Mountain-valley Circulations Research Articles

Lev Gutman—A Pioneer in Theoretical Mesoscale Meteorology

Abstract On 5 March 2023, Professor Lev Gutman would have been 100 years old. This article describes Professor Gutman’s legacy in the field of dynamic mesoscale meteorology and numerical weather prediction. Gutman developed his career as a mathematician and meteorologist in the Soviet Union, where he built a school of specialists in mesoscale meteorology from the 1950s through the 1970s. He primarily worked on analytical methods to solve complex nonlinear problems, such as the structure of sea breezes, mountain–valley circulations, and thermal convection over heated terrain. Gutman pioneered the development of theories of cumulus clouds, tornadoes, and other atmospheric phenomena. In the 1960s, he carried out numerous research investigations on these topics with his doctoral students and collaborators at the High-Altitude Geophysical Institute in Nalchik in the northern Caucasus and later at the Siberian Scientific Center near Novosibirsk. Gutman compiled the results from these studies into a monograph titled “Introduction to the Nonlinear Theory of Mesoscale Meteorological Processes,” which was published in Russian in 1969 and later translated into English, Chinese, and Japanese. This monograph became a major textbook for specialists in mesoscale meteorology, remaining relevant to this day. After Professor Gutman immigrated to Israel in 1978, his collaborations expanded to include Israeli and western scientists from Europe and the United States. Gutman did not receive the recognition he deserved due to the political realities of the time. His book and his seminal analytical solutions should still be useful for early career scientists in mesoscale meteorology and atmospheric dynamics.

Attenuation of mountain-valley circulations on PM2.5 pollution over the western Sichuan basin, southwest China

The Sichuan Basin (SCB), located immediately to the east of the Tibetan Plateau (TP) in southwest China, is identified as a region with severe PM2.5 pollution, especially in the western SCB region. To understand the terrain effect on the atmospheric environment change in detail, this study investigated the effect degree and meteorological mechanism of thermally driven mountain-valley breeze (MVB) circulations on wintertime PM2.5 in the western SCB region, based on the near-surface observations of PM2.5 and the ERA5 reanalysis data of meteorology. The results showed that the western SCB edge exhibited a significant diurnal change of MVB, shifting between daytime upslope easterly flows and nighttime downslope westerly flows. The frequency of the MVB circulations was accounted for 39% days in December 2017, with the mountain and valley breeze-controlling periods being from 01:00 to 05:00 and 14:00 to 17:00 local time, respectively. Notably, the hourly PM2.5 reductions of 13.9 ± 4.6 μg m−3 was averaged during the MVB days in the western SCB edge, resulting in a decrease of 46.4 ± 14.0% in PM2.5 pollution with the MVB, which indicates that the MVB could alleviate PM2.5 pollution in improving air quality over the western SCB region. The daytime MVB circulations drove the transport of PM2.5-rich air mass in the atmospheric boundary layer from the polluted western SCB edge to the surrounding regions, causing 22% attenuation in near-surface PM2.5 concentrations during the valley breeze-controlling period. The nocturnal MVB circulations carried clean TP air eastward downslope along the eastern slope of the TP into the polluted western SCB region, mitigating PM2.5 levels by 20% during the mountain breeze-controlling period.

Froude‐number‐based rainfall regimes over the Western Ghats mountains of India

AbstractVariations in the character of monsoonal rainfall over the Western Ghats region on the west coast of India are studied using radiosondes, satellite observations, and reanalysis products. Summer monsoon rainfall over this region occurs in alternate offshore and onshore phases. It is shown that these phases are controlled primarily by the strength of the low‐level westerly jet. Thus, a classification based on the Froude number, , of the onshore flow is proposed, where, is the mountain height, is the mean wind speed, and is the mean Brunt–Väisäla frequency over depth . At low (< 0.5), onshore winds are weak and the diurnal thermal fluctuation over the orography is strong; the land–sea and mountain–valley circulations are enhanced, leading to a stronger diurnal control over the rainfall. A nocturnal offshore propagation of rainfall from the west coast is seen during this phase. Rainfall over the rainshadow region to the east of the Western Ghats also increases, due to a weaker lee effect, while it decreases over the Western Ghats, due to a greater blocking effect. At high (> 1), orographic blocking of the low‐level winds is weak. Thus, rainfall is enhanced over the Western Ghats and reduced over the rainshadow region due to a stronger lee effect. In this phase, the diurnal thermal fluctuation over the orography is weak. The bulk Richardson number is less than 1, suggesting a dominance of vertical wind shear over the buoyancy forces. The level of free convection and convective inhibition over the west coast are also very low. Hence, at high , rainfall over the west coast results mainly from mechanical uplifting of the westerly winds by the Western Ghats, with no preference for a particular time of day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models.

Mesoscale Impacts on Cold Season PM<sub>2.5</sub> in the Yukon Flats

Near-surface PM2.5 and meteorological observations were performed in three rural communities in the high latitude Yukon Flats valley at various times during the cold season (October to April). These data were synthesized with data from other meteorological sites, NCEP reanalysis and MAIAC retrieved aerosol optical depths data to analyze the role of mesoscale processes and radiation on air quality. Under weak large-scale forcing mountain-valley circulations develop that are driven by the differences in insolation. During the long dark nights, radiative cooling occurs in the near-surface layer of the mountain slopes of the Brooks, Ogilvie and White Mountains Ranges and at the bottom of the valley. Here surface-based inversions (SBI)—known as roof-top inversions—forms, while the cold air drains from the slopes. A frontal wedge forms when the cold air slides over the relatively colder air in the valley. Drainage of cold air from the Brooks Range governed the circulation and cold air pooling in the valley. Concentrations during times with and without SBI differed significantly (at 95% confidence) at two sites indicating that local emissions were the major contributor. At the site, which is closest to the mountains, concentrations marginally changed in the presence of inversions. At all sites, 24-h mean PM2.5 remained below the National Ambient Air Quality Standard.

Seasonal variation of chemical characteristics of fine particulate matter at a high-elevation subtropical forest in East Asia

The aim of this study was to chemically characterize the fine particulate matter (PM2.5) at a subtropical forest in East Asia under the influences of anthropogenic and biogenic sources and a complex topographic setting. Four seasonal campaigns were conducted at the Xitou Experimental Forest in central Taiwan from the winter of 2013 to the autumn of 2014. The results indicated that the ambient levels and chemical features of PM2.5 exhibited pronounced seasonal variations. Non-sea-salt sulfate (nss-SO42-) constituted the major component of PM2.5, followed by ammonium (NH4+) and nitrate (NO3−) during winter, summer and autumn. However, it was revealed that the mass fraction of NO3− increased to be comparable with that of nss-SO42- in springtime. The mass contribution of secondary organic carbon (SOC) to PM2.5 peaked in summer (13.2%), inferring the importance of enhanced photo-oxidation reactions in SOC formation. Diurnal variations of O3 and SO2 coincided with each other, suggesting the transport of aged pollutants from distant sources, whereas CO and NOx were shown to be under the influences of both local and regional sources. Notably high sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were observed, which were 0.93 ± 0.05 and 0.39 ± 0.20, respectively. Precursor gases (i.e. SO2 and NOx) could be converted to sulfate and nitrate during the transport by the uphill winds. Furthermore, due to the high relative humidity at Xitou, enhanced aqueous-phase and/or heterogeneous reactions could further contribute to the formation of sulfate and nitrate at the site. This study demonstrated the significant transport of urban pollutants to a subtropical forest by the mountain-valley circulations as well as the long-range transport from regional sources, whereas the implications of which for regional climate change necessitated further investigation.

Impacts of sea-land and mountain-valley circulations on the air pollution in Beijing-Tianjin-Hebei (BTH): A case study.

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9-10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM2.5) and ozone (O3) concentrations compared to observations in BTH. The model results have shown that the offshore land breeze transports the pollutants formed in Shandong province to the Bohai Gulf in the morning, causing the formation of high O3 and PM2.5 concentrations over the gulf. The onshore sea breeze not only causes the formation of a convergence zone to induce upward movement, mitigating the surface pollution to some degree, also recirculates the pollutants over the gulf to deteriorate the air quality in the coastal area. The upward valley breeze brings the pollutants in the urban area of Beijing to the mountain area in the afternoon, and the downward mountain breeze transports the pollutants back during nighttime. The intensity of the mountain-valley breeze circulation is weak compared to the land-sea breeze circulation in BTH. It is worth noting that the local circulations play an important role when the large-scale meteorological conditions are relatively weak.

Mapping aerosol intrusion in Himalayan valleys using the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

Mapping the spatial and temporal distribution of aerosols along mountain ranges is an important step toward elucidating orographic aerosol–cloud–rainfall interactions. This requires high spatial resolution aerosol observations over complex topography, which are not currently available either from ground-based observing systems or from remote-sensing products. Here, a novel approach is presented that relies on visible channels from MODIS Rapid Response data at 250 m spatial resolution to extract the daytime aerosol run-up (intrusion length and height) from the Indo-Gangetic Plains to the High Himalaya. Intrusion length and height are determined from the intersection of topography with the MODIS-derived aerosol plume using an adaptive object-classification algorithm. The methodology is demonstrated for a case study of the Arun River in eastern Nepal. Results of run-up extraction are examined along with the Total Attenuated Backscatter (Level 1B at 532 nm) from CALIPSO to investigate the regional variability of aerosol. During the pre-monsoon season, CALIPSO nighttime profiles show the presence of a slanted dust layer following the envelope topography. This is consistent with upper level transport of aerosol by north-westerly winds associated with high-frequency dust storms. In the winter, the signal is weaker, and the nighttime elevated aerosol layer is flat and remains below the envelope orography consistent with blocking conditions. For both seasons, the daytime aerosol layer detected from MODIS observations is always below the ridges. This suggests that in addition to seasonal variability governed by synoptic conditions, there is a distinct diurnal cycle in the North–South transport of aerosol between the Himalayas and the IGP. ► High-resolution mapping of aerosol plumes in mountainous regions. ► Aerosol run-up (intrusion length and plume depth) in deep Himalayan valleys. ► Remote-sensing technique using MODIS RR and CALIPSO TBA data. ► Pre-monsoon nocturnal aerosol transport elevated above orography, blocked in winter. ► Diurnal cycle impacted by local sources, governed by mountain-valley circulations.

High-resolution simulations of atmospheric CO<sub>2</sub> over complex terrain – representing the Ochsenkopf mountain tall tower

Abstract. Accurate simulation of the spatial and temporal variability of tracer mixing ratios over complex terrain is challenging, but essential in order to utilize measurements made in complex orography (e.g. mountain and coastal sites) in an atmospheric inverse framework to better estimate regional fluxes of these trace gases. This study investigates the ability of high-resolution modeling tools to simulate meteorological and CO2 fields around Ochsenkopf tall tower, situated in Fichtelgebirge mountain range- Germany (1022 m a.s.l.; 50°1′48" N, 11°48′30" E). We used tower measurements made at different heights for different seasons together with the measurements from an aircraft campaign. Two tracer transport models – WRF (Eulerian based) and STILT (Lagrangian based), both with a 2 km horizontal resolution – are used together with the satellite-based biospheric model VPRM to simulate the distribution of atmospheric CO2 concentration over Ochsenkopf. The results suggest that the high-resolution models can capture diurnal, seasonal and synoptic variability of observed mixing ratios much better than coarse global models. The effects of mesoscale transports such as mountain-valley circulations and mountain-wave activities on atmospheric CO2 distributions are reproduced remarkably well in the high-resolution models. With this study, we emphasize the potential of using high-resolution models in the context of inverse modeling frameworks to utilize measurements provided from mountain or complex terrain sites.

An Observational and Modeling Study of Characteristics of Urban Heat Island and Boundary Layer Structures in Beijing

Abstract In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate these urban weather features for comparison with observations. WRF is also used to test the sensitivity of model simulations to different urban land use scenarios and urban building structures to investigate the impacts of urbanization on surface weather and boundary layer structures. Results show that the coupled WRF/Noah/UCM modeling system seems to be able to reproduce the following observed features reasonably well: 1) the diurnal variation of UHI intensity; 2) the spatial distribution of UHI in Beijing; 3) the diurnal variation of wind speed and direction, and interactions between mountain–valley circulations and UHI; 4) small-scale boundary layer convective rolls and cells; and 5) the nocturnal boundary layer lower-level jet. The statistical analyses reveal that urban canopy variables (e.g., temperature, wind speed) from WRF/Noah/UCM compare better with surface observations than the conventional variables (e.g., 2-m temperature, 10-m wind speed). Both observations and the model show that the airflow over Beijing is dominated by mountain–valley flows that are modified by urban–rural circulations. Sensitivity tests imply that the presence or absence of urban surfaces significantly impacts the formation of horizontal convective rolls (HCRs), and the details in urban structures seem to have less pronounced but not negligible effects on HCRs.

Annual and Diurnal Wind Patterns in the City of São Paulo

The major topographic, mesoscale, and urban influences on the wind patterns of the City of Sao Paulo are characterized using one year of surface wind velocity data observed at 11 surface stations within its urban limits. The data was used to study the diurnal and annual variations of wind velocity and horizontal wind divergence within the city. Results showed that the circulation over the investigated area is dominated by three major factors: sea breeze; mountain-valley circulations; and urban effects, such as roughness, building-barrier, and urban heat island. The sea breeze was found to be the dominant feature of the monthly-averaged diurnal variation of Sao Paulo surface winds during the eight warmest months of the year. The sea breeze front induces a velocity minimum at the time of its passage and a post-frontal afternoon velocity maximum. Mountain-valley thermal effects on the flow can be seen in the temporal divergence/convergence patterns. These thermal effects tend to be more important during colder months, at night, and when the wind velocities are low. Nighttime downslope convergent flows are present over the city during winter and spring and daytime upslope divergent flows are present over the city during summer months.

Cloud water chemistry in Sequoia National Park

Interception of cloudwater by forests in the Sierra Nevada Mountains may contribute significantly to acidic deposition in the region. Cloudwater sampled in Sequoia National Park had pH values ranging from 4.4 to 5.7. The advance of cold fronts into the Park appears to lead to higher aerosol and gas phase concentrations than are seen under normal mountain-valley circulations, producing higher cloud-water concentrations than might otherwise be expected. Estimates of annual deposition rates of NO 3 −, SO 4 2−, NH 4 + and H + due to cloudwater impaction are comparable to those measured in precipitation.

Numerical Simulation of Slope and Mountain Flows

Abstract Early descriptive models of mountain-valley circulations indicated that the mountain flow (i.e., the along-valley axis component out of the valley) is a true three-dimensional phenomenon. According to these descriptions, at night shallow-down slope flows on the valley sidewalls directly driven by temperature deficits near the surface produce a pooling of cool air in the valley. This deep pool of cool air in the valley compared with a much shallower surface inversion over the plains (to which the valley opens) produces a secondary flow (the mountain flow) out of the valley driven by a deep hydrostatic pressure gradient. It is this deep secondary flow which is most important to pollutant transport in deep valleys and which has not been previously investigated in a numerical model. It is the purpose of this investigation to numerically simulate the above-mentioned secondary circulation using a three-dimensional numerical model. The Colorado State University Hydrostatic Mesoscale Model-a hydrostatic, primitive equation model, forced by a surface energy budget-was utilized for the simulations. Both idealized topography and an actual Colorado valley were used in the investigation to emulate the classic mountain-plain configuration discussed above. Special attention was given to the development of the sidewall slope flows which produce the pooling of air in the valley. Results of the simulations indicated shallow slope flows less than 100 m deep started almost immediately after cooling began and reached their maximum velocity after approximately one hour and then slowly decreased through the night. A high level of turbulence was noted near the top of the slope flows. The turbulence was not only due to shear in the slope flow, but to destabilization of the temperature profile because of thermal advection. Definite pooling of cool air occurred in the valley with cooling occurring at all heights to near ridge tops. The secondary flow out of the valley lagged the development of the slope flows by approximately one hour, but was very deep, filling the whole valley to near ridge height.