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Model evaluations for winter orographic clouds with observations
XiaoFeng Lou,Dan Breed
Chinese Science Bulletin , 2011, DOI: 10.1007/s11434-010-4249-2
Abstract: The Real-Time Four-Dimensional Data Assimilation (RT-FDDA) system is used for orographic snowpack enhancement. The model has three nested domains with the grid spacing of 18, 6 and 2 km. To evaluate the simulations of winter orographic clouds and precipitation, comparisons are made between model simulations and observations to determine how the model simulates the cloud distribution, cloud height, cloud vertical profiles and snow precipitation. The simulated results of the 02:00 UTC cycling with 2-km resolution are used in the comparison. The observations include SNOTEL, ceilometer, sounding and satellite data, from the ground to air. The verification of these observations indicates that the Weather Research and Forecast (WRF) RT-FDDA system provides good simulations. It is better to use data within the forecast period of 2–16 h simulations. Although the horizontal wind component near the ground has some bias, and the simulated clouds are a little higher and have a little more coverage than observed, the simulated precipitation is a little weaker than observed. The results of the comparison show that the WRF RT-FDDA model provides good simulations and can be used in orographic cloud seeding.
A-train CALIOP and MLS observations of early winter Antarctic polar stratospheric clouds and nitric acid in 2008
A. Lambert, M. L. Santee, D. L. Wu,J. H. Chae
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2012,
Abstract: A-train Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Microwave Limb Sounder (MLS) observations are used to investigate the development of polar stratospheric clouds (PSCs) and the gas-phase nitric acid distribution in the early 2008 Antarctic winter. Observational evidence of gravity-wave activity is provided by Atmospheric Infrared Sounder (AIRS) radiances and infrared spectroscopic detection of nitric acid trihydrate (NAT) in PSCs is obtained from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Goddard Earth Observing System Data Assimilation System (GEOS-5 DAS) analyses are used to derive Lagrangian trajectories and to determine temperature-time histories of air parcels. We use CALIOP backscatter and depolarization measurements to classify PSCs and the MLS measurements to determine the corresponding gas-phase HNO3 as a function of temperature. For liquid PSCs the uptake of HNO3 follows the theoretical equilibrium curve for supercooled ternary solutions (STS), but at temperatures about 1 K lower as determined from GEOS-5. In the presence of solid phase PSCs, above the ice frost-point, the HNO3 depletion occurs over a wider range of temperatures (+2 to 7 K) distributed about the NAT equilibrium curve. Rapid gas-phase HNO3 depletion is first seen by MLS from from 23–25 May 2008, consisting of a decrease in the volume mixing ratio from 14 ppbv (parts per billion by volume) to 7 ppbv on the 46–32 hPa (hectopascal) pressure levels and accompanied by a 2–3 ppbv increase by renitrification at the 68 hPa pressure level. The observed region of depleted HNO3 is substantially smaller than the region bounded by the NAT existence temperature threshold. Temperature-time histories of air parcels demonstrate that the depletion is more clearly correlated with prior exposure to temperatures a few kelvin above the frost-point. From the combined data we infer the presence of large-size NAT particles with effective radii >5–7 μm and low NAT number densities <1 × 10 3 cm 3. This denitrification event is observed close to the pole in the Antarctic vortex before synoptic temperatures first fall below the ice frost point and before the widespread occurrence of large-scale NAT PSCs. An episode of mountain wave activity detected by AIRS on 28 May 2008 led to wave-ice formation in the rapid cooling phases over the Antarctic Peninsula and Ellsworth Mountains, seeding an outbreak of NAT PSCs that were detected by CALIOP and MIPAS. The NAT clouds formed at altitudes of 18–26 km in a polar freezing belt and appear to be composed of relatively small particles with estimated effective radii of around 1 μm and high NAT number densities >0.2 cm 3. This NAT outbreak is similar to an event previously reported from MIPAS observations in mid-June 2003.
Atmospheric nanoparticle observations in the low free troposphere during upward orographic flows at Iza a Mountain Observatory
S. Rodríguez,Y. González,E. Cuevas,R. Ramos
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2009,
Abstract: This study investigates the processes and conditions favouring the formation of nanoparticles (diameter<10 nm) which are frequently observed on high mountains reaching the low free troposphere. This was done through an analysis of a data set collected at Iza a Global Atmospheric Watch Observatory (Canary Islands; 2367 m above sea level). This high mountain supersite is located well above the stratocumulus layer characteristic of the subtropical oceanic tropospheres. At night, when the catabic flow regime is well established, free troposphere aerosols were measured. The development of orographic buoyant upward flows during daylight resulted in an increase of water vapour, SO2 and NOy concentrations. These ascending airflows perturbed the free troposphere and resulted in high concentrations of 3–10 nm particles (N3–10) due to new particle formation. An analysis of the 5-min average time series allowed the identification of two main types of N3–10 event. In Type I events a linear relationship between N3–10 and SO2 was observed (r2 coefficients 0.70–0.95 and a mean slope of 11 cm 3 ppt 1 for 5-min averaged data; SO2 concentrations from tens to hundreds of ppt). These particles seem to be formed during upward transport (probably within or after the outflows of clouds typically located below Iza a). During Type II events, no correlation between SO2 and N3–10 was observed and 3–10 nm particles were formed in-situ at noon and during the afternoon due to the condensation of vapours linked to photochemistry. New particle formation was observed almost every day owing to the favourable conditions associated with the entry of boundary layer air in the low free troposphere, even if SO2 concentrations are rather low at Iza a (tens to hundreds of ppt). The low surface area of pre-existing particles, low temperature and high radiation intensity clearly favoured the formation of nanoparticles. The low surface area of pre-existing particles in the upward flows is furthered by in-cloud particles scavenging in the stratocumulus layer typically located below Iza a. The higher temperature and the presence of coarse Saharan dust particles decrease the efficiency of the new particle formation mechanisms in summer. Thus, the "N3–10 versus SO2" slope (for r2>0.7 cases) was higher in autumn and winter (~15 cm 3 ppt 1 as average) than in summer (2–8 cm 3 ppt 1). These field observations suggest that elevated mounts that reaches the free troposphere may act as source regions for new particles.
Atmospheric nanoparticle observations in the low free troposphere during upward orographic flows at Iza a Mountain Observatory  [PDF]
S. Rodríguez,Y. González,E. Cuevas,R. Ramos
Atmospheric Chemistry and Physics Discussions , 2009,
Abstract: This study investigates the processes and conditions favouring the formation of nanoparticles (diameter <10 nm) which are frequently observed on high mountains reaching the low free troposphere. This was done through an analysis of a data set collected at Iza a Global Atmospheric Watch Observatory (Canary Islands; 2367 m a.s.l.). This high mountain supersite is located well above the stratocumulus layer characteristic of the subtropical oceanic tropospheres. At night, when the catabic flow regime is well established, free troposphere aerosols were measured. The development of orographic buoyant upward flows during daylight resulted in an increase of water vapour, SO2 and NOy concentrations. These ascending airflows perturbed the free troposphere and resulted in high concentrations of 3–10 nm particles (N3-10) due to new particle formation. An analysis of the 5-min average time series allowed the identification of two main types of N3-10 event. In Type I events a linear relationship between N3-10 and SO2 was observed (r2 coefficients 0.70–0.95 and a mean slope of 11 cm 3· ppt 1 for 5-min averaged data; SO2 concentrations from tens to hundreds of ppt). These particles seem to be formed during upward transport (probably within or after the outflows of clouds located below Iza a). During Type II events, no correlation between SO2 and N3-10 was observed and 3–10 nm particles were formed in-situ at noon and during the afternoon due to the condensation of vapours linked to photochemistry. New particle formation was observed almost every day owing to the favourable conditions associated with the entry of boundary layer air in the low free troposphere, even if SO2 concentrations are rather low at Iza a (tens to hundreds of ppt). The low surface area of pre-existing particles, low temperature and high radiation intensity clearly favoured the formation of nanoparticles. The low surface area of pre-existing particles in the upward flows is furthered by in-cloud particles scavenging in the stratocumulus layer typically located below Iza a. The higher temperature and the presence of coarse Saharan dust particles decrease the efficiency of the new particle formation mechanisms in summer. Thus, the "N3-10 versus SO2" slope (for r2>0.7 cases) was higher in autumn and winter (~15 cm 3· ppt 1 as average) than in summer (2–8 cm 3· ppt 1). These field observations suggest that elevated mounts that reaches the free troposphere may acts as source regions for new particles.
Observations and analysis of polar stratospheric clouds detected by POAM III and SAGE III during the SOLVE II/VINTERSOL campaign in the 2002/2003 Northern Hemisphere winter
J. Alfred, M. Fromm, R. Bevilacqua, G. Nedoluha, A. Strawa, L. Poole,J. Wickert
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: The Polar Ozone and Aerosol Measurement and Stratospheric Aerosol and Gas Experiment instruments both observed high numbers of polar stratospheric clouds (PSCs) in the polar region during the second SAGE Ozone Loss and Validation (SOLVE II) and Validation of INTERnational Satellites and Study of Ozone Loss (VINTERSOL) campaign, conducted during the 2002/2003 Northern Hemisphere winter. Between 15 November 2002 (14 November 2002) and 18 March 2003 (21 March 2003) SAGE (POAM) observed 122 (151) aerosol extinction profiles containing PSCs. PSCs were observed on an almost daily basis, from early December through 15 January, in both instruments. No PSCs were observed from either instrument from 15 January until 4 February, and from then only sparingly in three periods in mid- and late February and mid-March. In early December, PSCs were observed in the potential temperature range from roughly 375 K to 750 K. Throughout December the top of this range decreases to near 600 K. In February and March, PSC observations were primarily constrained to potential temperatures below 500 K. The PSC observation frequency as a function of ambient temperature relative to the nitric acid-trihydrate saturation point (using a nitric acid profile prior to denitrification) was used to infer irreversible denitrification. By late December 38% denitrification was inferred at both the 400–475 K and 475–550 K potential temperature ranges. By early January extensive levels of denitrification near 80% were inferred at both potential temperature ranges, and the air remained denitrified at least through early March.
Observations and analysis of polar stratospheric clouds detected by POAM III and SAGE III during the SOLVE II/VINTERSOL campaign in the 2002/2003 Northern Hemisphere winter  [PDF]
J. Alfred,M. Fromm,R. Bevilacqua,G. Nedoluha
Atmospheric Chemistry and Physics Discussions , 2006,
Abstract: The Polar Ozone and Aerosol Measurement and Stratospheric Aerosol and Gas Experiment instruments both observed high numbers of polar stratospheric clouds (PSCs) in the polar region during the second SAGE Ozone Loss and Validation Experiment (SOLVE II) and Validation of INTERnational Satellites and Study of Ozone Loss (VINTERSOL) campaign, conducted during the 2002/2003 Northern Hemisphere winter. Between 15 November 2002 (14 November 2002) and 18 March 2003 (21 March 2003) SAGE (POAM) observed 122 (151) aerosol extinction profiles containing PSCs. PSCs were observed on an almost daily basis, from early December through 15 January, in both instruments. No PSCs were observed from either instrument until 4 February, and sparingly in three periods in mid-and-late February and mid-March. In early December, PSCs were observed in the potential temperature range from roughly 375 K to 750 K. Throughout December the top of this range decreases to near 600 K. In February and March, PSC observations were primarily constrained to potential temperatures below 500 K. The PSC observation frequency as a function of ambient temperature relative to the NAT saturation point was used to infer irreversible denitrification. By late December 38% denitrification was inferred at both the 400–475 K and 475–550 K potential temperature ranges. By early January extensive levels of denitrification near 80% were inferred at both potential temperature ranges, and the air remained denitrified at least through early March.
Intercomparison of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds
A. Muhlbauer, T. Hashino, L. Xue, A. Teller, U. Lohmann, R. M. Rasmussen, I. Geresdi,Z. Pan
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: Anthropogenic aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is hypothesized to retard the cloud droplet coalescence and the riming in mixed-phase clouds, thereby decreasing orographic precipitation. This study presents results from a model intercomparison of 2-D simulations of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds. The sensitivity of orographic precipitation to changes in the aerosol number concentrations is analysed and compared for various dynamical and thermodynamical situations. Furthermore, the sensitivities of microphysical processes such as coalescence, aggregation, riming and diffusional growth to changes in the aerosol number concentrations are evaluated and compared. The participating numerical models are the model from the Consortium for Small-Scale Modeling (COSMO) with bulk microphysics, the Weather Research and Forecasting (WRF) model with bin microphysics and the University of Wisconsin modeling system (UWNMS) with a spectral ice habit prediction microphysics scheme. All models are operated on a cloud-resolving scale with 2 km horizontal grid spacing. The results of the model intercomparison suggest that the sensitivity of orographic precipitation to aerosol modifications varies greatly from case to case and from model to model. Neither a precipitation decrease nor a precipitation increase is found robustly in all simulations. Qualitative robust results can only be found for a subset of the simulations but even then quantitative agreement is scarce. Estimates of the aerosol effect on orographic precipitation are found to range from 19% to 0% depending on the simulated case and the model. Similarly, riming is shown to decrease in some cases and models whereas it increases in others, which implies that a decrease in riming with increasing aerosol load is not a robust result. Furthermore, it is found that neither a decrease in cloud droplet coalescence nor a decrease in riming necessarily implies a decrease in precipitation due to compensation effects by other microphysical pathways. The simulations suggest that mixed-phase conditions play an important role in buffering the effect of aerosol perturbations on cloud microphysics and reducing the overall susceptibility of clouds and precipitation to changes in the aerosol number concentrations. As a consequence the aerosol effect on precipitation is suggested to be less pronounced or even inverted in regions with high terrain (e.g., the Alps or Rocky Mountains) or in regions where mixed-phase microphysics is important for the climatology of orographic precipitation.
Intercomparison of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds  [PDF]
A. Muhlbauer,T. Hashino,L. Xue,A. Teller
Atmospheric Chemistry and Physics Discussions , 2010, DOI: 10.5194/acpd-10-10487-2010
Abstract: Anthropogenic aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is hypothesized to retard the cloud droplet collision/coalescence and the riming in mixed-phase clouds, thereby decreasing orographic precipitation. This study presents results from a model intercomparison of 2-D simulations of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds. The sensitivity of orographic precipitation to changes in the aerosol number concentrations is analyzed and compared for various dynamical and thermodynamical situations. Furthermore, the sensitivities of microphysical processes such as collision/coalescence, aggregation and riming to changes in the aerosol number concentrations are evaluated and compared. The participating models are the Consortium for Small-Scale Modeling's (COSMO) model with bulk-microphysics, the Weather Research and Forecasting (WRF) model with bin-microphysics and the University of Wisconsin modeling system (UWNMS) with a spectral ice-habit prediction microphysics scheme. All models are operated on a cloud-resolving scale with 2 km horizontal grid spacing. The results of the model intercomparison suggest that the sensitivity of orographic precipitation to aerosol modifications varies greatly from case to case and from model to model. Neither a precipitation decrease nor a precipitation increase is found robustly in all simulations. Qualitative robust results can only be found for a subset of the simulations but even then quantitative agreement is scarce. Estimates of the second indirect aerosol effect on orographic precipitation are found to range from –19% to 0% depending on the simulated case and the model. Similarly, riming is shown to decrease in some cases and models whereas it increases in others which implies that a decrease in riming with increasing aerosol load is not a robust result. Furthermore, it is found that neither a decrease in cloud droplet coalescence nor a decrease in riming necessarily implies a decrease in precipitation due to compensation effects by other microphysical pathways. The simulations suggest that mixed-phase conditions play an important role in reducing the overall susceptibility of clouds and precipitation with respect to changes in the aerosols number concentrations. As a consequence the indirect aerosol effect on precipitation is suggested to be less pronounced or even inverted in regions with high terrain (e.g., the Alps or Rocky Mountains) or in reg
Single particle analysis of ice crystal residuals observed in orographic wave clouds over Scandinavia during INTACC experiment
A. C. Targino, R. Krejci, K. J. Noone,P. Glantz
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2006,
Abstract: Individual ice crystal residual particles collected over Scandinavia during the INTACC (INTeraction of Aerosol and Cold Clouds) experiment in October 1999 were analyzed by Scanning Electron Microscopy (SEM) equipped with Energy-Dispersive X-ray Analysis (EDX). Samples were collected onboard the British Met Office Hercules C-130 aircraft using a Counterflow Virtual Impactor (CVI). This study is based on six samples collected in orographic clouds. The main aim of this study is to characterize cloud residual elemental composition in conditions affected by different airmasses. In total 609 particles larger than 0.1 μm diameter were analyzed and their elemental composition and morphology were determined. Thereafter a hierarchical cluster analysis was performed on the signal detected with SEM-EDX in order to identify the major particle classes and their abundance. A cluster containing mineral dust, represented by aluminosilicates, Fe-rich and Si-rich particles, was the dominating class of particles, accounting for about 57.5% of the particles analyzed, followed by low-Z particles, 23.3% (presumably organic material) and sea salt (6.7%). Sulfur was detected often across all groups, indicating ageing and in-cloud processing of particles. A detailed inspection of samples individually unveiled a relationship between ice crystal residual composition and airmass origin. Cloud residual samples from clean airmasses (that is, trajectories confined to the Atlantic and Arctic Oceans and/or with source altitude in the free troposphere) were dominated primarily by low-Z and sea salt particles, while continentally-influenced airmasses (with trajectories that originated or traveled over continental areas and with source altitude in the continental boundary layer) contained mainly mineral dust residuals. Comparison of residual composition for similar cloud ambient temperatures around –27°C revealed that supercooled clouds are more likely to persist in conditions where low-Z particles represent significant part of the analyzed cloud residual particles. This indicates that organic material may be poor ice nuclei, in contrast to polluted cases when ice crystal formation was observed at the same environmental conditions and when the cloud residual composition was dominated by mineral dust. The presented results suggest that the chemical composition of cloud nuclei and airmass origin have a strong impact on the ice formation through heterogeneous nucleation in supercooled clouds.
The suppression of aerosols to the orographic precipitation in the Qinling Mountains
秦岭地区气溶胶对地形云降水的抑制作用

Dai Jin,Yu Xing,Rosenfeld Daniel and,
戴进
,余兴,Rosenfeld Daniel,

大气科学 , 2008,
Abstract: Based on the dataset of observations of precipitation,visibility and winds since 1954 at the top of Huashan Mountain,the ratio between the precipitation at Huashan Mountain and at the nearby plain stations,which is defined as the orographic enhancement factor(Ro),and the relationship between Ro and visibility,were used to quantitively study the ways that air pollution aerosols suppress orographic precipitation.Ro decreased 14%-20% gradually during the measurement period,which means that the precipitation at the top of Huashan Mountain decreased 14%-20% compared with the precipitation at the plains stations.The indicated trend of Ro matched well with the decreasing visibility and increasing aerosol,which suggests that enhanced pollution aerosols suppress the orographic precipitation.The decreasing trend of Ro is mainly caused by days of the light and moderate rain(daily precipitation less than 30 mm),but not by days with precipitation more than 30 mm,which suggested that the thin short living orographic clouds are much more susceptible to precipitation suppression by air pollution aerosols.The precipitation less than 30 mm and 5 mm,respectively,can be affected by the aerosols entering the clouds for Huashan Mountain and Xi'an stations,which suggests that the more aerosols enter the clouds,the deeper precipitation clouds will be influenced by the aerosols to suppress the precipitation.In the spring and autumn when dynamical lifting plays the leading role,the suppression of aerosol to clouds at the mountain top is stronger than that in plains,and causes about 20% decrease with a maximum of 25% in precipitation at Huashan Mountain.In the summer for the thermodynamically driven clouds,the suppression of aerosol to the clouds at the mountain top and in plains is equivalent.The variations of Ro in different wind directions show that the decreasing trend of Ro increases with the wind speed,and the decrease of orographic precipitation exceeds 30% for 240-030 wind direction and wind speed greater than 5 m/s.The quantitative analyses of Ro and visibility show that Ro decreases from about 1.8 to 1.2 when the visibility changes from 14 to 8 km,and the decrement exceeds 30%.Ro at Huashan Mountain to Huayin stations is linearly correlated with the visibility,and the regression coefficient is 0.81.Finally,a brief physical model that aerosols suppress orographic precipitation is summarized based on the results.
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