Valley Wind Research Articles

Representation of Stony Surface‐Atmosphere Interactions in WRF Reduces Cold and Wet Biases for the Southern Tibetan Plateau

AbstractCurrent regional and global climate models often have significant cold and wet biases in the simulation of climate for the Southern Tibetan Plateau (STP). This region has complex terrain and steep slopes, and the underlying surface in many areas is stony with little soil and vegetation, which causes negligible infiltration and rapid runoff. The default soil type (loam) in the Weather Research and Forecasting (WRF) is thus replaced with bedrock to improve the representation of the mountainous hydrological processes in this region. The domain of bedrock is identified by using satellite‐based Normalized Difference Vegetation Index (NDVI) and terrain elevation standard deviation. The model results demonstrate that the WRF‐simulated precipitation and temperature are sensitive to soil type setting in this region. After the soil type is corrected, the mean precipitation bias at weather stations in the STP is reduced from 35 mm to −6 mm and the cold bias of 2 m air temperature is reduced from 1.5°C to 0.8°C when evaluated against weather station data. The relevant land and atmospheric processes are explored. The revision of the soil type from loam to bedrock results in increased sensible heat flux and decreased evaporation. A higher sensible heat flux results in higher air temperature, and the suppressed evaporation attenuates precipitation‐evaporation feedback. The altered energy budget enhances the valley wind, resulting in a sinking motion anomaly and reduced precipitation over lower elevations. The study highlights the critical role of soil type in the realistic representation of land‐atmosphere interactions by numerical models for the unique region.

Late Pleistocene-Holocene aeolian loess-paleosol sections in the Yellow River source area on the northeast Tibetan Plateau: chronostratigraphy, sediment provenance, and implications for paleoclimate reconstruction

Two late Pleistocene-Holocene aeolian loess-paleosol sections were identified in the Yellow River source area on the northeast (NE) Tibetan Plateau. Based on the comprehensive study on multiple proxies, AMS 14C and optically stimulated luminescence (OSL) dates, these two sections have a basal age of 14000–11500 yr BP, suggesting that they accumulated since the last deglaciation. Four stages of paleoclimate evolution can be summarized as follows. (1) A cold-dry period was dominated by the Westerlies in the late Pleistocene (ca. 14000–11500 yr BP). (2) A gradual transition to warm-wet with the weakened Westerlies and strengthened East Asian summer monsoon (EASM) occurredin the early Holocene (ca. 11500–9000 yr BP). (3) A warm-wet episode was under strong EASM in the middle Holocene (ca. 9000–3100 yr BP), however, a cold event was identified at 5000–3500 yr BP, which was caused by the abrupt weakening of the EASM and the strengthening of the Westerlies. (4) A phase of gradually shifting to dry-cold was controlled by the weakened EASM and strengthened Westerlies (after ca. 3100 yr BP). The assemblage of grain-size end-member modelling analysis (EM1-EM5) and heavy minerals indicated that aeolian deposit units have multiple sediment provenances and transport processes. EM1 is clay deposits composed of secondary clay minerals produced during weathering and pedogenesis, whereas EM2 is fine silts derived from distant dust transported through long-existing high-level Westerlies. EM3 and EM4 are coarse silts that originated from the moraines and periglacial deposits in the middle and distant source areas transported by the Westerlies and Tibetan Plateau Monsoon. Lastly, EM5 is sand from fluvial deposits and slope clastics transported by strong valley wind within the local area. This study is helpful to understand the mechanisms of paleoclimate evolution on the NE Tibetan Plateau. It also has important implications for characterizing and subdividing aeolian loess in China.

Similarities and Differences of Valley Winds in the Beijing Plain and Yanqing Areas and Its Impact on Pollution

Under certain terrain and weather conditions, mountain-valley circulation is one of the main meteorological factors affecting aerosol pollution in plain-mountain area. Based on environmental monitoring data and multi-source meteorological data for the Beijing-Tianjin-Hebei region between 2015 and 2019, the characteristics, similarities, and differences of mountain-valley winds in the Beijing Plain and Yanhuai Basin regions were compared. The results show that the mountain-valley winds recorded at the Beijing Observatory are from southwest to northeast compared to from the southeast to northeast at Yanqing station. With the aggravation of pollution levels, the mountain-valley wind intensity decreased by 17.7%-32.4%. When the wind speed at Beijing Observatory was 2-6 m·s-1, the maximum PM2.5 concentration in southeast was 83 μg·m-3, which was higher than in the southwest. When the wind speed at the Yanqing station was 2-6 m·s-1, the PM2.5 concentrations in SE-SSE area was 20-40 μg·m-3 higher than in other directions, and the concentrations in the valley winds were 10-12 μg·m-3 higher than the average value for the last five years. Taking the typical heavy pollution event on March 5-8, 2015, as an example, the influence of mountain-valley winds is mainly reflected in the high humidity and regional transmission of southeast winds during the valley wind stage. The PM2.5 concentrations at the Yanqing station increased by 100-130 μg·m-3 during the valley wind stage on March 6 and 7, 2015. The inversion temperature developed to 1000 m during the mountain wind stage, the local dew point at the Beijing Observatory and the Yanqing station rose by approximately 18℃. The peak dew point at the Yanqing station occurring 2 hours after the Beijing Observatory, and the concentrations of PM2.5 rose slightly under high humidity conditions. Meanwhile, the thermal gradient between the 400-m-high Yanqing Station and Yudu Mountain gradually decreased, and the mountain-valley wind decreased by 8% and 6%, respectively. The weakening of local circulation may be related to the bidirectional feedback mechanism of the boundary layer and high concentrations of aerosols.

Using an Artificial Neural Network to improve operational wind prediction in a small unresolved valley

AbstractThis study improves surface wind predictions in an unresolved valley using an artificial neural network (ANN). Forecasting winds in complex terrain with a mesoscale model is challenging. This study assesses the quality of 3-km wind forecasts by the Weather Research and Forecasting (WRF) model and the potential of post-processing by an ANN within the 1-2 km wide Cadarache Valley in southeast France. Operational wind forecasts for 110m above ground level and the near-surface vertical potential temperature gradient with a lead time of 24-48h were used as ANN input. Observed horizontal wind components at 10m within the valley were used as targets during ANN training. We use the Directional ACCuracy (DACC45, wind direction error ≤ 45°) and mean absolute error to evaluate the WRF direct model output and the ANN results. By post-processing, the score for DACC45 improves from 56% in the WRF direct model output to 79% after applying the ANN. Furthermore, the ANN performed well during the day and night, but poorly during the morning and afternoon transitions. The ANN improves the DACC45 at 10m even for poor WRF forecasts (direction bias ≥ 45°) from 42% to 72%. A shorter lead time and finer grid spacing (1 km) showed negligible impact which suggests that a 3 km grid spacing and a 24-48h lead time is effective and relatively cheap to apply. We find that WRF performs well in near-neutral conditions and poorly in other atmospheric stability conditions. The ANN post-treatment consistently improves the wind forecast for all stability classes to a DACC45 of about 80%. The study demonstrates the ability to improve Cadarache valley wind forecasts using an ANN as post-processing for WRF daily forecasts.

Field measurement study of wind characteristics at different measuring positions along a bridge in a mountain valley

Wind field characteristics were investigated with a non-stationary wind speed model at a bridge in a mountain valley, where five anemometers were set along the bridge span for long-term field measurements. The mean wind characteristics, such as mean wind speed and direction, were studied with a traditional steady wind-speed model, which showed the significant influence of the terrain on the mean wind characteristics. In addition, data for certain strong-wind weather conditions were measured, and the non-stationary analysis of strong wind samples were conducted using the run test and wavelet analysis. Subsequently, the characteristics of the fluctuating wind—time-varying mean wind speed, turbulence intensity, turbulence integral scale, and power spectral density (PSD)—were compared based on the two wind speed models. The comparative results indicated that the mountain valley wind field was substantially affected by the terrain and exhibited a strong non-stationarity. However, there were certain differences between the wind field characteristics of the two wind speed models, and the recommended value was not completely consistent for the field measurements in the current specification.

Early Peak of Latent Heat Fluxes Regulates Diurnal Temperature Range in Montane Cloud Forests

AbstractHydro-climate in the montane cloud forest (MCF) regions is unique for its frequent fog occurrence and abundant water interception by tree canopies. Latent heat (LH) flux, the energy flux associated with evapotranspiration (ET), plays an essential role in modulating energy and hydrological cycles. However, how LH flux is partitioned between transpiration (stomatal evaporation) and evaporation (non-stomatal evaporation), and how it impacts local hydro-climate remain unclear. In this study, we investigate how fog modulates the energy and hydrological cycles of MCF by using a combination of in-situ observations and model simulations. We compare LH flux and associated micrometeorological conditions at two eddy-covariance sites—Chi-Lan (CL), a MCF, and Lien-Hua-Chih (LHC), a non-cloud forest in Taiwan. The comparison between the two sites reveals an asymmetric LH flux with an early peak at 9:00 in CL as opposed to LHC, where LH flux peaks at noon. The early peak of LH flux and its evaporative cooling dampen the increase in near-surface temperature during the morning hours in CL. The relatively small diurnal temperature range, abundant moisture brought by the valley wind, and local ET result in frequent afternoon fog formation. Fog water is then intercepted by the canopy, sustaining moist conditions throughout the night. To further illustrate this hydrological feedback, we used a land surface model to simulate how varying canopy water interception can affect surface energy and moisture budgets. Our study highlights the unique hydro-climatological cycle in MCF and, specifically, the inseparable relationship between the canopy and near-surface meteorology during the diurnal cycle.

Modern Pollen-vegetation Relationships along the Vegetation Gradient in the Bhagirathi Valley, Western Himalaya, India

Abstract This study highlights the modern pollen-vegetation relationships along an altitudinal gradient covering the lower temperate (~2,000 masl) to alpine vegetation zones (~3,800 masl) in the Bhagirathi valley, western Himalaya. The pollen dispersal dataset compared with the altitudinal vegetation distribution in the valley shows incoherency between the arboreal taxa and their respective pollen proportions. Discriminant analysis (DA) revealed the role of diurnal valley winds in pollen mixing between altitudinal vegetation zones. Pollen of arboreal taxa are transported profusely from the lower temperate vegetation zones to alpine zone. Whereas pollen of non-arboreal taxa, dominantly growing in the sub-alpine and alpine meadows, remain close to their source and represent the prevailing localized environmental conditions in the valley. Study revealed Pinus and Quercus as dominant arboreal pollen taxa throughout the altitudinal transect with a distinct transition in their representation between ~2,500 and 2,600 masl. High percentage of Pinus (Quercus) pollen below (above) this transition zone nearly corresponds with the present distribution of conifer (pine-oak) and broadleaved (oak-pine) forests in the valley and can be taken as a marker zone to infer palaeoecological changes. The sub-alpine and alpine krummholz, Juniperus and Ericaceae are found autochthonous in their pollen distribution that can serve as good indicators of relative tree-line and glacier dynamics in past. This pollen-vegetation relationship dataset could be an analogue for the interpretation of fossil pollen records in terms of extant vegetation in the mountain regions.

Observational Signal of the Interaction Between Mountain–Plain Wind and Urban Breeze Under Weak Synoptic Systems

AbstractValley winds and urban breezes induced by thermal circulation have been the subject of numerous studies; however, their interactions deserve further observational study under different geographical conditions globally. We propose quantitative criteria for analysis of observations, including identification of valley and mountain–plain wind episodes, determination of the direction and duration of valley winds, and exclusion of suspect cases induced by strong synoptic systems. We selected a subset of 345 valley wind days that occurred during 2013–2015 in Hangzhou, China. Valley winds in the study area occur mainly in autumn and early winter (annual frequency: 32%). To the west (east) of the urban center of Hangzhou are small mountains (is a plain). At night, the directions of mountain–plain circulation and urban thermal circulation are consistent in the region from the mountains to western parts of the city, resulting in a strengthened mountain wind at the surface. Their interaction also delays the reversal of the mountain wind to the plain wind in the morning, and slightly advances the reversal of the plain wind to the mountain wind in the evening. Conversely, in the plain area to the east of Hangzhou, the interaction delays the time of reversal of the plain wind to the mountain wind in the evening. Our results indicate that interactions between the urban breeze and local thermal circulations could have distinct impact on the environment of the city and surrounding areas, and thus they should be taken into consideration in relation to urban planning and management.

Vertical Structures of Meteorological Elements and Black Carbon at Mt. Tianshan Using an Unmanned Aerial Vehicle System

As the largest independent east–west-trending mountain in the world, Mt. Tianshan exerts crucial impacts on climate and pollutant distributions in central Asia. Here, the vertical structures of meteorological elements and black carbon (BC) were first derived at Mt. Tianshan using an unmanned aerial vehicle system (UAVS). Vertical changes in meteorological elements can directly affect the structure of the planet boundary layer (PBL). As such, the influences of topography and meteorological elements’ vertical structure on aerosol distributions were explored from observations and model simulations. The mass concentrations of BC changed slightly with the increasing height below 2300 m above sea level (a.s.l.), which significantly increased with the height between 2300–3500 m a.s.l. and contrarily decreased with ascending altitude higher than 3500 m. Topography and mountain–valley winds were found to play important roles in the distributions of aerosols and BC. The prevailing valley winds in the daytime were conducive to pollutant transport from surrounding cities to Mt. Tianshan, where the aerosol number concentration and BC mass concentration increased rapidly, whereas the opposite transport pattern dominated during nighttime.

Impacts of the Westerlies on Planetary Boundary Layer Growth Over a Valley on the North Side of the Central Himalayas

AbstractThis study focused on planetary boundary layer (PBL) growth in winter under the influence of the westerlies over the Rongbuk Valley on the north side of the central Himalayas. Two sunny November days in 2014 with different synoptic conditions in terms of large‐scale wind direction and speed were selected to investigate the ways in which large‐scale synoptic forcing affected the vertical structure of the PBL, atmospheric stability, surface wind field, and land surface energy fluxes. This was done by using radiosonde and in‐situ measurements, the European Centre for Medium‐Range Weather Forecasts ERA5 reanalysis data set, and a one‐dimensional model. The results revealed that the valley winds and PBL growth were strongly influenced by the variations of the westerlies, which prevail in this region during the winter. When the synoptic wind direction at the height of the mountain ridges was parallel to the axis of the valley and strong, the downward transmission of the westerlies to the valley floor (DTWTV) was strong, causing high near‐surface wind speeds and sensible heat flux values. These conditions, together with a deep residual layer, unstable atmosphere, and thermally driven local wind, produced an extremely deep PBL (9 km above sea level) in the early afternoon of November 23. When the synoptic wind direction at the ridge height intersected the axis of the valley and was weak, the DTWTV was weak, and the PBL became relatively low on November 28. These results demonstrate that the interaction between the topography and synoptic circulation plays a critical role in PBL growth.

Valley Winds at the Local Scale: Correcting Routine Weather Forecast Using Artificial Neural Networks

In regions of complex topography, local flows are difficult to forecast on a routine basis, especially in stable conditions, due to the coarse resolution of operational models. The Cadarache valley (southeastern France) features this sort of complex topography. The Weather Research and Forecasting (WRF) model is run daily to forecast the weather in this region with a horizontal resolution of 3 km. Such a resolution cannot resolve all topography details of the small Cadarache valley, and therefore its local wind patterns. Other variables, however, that are less dependent on the subgrid topography, are satisfactorily forecasted, and used as inputs to an artificial neural network (ANN) designed to reproduce wind observations inside the valley from WRF forecasts. A variable selection procedure identified 5 key input variables that best drive the ANN. With respect to the WRF output, the ANN significantly improves forecasted low-level winds, both for speed and direction. This study demonstrates the potential for the ANN technique to be used as a correcting tool to forecast weather conditions at the local scale when numerical modeling is performed at a resolution too coarse to take into account the effect of local topography.

Climatology of nocturnal low-level wind maxima at a topographically complex coastal site in Boseong

We examined the characteristics of nocturnal low-level wind maximum (NLWM) at a topographically complex site in Boseong, South Korea. The study site is located on the southern coast of the Korean peninsula. Although the area within a few kilometers of the site is flat, it is surrounded by mountains and faces the ocean on the southeast side. We defined NLWM as the lowest wind speed maximum within the lowest 300 m of the atmosphere that is at least 2 m s−1 greater than the next minimum above during nighttime. Climatology of NLWMs was derived from a 4-year half-hourly database of wind profiles (11 levels) obtained from a 300-m tower. The half-hourly data showed that NLWM occurred on approximately 18% of the nights. NLWMs were typically situated at 40 m above the ground. Half of the NLWMs had a speed of 3–5 m s−1, and its direction was predominantly west-northwesterly regardless of the height or season. The occurrence of NLWM showed a maximum in October and a minimum in July. The cause of NLWM at this site was discussed. It was found that the NLWM at this site occurred in the presence of valley wind forcing on nights with weak northerly geostrophic winds at 850 hPa.

Cable-car measurements of vertical aerosol profiles impacted by mountain-valley breezes in Lushan Mountain, East China

In-situ field observations of vertical aerosol profiles for one month in complex terrain (Lushan Mountain, China) were carried out using a cable car, which resolved detailed vertical distributions of mountain aerosols with low-cost operation. Cable-car observations were conducted during the early morning and late afternoon, when mountain and valley winds dominated, respectively. The diurnal aerosol variations at the top and foot of Lushan Mountain were analyzed based on environmental and meteorological stations. The observations indicated that the mountain-valley breezes notably impacted the mountain-area aerosol distribution under weak weather conditions. More uniform aerosol profiles for the afternoon than the morning, with their decreasing rates of PM2.5 (particles with diameters less than 2.5 μm) were 1.64 and 2.28 μg m−3/100 m, respectively. The PM2.5/PM10 ratio at the mountain top increased from 0.69 to 0.81, and that at the mountain base decreased from 0.75 to 0.70 from morning to afternoon. The PM2.5 concentration decreased in and around Lushan Mountain from daytime to nighttime, with the impacted diameter of the 300-m topography line being smaller than ~5 km, while the concentration increased in Jiujiang City. The relative decreasing rate of PM2.5 was higher at the mountain top site (~20%) than at the base site (~2%) from daytime to nighttime. Moreover, uniform aerosol profiles could have been caused by regional transport through a relatively strong low-level synoptic flow (~5 m s−1) and the mountain's dynamic lifting effect.

The Alaiz experiment: untangling multi-scale stratified flows over complex terrain

Abstract. We present novel measurements from a field campaign that aims to characterize multi-scale flow patterns, ranging from 0.1 to 10 km in a time-resolved manner, in a mountainous region in northwestern Spain with a mountain–valley–ridge configuration. We select two flow cases where topographic-flow interactions were measured by five synchronized scanning Doppler wind lidars along a 10 km transect line that includes a cross section of the valley. We observed a hydraulic jump in the lee side of the mountain. For this case, the Froude number transition from supercritical (>1) at the mountain to subcritical (<1) at the valley is in agreement with previous experiments at a smaller scale. For a 1-year period, the measurements show such a transition about 10 % of the time, indicating a possible high occurrence of hydraulic jumps. The second flow case presents valley winds that are decoupled from the northerly flow aloft and show a stratified layered pattern, which is well captured by the lidar scans and complementary ground-based observations. These measurements can aid the evaluation of multi-scale numerical models as well as improve our knowledge with regards to mountain meteorology.

Characteristics of Clouds and Raindrop Size Distribution in Xinjiang, Using Cloud Radar Datasets and a Disdrometer

Observation data from March to May 2020 of the Ka-band millimeter-wave cloud radar and disdrometer, located in Xinjiang, a typical arid region of China, were used to study the diurnal variation of clouds and precipitation, raindrop size distribution (DSD), and the physical parameters of raindrops. The results showed that there are conspicuous diurnal changes in clouds and precipitation. There is a decreasing trend of the cloud base height (CBH) from 05:00 to 19:00 CST (China Standard Time, UTC +8) and a rising trend of CBHs from 20:00 to 04:00 CST. The cloud top height (CTH) and the cloud thickness show a rising trend from 03:00 to 05:00 CST, 12:00 to 14:00 CST, and 20:00 to 01:00 CST. The diurnal variation of clouds is mainly driven by wind and temperature closely related to the topography of the study area. There are three apparent precipitation periods during the day, namely, 02:00–09:00 CST, 12:00 CST, and 17:00–21:00 CST. The changes in the physical parameters of raindrops are more drastic and evident with a lower CBH, lower CTH, and higher number of cloud layers from 12:00 to 21:00 CST than other times, which are closely related to day-to-day variations of systems moving through, and incoming solar radiation and the mountain–valley wind circulation caused by the trumpet-shaped topography that opens to the west played a secondary role. The DSD is in agreement with a normalized gamma distribution, and the value of the shape factor μ is significantly different from the fixed μ value in the Weather Research and Forecasting (WRF) Model. The rain in arid Xinjiang had a higher concentration of raindrops and a smaller average raindrop diameter than the rain in other humid regions of the Central and Southeast Asian continent. In the Z−R (radar reflectivity–rain rate) relationship, Z=249R1.20 is derived for stratiform rain, and it is significantly different from humid regions. Using Z/Dm (mass–weighted mean diameter) and R, a new empirical relationship Z/Dm=214R1.20 is established, and improvement is obtained in rain retrieval by using the Z/Dm−R relation relative to the conventional Z−R relation. Additionally, the Nt−R, Dm−R, Nw−R, and Nt−Nw relationships with larger differences from humid regions are established by fitting the power-law equations. These results are useful for improving the data parameters of microphysical processes of WRF and the accuracy of quantitative precipitation estimation in arid regions.

The influence of upslope fog on hygroscopicity and chemical composition of aerosols at a forest site in Taiwan

This study investigated the influence of upslope fog formation on the chemical composition and single hygroscopicity parameter (κ) of rural aerosols from Dec. 1st to Dec. 24th, 2018 at the Xitou forest site (23.67°N, 120.80°E, 1178 m above sea level (a.s.l.)) in central Taiwan. The aerosol chemical compositions were monitored using a mini compact time-of-flight aerosol mass spectrometer (mini-C-ToF-AMS), and the ambient aerosol particles were collected by a 13-stage micro-orifice uniform deposit impactor (MOUDI) for a Fourier-transform infrared spectrometer with an attenuated total reflectance accessory (FTIR-ATR) analysis. κ of aerosols was derived from the comparison of AMS pToF size distribution and FTIR-ATR measurement using the κ-Köhler equation. Our results show that the moderate correlation between OOA and CO provided evidence of upstream anthropogenic emission and time-lagged aged secondary organic aerosol transport by the daytime sea breeze and valley wind. Larger particles (Dva ~800 nm–1.0 μm) had a higher fog scavenging removal efficiency (62% for organic and 81% for nitrate, respectively), and smaller particles (Dva ~100 nm–700 nm) could remain growth through gas-to-aqueous partition. The submicrometer nitrate particles were likely significantly formed via gas-particle partitioning as HNO3(aq) or NH4NO3 while the larger micrometer-sized sea salt-like nitrate particles might be formed via heterogeneous reaction of gaseous HNO3 with deliquesced sea salt particles. The inconsistency of submicrometer nitrate between real-time AMS and offline FTIR-ATR measurements indicates that the evaporation loss of HNO3 or NH4NO3 during MOUDI filter sampling could lead to the unavailable κp−NO3 (κ for nitrate-containing particles) retrieval. The average κp-org (organics-containing particles) was 0.19 ± 0.12 (r2 ≥ 0.8) and nearly overlapped with κp−SO4 (0.22 ± 0.08) (sulfate-containing particles) during misty daytime, indicating that aerosols were more likely internally mixed particles to have similar hygroscopicity and physical mixing state property.

Distribution Characteristics of Aerosol Size and CCN during the Summer on Mt. Tian and Their Influencing Factors

The aerosol size distribution and cloud condensation nuclei (CCN) number concentration were measured using a wide-range particle spectrometer (WPS) and a cloud condensation nuclei counter (CCNC) on Mt. Tian from 31 July to 9 September, 2019. Combined with meteorological data, distribution characteristics of aerosol size and CCN and their influencing factors were analyzed. The results indicated that the mean aerosol number concentration was 5475.6 ± 5636.5 cm−3. The mean CCN concentrations were 183.7 ± 114.5 cm−3, 729.8 ± 376.1 cm−3, 1630.5 ± 980.5 cm−3, 2162.5 ± 1345.3 cm−3, and 2575.7 ± 1632.9 cm−3 at supersaturation levels of 0.1%, 0.2%, 0.4%, 0.6%, and 0.8%, respectively. The aerosol number size distribution is unimodal, and the dominant particle size is 30–60 nm. Affected by the height of the boundary layer and the valley wind, the diurnal variation in aerosol number concentration shows a unimodal distribution with a peak at 17:00, and the CCN number concentration showed a bimodal distribution with peaks at 18:00 and 21:00. The particle size distribution and supersaturation have a major impact on the activation of the aerosol into CCN. At 0.1% supersaturation (S), the 300–500 nm particles are most likely to activate to CCN. Particles of 100–300 nm are most easily activated at 0.2% (S), while particles of 60–80 nm are most likely activated at high supersaturation (≥0.4%). The concentrations of aerosol and CCN are higher in the northerly wind. Ambient relative humidity (RH) has little relationship with the aerosol activation under high supersaturation. According to N = CSk fitting the CCN spectrum, C = 3297 and k = 0.90 on Mt. Tian, characteristic of the clean continental type.

New insights into particle-bound trace elements in surface snow, Eastern Tien Shan, China

Trace elements (TEs) in the insoluble particles of surface snow are less affected by melting processes and can be used as environmental proxies to reveal natural and anthropogenic emissions. Here the first comprehensive study of the 16 TEs (Al, As, Ba, Bi, Cr, Cu, Fe, Mn, Ni, Pb, Sn, Sr, Ti, U, V, and Zn) in insoluble particles (>0.45 μm) from surface snow samples collected at Urumqi Glacier No. 1 (UG1), Eastern Tien Shan, China, from February 2008 to January 2010 were presented. Results show that concentrations of most insoluble particulate TEs (TEs insol) in the snow were higher in summer while lower in winter, due to the increasing particle inputs and melting processes. The abundances of As, Cr, Cu, Ni, Pb, and Zn in some samples were higher than those in surrounding urban soils, which might due to these TEs have further anthropogenic input beyond the already contaminated re-suspended urban soil particles and TEs were mainly enriched in particles with small grain size. Based on enrichment factor (EF) and principal component analysis (PCA), our results suggest that eight TEs (Al, Fe, Ti, Ba, Mn, Sr, U, and V) mainly came from mineral dust, while the remaining eight TEs (As, Bi, Cr, Cu, Ni, Pb, Sn, and Zn) were affected by coal combustion, mining and smelting of non-ferrous metals, traffic emissions, and the steel industry. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model suggests that pollutants might originate from Xinjiang province, Kazakhstan, and Kyrgyzstan. Moreover, UG1 received more significant inputs of particle-bound pollutants in summer than in winter due to the stronger convection and the prevailing valley wind that transports pollutants from the city of Urumqi.

Grain size characteristics of aeolian sands and their implications for the aeolian dynamics of dunefields within a river valley on the southern Tibet Plateau: A case study from the Yarlung Zangbo river valley

Typical aeolian landforms have developed in China’s Yarlung Zangbo River valley, which is on the Qinghai–Tibet Plateau. Fully understanding the grain size characteristics of these dunefields is the first step to reveal the region’s aeolian dynamics. We selected three distinctive dunefields for grain size analysis: the Maquan, Shannan, and Mainling valley dunefields. At the scale of individual dunes, we found a trend for sands on the dune surface, with the sediments gradually becoming finer towards the crest, gradually becoming coarser, or exhibiting an asymmetric M-shaped distribution. In Maquan valley, these patterns resulted from interactions between the local airflow and local sands with a diameter of 2.0–2.5 Φ (177–250 μm). The asymmetric M-shaped distribution in the Shannan valley was mainly controlled by the positions on a dune and valley winds. The slightly coarser crests in Mainling valley were controlled by sands that are easily entrained by the wind (2.00–3.00 Φ or 125–250 μm) and that undergo saltation. At a dunefield scale, strong winds that entrained medium sands cause these sands to accumulate downwind in Maquan valley. Multi-directional winds combined with a limited sand supply (1.74–2.30 Φ or 203–299 μm) favored a stable aeolian environment in Shannan valley. The saltating sands produced by these winds build dunes under a constantly erosive wind regime in Mainling valley. Wind transported interdune sediments is an important local source for building dunes. The gradual variation of grain sizes moving downstream and the distinct evolution history of dunefield in Maquan valley as compared to Shannan and Mainling valleys may suggest the existence of a spatial connection among dune sands along the main stream of the Yarlung Zangbo River.

Diurnal Cycle of Precipitation Over the Maritime Continent Under Modulation of MJO: Perspectives From Cloud‐Permitting Scale Simulations

AbstractCloud‐permitting scale (3 km) simulations are made to investigate the diurnal cycle of precipitation (DCP) over the Indo‐Pacific Maritime Continent under the modulation of the Madden‐Julian Oscillation (MJO), focusing on January–February 2018 during the Years of the Maritime Continent (YMC). Comparisons with satellite precipitation and global reanalysis products show that the simulations reasonably capture main features of diurnal wind and precipitation. The MJO modulates the amplitude, timing, and propagation of the DCP through affecting both large‐scale and local circulation and convection. Under local suppressed conditions, much strengthened sea/valley breezes are generally responsible for the enhanced and earlier‐triggered (2‐ to 3‐hr lead) DCP over land. Meanwhile, stronger cooling induced by enhanced stratiform rainfall excites well‐developed gravity waves spreading outward and upward, accompanied by continuous decreases in low‐level temperature and amplified land breezes, which cause stronger and further offshore‐propagating DCP. The opposite is true under local active conditions. Modulated by the passing of MJO, the enhanced DCP also displays a systematic shift from the west to the east coasts of large islands, forming a unique dipolar structure in the anomalous island DCP. The DCP exhibits island‐dependent characteristics: With topographical elevation, the triggering mechanism of island convection transforms from later penetrating sea breeze fronts over mountain foothills to early‐excited valley winds over mountain tops. Steep topography also supports stronger intraseasonal variations of the DCP, which can be well explained by regional variations in land‐sea/mountain‐valley breezes, terrain lifting, and ambient wind‐induced advective and vertical wind shear effects.