newly formed particles to act as CCN is critical for predicting the effects of NPF on orographic cloud formation during winter storms along the Sierra Nevada Mountain range. The potential impact of newly formed particles in remote regions needs to be compared with that of transported urban aerosols when evaluating the impact of aerosols on clouds and climate.

Impact of land use on Costa Rican tropical montane cloud forests: Sensitivity of orographic cloud formation to deforestation in the plains

The current study provides new insights into the coupling of land use in lowland and premontane regions (i.e., regions below 1000 m) and orographic cloud formation over the Monteverde cloud forests. Rawinsondes launched during the Land Use Cloud Interaction Experiment (LUCIE) together with those from the National Centers for Environmental Prediction (NCEP) provided profiles that were used to drive the Colorado State University Regional Atmospheric Modeling System (CSU RAMS) model, which simulated three realistic land use scenarios (pristine forests, current conditions and future deforestation). For current conditions, the model‐simulated clouds were compared against those observed at hourly intervals by the Geostationary Environmental Observational Satellite–East (GOES E) satellite. The model performed best on 6 different days. The model‐simulated profiles of dew point and air temperatures were compared with the observed profiles from rawinsondes for these days. There was generally very good agreement below 700 mb, the region of the atmosphere most crucial to the cloud forests. The average model simulations for the 6 days show that when the lowland and premontane regions were completely forested, the orographic cloud bank intersected the mountains at the lowest elevations, covered the largest land surface area and remained longest on the surface in the montane regions. Deforestation has decreased the cloud forest area covered with fog in the montane regions by around 5–13% and raised the orographic cloud bases by about 25–75 m in the afternoon. The model results show that further deforestation in the lowland and premontane regions would lead to around 15% decrease in the cloud forest area covered with fog and also raise the orographic cloud base heights by up to 125 m in the afternoon. The simulations show that deforestation in the lowland and premontane regions raises surface sensible heat fluxes and decreases latent heat fluxes. This warms the air temperature and results in a lower dew point temperature of air masses that blow over the lowland and premontane regions. These air masses when lifted form the orographic cloud bank at higher elevations.

Future Climate Change of the Subtropical North Atlantic: Implications for the Cloud Forests of Tenerife

This paper is concerned with climate change in the region of the Canary Islands and the potential implications for the laurel forests of Tenerife. Frequent orographic cloud formation during the dry season is of vital importance to the altitudinal distribution of the laurel forests, because it maintains a semi-humid environment in the otherwise semi-arid climate of the Canary Islands. The distinctive environmental conditions in conjunction with the location of the Canary Islands on the Northern poleward edge of the Hadley Circulation make these ecosystems potentially highly sensitive to regional changes in climatic conditions. To explore this sensitivity, we first quantify observed trends in humidity and temperature across an altitudinal transect at the base of the Anaga peninsular, and second, analyse the results of three GCM experiments (CGCM1, ECHAM4 and CSIRO) to develop alternative climate change scenarios, and third, use these data to assess likely shifts in the elevational distribution of the laurel forest climate envelope. We report a significant increase in relative humidity and decreases in the diurnal temperature range on Tenerife at altitudes below the trade wind inversion within the last 30 years during the dry season, which suggests an increased occurrence of low-level clouds. There is also partial evidence for a drying trend across the trade wind inversion, which may be linked to an increased subsidence. Overall, the models suggest a downward shift of the area climatically suitable for laurel forests, which may be driven by changes in temperature and moisture supply in the region as well as by larger-scale changes in the atmospheric circulation. Our findings contrast with previously published findings for a tropical montane cloud region, which predict an upward shift of the cloud base. This suggests, following the assumptions inherent in the models applied, that the ecological consequences of climate change for cloud forests may be linked to their relative location in the Hadley Circulation.

EPIC Simulations of Bright Companions to Neptune's Great Dark Spots

We present a three-dimensional model of the bright companion clouds associated with Neptune's Great Dark Spots (GDSs). Our results support the hypothesis that the bright companions of the southern-hemisphere GDS discovered in 1989 and of the northern-hemisphere GDS discovered in 1994 are methane clouds that form at or just below the tropopause and that they are caused by lifting in a manner analogous to the formation of orographic clouds. We vary the vertical position of the GDS and find that the companion cloud is a robust feature except when the anticyclone itself does not survive. When a GDS is started with its top in the stratosphere it drifts much too rapidly toward the equator and quickly disperses. On the other hand, if its top is well below the tropopause there is a tendency for the companion clouds to be too large. Hence the top of a GDS is probably at the tropopause. Along an open streamline that threads a bright companion, the typical pressure and temperature drops are about 3 mb and 1K, respectively, corresponding to a lift of about half a kilometer or 4% of the pressure scale height, and the relative horizontal wind speed is about 45 m s −1 eastward through the cloud.

Solar radiation at Rome, Italy, and its environment

Routinely collected data of solar radiation over a 12 year period have been studied for different zones near Rome. The region under consideration extends between Tyrrhenian Sea and Apennines Mountains and is characterized by three stations situated in urban, rural and mountain sites. Again, latitude, sunshine duration and cloud cover are midway among the others in Italy, so that this zone can be considered as representative for many regions of this country. Variations in time and space on the falling radiation, due to different climatological effects are evidenced. Averaged and extreme values of sunshine duration and global radiation are discussed, especially with reference to the local cloudiness. As the formation of orographic clouds over the Apennines cause a depletion on solar energy greater than the effect of a layer of about 2 km of turbid air over the plain, the latter seems more convenient in order to use solar energy for practical applications.

UNUSUAL OROGRAPHIC CLOUD FORMATION

WeatherVolume 12, Issue 10 p. 320-322 UNUSUAL OROGRAPHIC CLOUD FORMATION First published: October 1957 https://doi.org/10.1002/j.1477-8696.1957.tb00393.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume12, Issue10October 1957Pages 320-322 RelatedInformation