Air Advection Research Articles

Влияние пыльных и песчаных бурь на работу авиационного транспорта и оценка условий их возникновения на аэродромах европейской части России

A review of literature on the impact of dust and sand storms on the air transport operation is presented. Observational data on dust storms at the aerodromes of European Russia for the period of 2001-2019 are analyzed. The seasonal variations in dust transport episodes at aerodromes and its relationship with visibility changes are discussed. The characteristics of dusty air masses and advection are given. It is concluded that the frequency of dust transfer episodes for the aerodromes under study has decreased over the past five years, except for Gumrak aerodrome (Volgograd). Keywords: dust storm, sand storm, aviation, visibility, seasonal variations, aerodrome оf European Russia

Bioindication of the dangerous geomorphological processes’ activity centers (on the example of the central part of the Chornohora massif)

Rapid climate change in the Ukrainian Carpathians has triggered the intensity and recurrence of geomorphological processes that in turn has changed the vertical structure of landscape complexes. Forest ecosystems serve apparent indicators of the processes. Accordingly, the study aims to identify the spatio-temporal features of avalanche processes, landslides, rock falls using bioindication in the central part of the Chornogora massif in the Ukrainian Carpathians. The main reasons for the intensification of current geomorphological processes include 1) climatic changes and variability in the course of precipitation and air temperatures in the cold season, 2) terrain features (slope, slope exposure and surface type), 3) vegetation features, etc. Snow avalanche activity is influenced by the fluctuation of diurnal temperature within -1 to +5 ° C. Temperature features of the territory have a direct impact on the formation of the structure of snow, stratigraphy, and thickness. One of the important factors triggering snow avalanches are the steepness of the slopes equal to 30-45. The main reasons for the manifestation of such are the force of gravity, which itself affects the movement of rocks from the slopes from elevated to low terrain. The causes of landslides and rock falls are sharp daily changes in air temperature, heavy rainfall, permafrost, melting snow, weathering and more. In mountainous areas, such processes are quite common, and they are associated with morphological, lithological, climatic and other conditions associated with the passage of these processes. The description of the five most intense centers of avalanches was identified and compiled based on the surveys of the Bystrets local residents and the detected deformations and damages of forest formations. The most intensive events were distinguished in 1977, 1995, 1998 and 2001. These avalanche events have caused the greatest changes in the landscape complexes of the central part of the Chornohora. The area occupied by the avalanche flows and the area of damaged forests were detected with the satellite images. According to the remaining tree material (trunks up to 1 m high), the main tree species damaged by the destructive force of the avalanche were identified. In addition, in the period of 2014-2018, 7 avalanche streams have taken place in the central part of the Chornohora. We also discovered a new avalanche centre, in addition to those mentioned earlier. This centre is located on the northeastern slope of the Gomul peak (1788 m). 57% of all the detected snow avalanches are considered epigenetic, 29% are syngenetic and 14% are polygenic. Most of them are associated with either the advection of warm Mediterranean air or local radiation warming of the surface air layer. The results of the survey of avalanche sites indicate a determinative role of tree-shrub formations in the indication of the destructive forces and consequences of avalanches. The restoration of landscapes after snow avalanche processes is also accompanied by the replacement of forest ecosystems with shrubs and the loss of valuable ecosystems, which may lead to the search for climatic explanations for such changes. Other geomorphological processes studied during the expeditions in 2019-2020 are landslides and rock falls. During the survey of the central part of the Chornohora massif for the manifestation of the processes, one active landslide was detected within the forest belt at the altitudes of 1000-1600 meters a.s.l l. Traces of bifurcations, scars and tilted tree species witness the landslide activity. The number of detected damaged trees reaches 20 units per 40 m2. Identified damaged coniferous formations serve valuable data for perspective dendrogeomorphological studies to reconstruct snow avalanches and other geomorphological processes of the past, as well as to distinguish local climate features.

An Analysis of Fog in the Mainland Portuguese International Airports

An analysis of 17 years of half-hourly aeronautic observations (METAR) and special observations (SPECI) in the three international airports of mainland Portugal indicates strong variations in fog properties. Fog is a rare event at Faro, a winter phenomenon in Lisbon and mainly a summer process at Porto. At both Lisbon and Porto, fog is favoured by specific synoptic circulations, here classified into a set of weather types, compatible with the strict requirements of fog formation. At the same time, however, a detailed analysis of the distribution of fog, and the classification of its onset processes, reveal a crucial dependence on local wind. This suggests that the advection of moist air from nearby sources, from the Tagus estuary at Lisbon and from the ocean at Porto, is the dominant process at both locations, despite the large differences found in the timing of those fog processes. The observational data (METAR) prior to the fog formation is used to classify the fog generation mechanism for 96.9% of the fog events at Porto, and 98.9% at Lisbon. Among the five fog types identified using a classification algorithm, cloud base lowering is the most common one at both locations, gathering half of the classified fog events, followed by advection, precipitation, and radiation. No fog event of the evaporation type was detected at both airports. The analysis of the observed horizontal visibility during the fog events revealed that cloud base lowering and radiation fog are the most intense events. The median of the minimum horizontal visibility of these two types of fog varies between 150 and 250 m, as the average ranges between 217.8 and 312.9 m. The study results have revealed a promising prefog diagnosis tool to be explored in detail in further operational context studies.

The Observed Water Vapor Budget in an Atmospheric River over the Northeast Pacific

AbstractCombined airborne, shipboard, and satellite measurements provide the first observational assessment of all major terms of the vertically integrated water vapor (IWV) budget for a 150 km × 160 km region within the core of a strong atmospheric river over the northeastern Pacific Ocean centered on 1930 UTC 5 February 2015. Column-integrated moisture flux convergence is estimated from eight dropsonde profiles, and surface rain rate is estimated from tail Doppler radar reflectivity measurements. Dynamical convergence of water vapor (2.20 ± 0.12 mm h−1) nearly balances estimated precipitation (2.47 ± 0.41 mm h−1), but surface evaporation (0.0 ± 0.05 mm h−1) is negligible. Advection of drier air into the budget region (−1.50 ± 0.21 mm h−1) causes IWV tendency from the sum of all terms to be negative (−1.66 ± 0.45 mm h−1). An independent estimate of IWV tendency obtained from the difference between IWV measured by dropsonde and retrieved by satellite 3 h earlier is less negative (−0.52 ± 0.24 mm h−1), suggesting the presence of substantial temporal variability that is smoothed out when averaging over several hours. The calculation of budget terms for various combinations of dropsonde subsets indicates the presence of substantial spatial variability at ~50-km scales for precipitation, moisture flux convergence, and IWV tendency that is smoothed out when averaging over the full budget region. Across subregions, surface rain rate is linearly proportional to dynamical convergence of water vapor. These observational results improve our understanding of the thermodynamic and kinematic processes that control IWV in atmospheric rivers and the scales at which they occur.

On the control of subantarctic stratification by the ocean circulation

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.

Daily and seasonal variabilities of thermal stress (based on the UTCI) in air masses typical for Central Europe: an example from Warsaw

The objective of research involved the comparison of daily and seasonal courses of thermal stress occurring in Central Europe depending on the inflowing air mass. The analysis used data from Warsaw (1991–2000), including air temperature (°C), water vapour pressure (hPa), wind speed (m s−1) and cloud cover (%). Universal thermal climate index was calculated and subsequently averaged for the individual months and four types of atmospheric air masses: polar maritime (mP), arctic(A), polar continental (cP) and tropical (T). The studies analysed differences in daily patterns of the averaged values of universal thermal climate index between air masses and determined the frequency of days with various types of thermal stress in individual air masses. The analyses indicated that under the conditions of Central Europe, the highest daily variance of biothermal conditions occurs between the masses of cP and T in the spring and autumn. Considerably greater diversity of biothermal conditions was observed between the masses during daytime compared with nighttime, especially in the warm half of the year. The thermal stress, which can be encountered in Central Europe, ranges from an “extreme cold stress” in winter at night and early morning hours to “very strong heat stress” in summer at noon. Extreme thermal stress is related primarily to the masses of cP, A and T. The most optimal biothermal conditions occur during the advection of mP air.

Quantifying the impact of synoptic weather types and patterns on energy fluxes of a marginal snowpack

Abstract. Synoptic weather patterns are investigated for their impact on energy fluxes driving melt of a marginal snowpack in the Snowy Mountains, southeast Australia. K-means clustering applied to ECMWF ERA-Interim data identified common synoptic types and patterns that were then associated with in situ snowpack energy flux measurements. The analysis showed that the largest contribution of energy to the snowpack occurred immediately prior to the passage of cold fronts through increased sensible heat flux as a result of warm air advection (WAA) ahead of the front. Shortwave radiation was found to be the dominant control on positive energy fluxes when individual synoptic weather types were examined. As a result, cloud cover related to each synoptic type was shown to be highly influential on the energy fluxes to the snowpack through its reduction of shortwave radiation and reflection/emission of longwave fluxes. As single-site energy balance measurements of the snowpack were used for this study, caution should be exercised before applying the results to the broader Australian Alps region. However, this research is an important step towards understanding changes in surface energy flux as a result of shifts to the global atmospheric circulation as anthropogenic climate change continues to impact marginal winter snowpacks.

The Balearic rissaga: from pioneering research to present-day knowledge

The pioneering work done in late 1970s and early 1980s on research into rissaga events or meteotsunamis in the Balearic Islands is briefly reviewed in this paper and contrasted with the present state of the art on the phenomenon. The early research phase used existing observational evidence to construct a doctrine for the initial comprehension and forecasting of the phenomenon. By the beginning of the 1980s, it had already been established that the rissaga is a marine response to quick atmospheric pressure oscillations in resonance conditions, initially attributed exclusively to internal atmospheric gravity waves and later to convective pressure jumps too. Forty years later, new observational methods have provided a much richer database, with theoretical research and the use of forecasts using numerical models underpinning a much more solid perspective. With regard to predictability, quick atmospheric pressure oscillations were associated with a three-layer atmospheric vertical structure from the beginning: a thermodynamically neutral lower layer of Mediterranean air, a thermal inversion layer (Saharan air lying above and Mediterranean air below) and a deep upper layer with conditional instability in at least some strata. This structure is compatible with a high-level trough or cut-off low over the Iberian Peninsula; strong south-westerly flow over the Western Mediterranean area; jet stream speed being reached at the highest tropospheric levels, and forced Saharan air advection at the low/medium levels. A weak low-pressure area is usually present at low levels in the Mediterranean. The identification of the meteorological framework favourable to a rissaga event soon led to a probabilistic and purely meteorological forecasting method for the rissaga phenomenon. This method is still used today, albeit in combination with objective methods, some of which are based on the use of coupled atmospheric and oceanic forecasting models.

Reasons for the Weakening of Tropical Depressions in the South China Sea

AbstractThe frequency and location distribution of tropical depressions (TDs) from 1979 to 2017 in the South China Sea (SCS) are statistically analyzed based on the best track data of tropical cyclones (TCs) from the Shanghai Typhoon Institute, China Meteorological Administration (CMA-STI). ECMWF interim reanalysis data (ERA-Interim) are used to investigate the reasons for the weakening of TDs in this study. The results show that there are 4.8 TDs formed in the SCS per year, and these TDs can be separated into 3.2 developing cases (DTDs) and 1.6 nondeveloping cases (NTDs) according to whether they intensify into tropical storms. Further objective classification by the multivariable-time empirical orthogonal function (MVT-EOF) method finds that the weakening cases in the positive-PC1 (the first principle component) mode occur in May–September, with the reason for weakening being a shortage of moisture. The decrease of westerly wind south of the NTDs reduces the water vapor transportation from the Indian Ocean. Binary TCs in the northwestern Pacific acquire water vapor from the eastern boundary of the SCS NTDs. Meanwhile, the weak high-level divergence and low-level convergence are not enough for the accumulation of local moisture and maintaining local convections inside the NTDs. The weakening cases in negative-PC1 mode occur in October–December with the reason for weakening being the invasion of cold air from the north. Strong cold air advection in the lower troposphere increases the vertical wind shear in front of the NTDs, and sharply reduce sensible and latent heat flux as well. Seasonal dependence exists in the causes of the SCS NTDs weakening.

Fast responses on pre-industrial climate from present-day aerosols in a CMIP6 multi-model study

Abstract. In this work, we use Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth system models (ESMs) and general circulation models (GCMs) to study the fast climate responses on pre-industrial climate, due to present-day aerosols. All models carried out two sets of simulations: a control experiment with all forcings set to the year 1850 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. In response to the pattern of all aerosols effective radiative forcing (ERF), the fast temperature responses are characterized by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India, sulfate being the dominant aerosol surface temperature driver for present-day emissions. In the Arctic there is a warming signal for winter in the ensemble mean of fast temperature responses, but the model-to-model variability is large, and it is presumably linked to aerosol-induced circulation changes. The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by a reduction over continental regions and presumably a southward shift of the tropical rain belt. This is a characteristic and robust feature among most models in this study, associated with weakening of the monsoon systems around the globe (Asia, Africa and America) in response to hemispherically asymmetric cooling from a Northern Hemisphere aerosol perturbation, forcing possibly the Intertropical Convergence Zone (ITCZ) and tropical precipitation to shift away from the cooled hemisphere despite that aerosols' effects on temperature and precipitation are only partly realized in these simulations as the sea surface temperatures are kept fixed. An interesting feature in aerosol-induced circulation changes is a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.

Atmospheric Structure for Convective Development in the Events of Cloud Clusters over the Korean Peninsula

This study examines atmospheric structures causing convective development in the events of cloud cluster (CC) over the Korean peninsula using the analysis and forecast data of National Centers for Environmental Prediction (NCEP) climate forecast system reanalysis (CFSR) and observation data. Two CC types—CCs associated with meso-α-scale lows (CCMLs) and mesoscale troughs (CCMTs)—were investigated. The common atmospheric structure for convective development in CC events is comprised of i) a strong southwesterly band (SWB; a region with southwesterly wind speeds >12.5 m s−1) in the lower troposphere upstream of CCs with a mesoscale convergence zone in its exit area, ii) a layer of high-θe air in the lower troposphere near the surface extending from the southwest to SWB exit, iii) elevated height of maximum θe in the lower troposphere near and over the convergence zone, above which a convectively unstable layer exists. Generality of the above-described structure has been demonstrated via examination of composite fields. SWB plays a major role in producing the structure for convective development in CC events over the Korean peninsula mainly through i) advection of high-θe air from the southwest, and ii) significant horizontal convergence in the exit area, which can facilitate convection initiation. The two types of CC show notable differences in atmospheric structure across the boundary between high-θe air from the southwest and low-θe air in the northeast and in the mode of high-θe air transport to the region of convective development. The boundary is generally tilted northeastward with height for CCML cases, whereas it is nearly vertical for the majority of CCMT cases. This study indicates that, despite the above-mentioned differences, convective developments in both CC types can be considered as elevated convection that occurs as air parcels in an elevated layer of convective instability are lifted by upward motion in the convergence zone. For both types of CC, differential θe advection plays the key role for the occurrence of elevated layer of convective instability. And θe front in CCML events indicates the presence of elevated convective instability above it and the possibility of elevated convection provided that a lifting mechanism is available.

A More General Paradigm for Understanding the Decoupling of Stratocumulus‐Topped Boundary Layers: The Importance of Horizontal Temperature Advection

AbstractMost prior studies on decoupling of a stratocumulus‐topped boundary layer (STBL) are focused on subtropics where cold air advection with moderate strength is dominant. This study expands across a wider spectrum of temperature advection spanning from moderately strong warm air advection to extremely strong cold air advection. A STBL undergoing warm advection is found to be more mixed than a STBL undergoing cold advection. This finding is consistent with the cold advection facilitating turbulent mixing in the boundary layer. When cold advection becomes sufficiently strong (<−5 K/day), the STBL becomes more stably stratified again because of emergence of the cumulus‐coupled STBL regime induced by the “deepening‐warming” mechanism. Such a “deepening‐warming” induced STBL decoupling, however, is still much weaker than that caused by warm advection flows (even weak ones), suggesting that the direction and strength of temperature advection must be considered for any STBL decoupling studies.

Aerosol Layers in the Troposphere: Peculiarities of Variations in Aerosol Parameters at a Change in the Advection Direction

Aerosol layers with different scattering and absorption properties are studied on the basis of the data of multifrequency sounding. The effect of air advection on the aerosol optical and microphysical parameters in the lower and middle troposphere is analyzed. It is revealed that the low values of the extinction and backscattering coefficients, as well as the imaginary part of the refractive index and mean geometric radius of fine particles are observed at the north transfer direction, and the high values of these parameters are at the south direction. On the contrary, the lidar ratio and the contribution of fine fraction into the total concentration of particles decrease when the direction has been changed from north to south.

Northern Hemisphere Winter Air Temperature Patterns and Their Associated Atmospheric and Ocean Conditions

AbstractThe Northern Hemisphere (NH) has experienced winter Arctic warming and continental cooling in recent decades, but the dominant patterns in winter surface air temperature (SAT) are not well understood. Here, a self-organizing map (SOM) analysis is performed to identify the leading patterns in winter daily SAT fields from 1979 to 2018, and their associated atmospheric and ocean conditions are also examined. Three distinct winter SAT patterns with two phases of nearly opposite signs and a time scale of 7–12 days are found: one pattern exhibits concurrent SAT anomalies of the same sign over North America (NA) and northern Eurasia, while the other two patterns show SAT anomalies of opposite signs between, respectively, NA and the Bering Sea, and the Kara Sea and East Asia (EA). Winter SAT variations may arise from changes in the SOM frequencies. Specifically, the observed increasing trends of winter cold extremes over NA, central Eurasia, and EA during 1998–2013 can be understood as a result of the increasing occurrences of some specific SAT patterns. These SOMs are closely related to poleward advection of midlatitude warm air and equatorward movements of polar cold airmass. These meridional displacements of cold and warm airmasses cause concurrent anomalies over different regions not only in SAT but also in water vapor and surface downward longwave radiation. Anomalous sea surface temperatures in the tropical Pacific, midlatitude North Pacific, and North Atlantic and anomalous Arctic sea ice concentrations also concur to support and maintain the anomalous atmospheric circulation that causes the SAT anomalies.

Did the Climate Forecast System Anticipate the 2015 Caribbean Drought?

AbstractIn groundwater-limited settings, such as Puerto Rico and other Caribbean islands, societal, ecological, and agricultural water needs depend on regular rainfall. Though long-range numerical weather predication models explicitly predict precipitation, such quantitative precipitation forecasts (QPF) critically failed to detect the historic 2015 Caribbean drought. Consequently, this work examines the feasibility of developing a drought early warning tool using the Gálvez–Davison index (GDI), a tropical convective potential index, derived from the Climate Forecast System, version 2 (CFSv2). Drought forecasts are focused on Puerto Rico’s early rainfall season (ERS; April–July), which is susceptible to intrusions of strongly stable Saharan air and represents the largest source of hydroclimatic variability for the island. A fully coupled atmosphere–ocean–land model, the CFSv2 can plausibly detect the transatlantic advection of low-GDI Saharan air with multimonth lead times. The mean ERS GDI is calculated from semidaily CFSv2 forecasts beginning 1 January of each year between 2012 and 2018 and monitored as the initialization approaches 1 April. The CFSv2 demonstrates a broad region of statistically significant correlations with observed GDI across the eastern Caribbean up to 30 days prior to the ERS. During 2015, the CFSv2 forecast a low-GDI tongue extending across the Atlantic toward the Caribbean with 60–90 days lead time and placed Puerto Rico’s 2015 ERS beneath the 15th percentile of all 1982–2018 ERS forecasts with up to 30 days lead time. A preliminary GDI-based QPF tool tested herein is a statistically significant improvement over climatology for the driest years.

Characteristics of the atmospheric vertical structure with different sea ice covers over the Pacific sector of the Arctic Ocean in summer

During the Chinese National Arctic Research Expedition in 2018 (CHINARE 2018), 84 radiosondes were released over the Pacific sector of the Arctic Ocean. Using these data, the characteristics of the atmospheric vertical structure with different sea ice covers were studied. The results showed that (1) typical temperature inversions were found in the low-level atmosphere with various characteristics over different regions. The inversions over the open water were the weakest and elevated. Oppositely, the inversions over the very pack ice zone were the strongest and located at the lowest altitude. Warm air advection, radiative cooling and surface melt together provided favorable conditions for the occurrence of an inversion over the very close pack ice zone. As a transition region, the marginal ice zone presented relatively moderate inversions that were mainly controlled by warm air advection and surface melt. (2) The upper-level jet over the very close pack ice zone was the weakest, and large differences were observed in the characteristics of low-level jets (LLJs) over different regions. With increasing sea ice, the frequency and strength of the LLJ increased and tended to enhance, respectively. Most LLJs were located below the tops of the inversions over the very close pack ice zone. Additional observation data are required to confirm that sea ice has significant impacts on the atmospheric vertical structure.

Shifts in the physical environment in the Pacific Arctic and implications for ecological timing and conditions

The northern Bering Sea and Chukchi Sea represent the gateway from the Pacific to the Arctic. This contiguous marine system encompasses one of the largest continental shelves in the world and serves as the sole point of connection between the North Pacific and Arctic Ocean. This region has unique attributes and complex dynamics, driven by the convergence of distinct water masses, dynamic currents, advection between Pacific and Arctic systems, and important latitudinal gradients relevant to stratification and water mass structure, water temperature, and seasonal ice cover. Many processes and interactions in the region appear to be changing with important implications for both hydrography and ecology. Our analyses access remote and local data sources in US and Russian waters to characterize oceanographic conditions and analyze the implications of dramatic shifts in recent years. Previously, this region appeared resistant to trends apparent elsewhere in the greater Arctic. Now, the Pacific Arctic also appears to be in rapid transition. The conditions observed in 2017–2019 are unprecedented. We note important shifts in the phenology and magnitude of physical variables, including sea-ice extent, concentration, and duration, as well as extreme reduction in the extent and intensity of the related Bering Sea cold pool. We also note distinct regional dynamics in sea surface temperature in the Bering-Chukchi system, distinguishing western, eastern and northern areas of the Bering Sea. Specifically, our analyses distinguish the northern Bering Sea as an important transition zone between the Pacific and Arctic with higher frequency variability in sea surface temperature anomalies. Our results suggest that the strength and position of the Aleutian Low may be linked to warm and cold phases in the Bering Sea and has an important role in large-scale circulation. While cold winds out of the north are necessary to form ice in the northern Bering Sea, strong winds may be associated with weak sea ice, as wind action may break ice and enhance vertical mixing, counteracting enhanced sea-ice production from the advection of cold air. Research in this important region is complicated by international borders but may be enhanced through international collaboration. This analysis represents an attempt to integrate data across Russian and US waters to more fully represent system-wide processes, to contrast regional trends, and to better understand physical interactions.

Small-scale structure of thermodynamic phase in Arctic mixed-phase clouds observed by airborne remote sensing during a cold air outbreak and a warm air advection event

Abstract. The combination of downward-looking airborne lidar, radar, microwave, and imaging spectrometer measurements was exploited to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the thermodynamic phase of low-level Arctic mixed-layer clouds. Two cloud cases observed in a cold air outbreak and a warm air advection event observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign were investigated. Both cloud cases exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The horizontal, small-scale distribution of the thermodynamic phase as observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability in the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability in this warm air advection. Large eddy simulations (LESs) were performed to reconstruct the observed cloud properties, which were used subsequently as input for radiative transfer simulations. The LESs of the cloud case observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated ice water content (IWC) was systematically lower than the measured IWC. Nevertheless, the LESs revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability in the cloud field. It is concluded that the cloud top small-scale horizontal variability is directly linked to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m miss the small-scale cloud top structures.

On a Flood-Producing Coastal Mesoscale Convective Storm Associated with the Kor’easterlies: Multi-Data Analyses Using Remotely-Sensed and In-Situ Observations and Storm-Scale Model Simulations

A flood-producing heavy rainfall event occurred at the mountainous coastal region in the northeast of South Korea on 5–6 August 2018, subsequent to extreme heat waves, through a quasi-stationary mesoscale convective system (MCS). We analyzed the storm environment via a multi-data approach using high-resolution (1-km) simulations from the Weather Research and Forecasting (WRF) and in situ/satellite/radar observations. The brightness temperature, from the Advanced Himawari Imager water vapor band, and the composite radar reflectivity were used to identify characteristics of the MCS and associated precipitations. The following factors affected this back-building MCS: low-level convergence by the Korea easterlies (Kor’easterlies), carrying moist air into the coast; strong vertical wind shear, making the updraft tilted and sustained; coastal fronts and back-building convection bands, formed through interactions among the Kor’easterlies, cold pool outflows, and orography; mid-level advection of cold air and positive relative vorticity, enhancing vertical convection and potential instability; and vigorous updraft releasing potential instability. The pre-storm synoptic environment provided favorable conditions for storm development such as high moisture and temperature over the coastal area and adjacent sea, and enhancement of the Kor’easterlies by expansion of a surface high pressure system. Upper-level north-northwesterly winds prompted the MCS to propagate south-southeastward along the coastline.

Influence of wintertime surface sensible heat flux variability over the central and eastern Tibetan Plateau on the East Asian winter monsoon

Analyses of in situ and reanalysis output are performed to examine linkages between surface sensible heat fluxes over the central and eastern Tibetan Plateau (CETP) and indices of the East Asian winter monsoon (EAWM) and wintertime near-surface air temperatures over China. The results demonstrate that the sensible heat fluxes over CETP exhibit substantial decadal variability with positive, negative and positive phase during 1980–1987, 1988–2002 and 2003–2014, respectively. This decadal variability exhibits statistically significant associations with sub-components of EAWM and surface temperature anomalies over eastern China. The recovery of decadal change in wintertime sensible heat fluxes from negative to positive phase over CETP over the past decade (since 2003) has been associated with intensification and northward displacement of the East Asian subtropical jet (EASJ), enhanced and westward displacement of East Asian trough and strengthening of the Siberian High. Such processes are associated with the strengthening of EAWM, as well as cold air advection from high latitudes towards the south. Increased wintertime sensible heat fluxes over CETP is associated with markedly changes in the meridional temperature gradient that in term intensifies upper-level zone flow in the EASJ region, which provides a key physical factor linking the anomalies of sensible heat fluxes and EAWM.