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An initially well‐mixed stratocumulus deck can remain overcast for several tens of hours under warm‐advection conditions, although moisture supply is cut off from the ocean due to surface‐atmosphere decoupling (stabilization of the surface‐atmosphere interface). In this study, a set of idealized large‐eddy simulations were performed to investigate the physical mechanism of how warm‐air advection impacts the evolution of a pre‐existing stratocumulus deck. To mimic warm‐air advection, we decrease the sea surface temperature linearly over time in a doubly periodic domain. Given the same initial conditions, the stratocumulus deck is more persistent when experiencing warm‐air advection than cold‐air advection. This persistence is caused by reduced cloud‐top entrainment drying due to decoupling, a process more influential than the decoupling‐induced cutoff of moisture supply. This mechanism is more notable when the free troposphere becomes more humid. The relevance of the mechanism to previous observations of less low‐level cloudiness under warm‐advection conditions is discussed. Plain Language Summary Marine stratocumulus clouds exert strong radiative cooling on Earth's climate because they reflect much solar radiation back to space. It is important to understand what factors control the cloud properties, or cloud‐controlling factors. Among all cloud‐controlling factors, the least understood is warm‐air advection, meaning winds mobilizing clouds from over warm water to over cold water. A high‐resolution numerical model was used to investigate the response of the stratocumulus evolution to warm‐air advection. A stratocumulus deck was found to persist longer under warm‐advection conditions than its cold counterpart, inconsistent with the decoupling‐induced dissipation mechanism. This persistence is primarily due to the weaker mixing of clouds with the overlying dry air when the atmosphere become decoupled from the sea surface, a consequence of warm‐air advection. This work revises our conventional understanding of how clouds respond to changes in temperature advection that might change as the planet warms, contributing to a more confident projection of future climates. Key Points A preexisting stratocumulus deck is more persistent when experiencing warm‐air advection than cold‐air advection This persistence is due to reduced entrainment drying as a result of decoupling, which outweighs decreased cloud‐base moisture transport The mechanism is more notable when free‐tropospheric humidity is higher

Warm Airmass Intrusions (WAIs) from the mid‐latitudes significantly impact the Arctic water budget. Here, we combine water vapor isotope measurements from the MOSAiC expedition, with a Lagrangian‐based process attribution diagnostic to track moisture transformation in the central Arctic Ocean during two WAIs, under contrasting sea‐ice concentrations (SIC). During winter with high SIC, two moisture supplies are identified. The first is Arctic moisture, locally‐sourced over the sea ice, with isotopic composition influenced by kinetic fractionation during ice‐cloud formation and vapor deposition. This moisture is rapidly overprinted by low‐latitude moisture advected poleward during WAI. In summer under low SIC, moisture is supplied through evaporation from land and ocean, with moisture removal via liquid‐cloud and dew formation. The isotopic composition reflects the influence of higher relative humidity at the evaporation sites. Given the projected increase of frequency and duration of WAIs, our study contributes to assessing process changes in the Arctic water cycle. Plain Language Summary The movement of warm and moist airmasses from lower latitudes has a big effect on the Arctic climate system. We used data from the MOSAiC drift expedition, where we measured the isotopic composition of water vapor. Water isotopes are powerful tracers of where moisture came from and how it changed during the transport. We focused on two specific warm air intrusions, occurring in February and September 2020 respectively, when the amount of sea ice was different. During the winter, the isotopic composition of the airmasses was primarily influenced by in‐Arctic moisture exchanges over sea ice. This local moisture was swiftly replaced by isotopically‐distinct warmer and moister airmasses coming from lower latitudes during the warm intrusion. In summer, when there was less sea ice, we found that water came mainly from ocean evaporation with additional land evaporation during the air intrusion. The isotopic composition of vapor was influenced by how humid the places it came from were. As warm air intrusions are expected to happen more often and last longer in the future, our study helps us understand how they affect the Arctic water cycle. Key Points Transformation of moist airmasses and their isotopic composition during warm air intrusions depends on sea‐ice extent In winter, warm air intrusions suppress ice‐cloud formation and kinetic isotopic fractionation over sea ice In summer, d‐excess is driven by vapor pressure gradients between ocean skin layer and the lower atmosphere at the evaporative sites

Most wildland fires in boreal forests occur during summer, but major fires in the lower Amur River Basin of the southern Khabarovsk Krai (SKK) mainly occur in spring. To reduce active fires in the SKK, we carried out daily analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) hotspot (HS) data and various weather charts. HS data of 17 years from 2003 were used to identify the average seasonal fire occurrence. Active fire-periods were extracted by considering the number of daily HSs and their continuity. Weather charts, temperature maps, and wind maps during the top 12 active fire-periods were examined to clarify each fire weather condition. Analysis results showed that there were four active fire-periods that occurred in April, May, July, and October. Weather charts during the top active fire-periods showed active fires in April and October occurred under strong wind conditions (these wind velocities were over 30 km h−1) related to low-pressure systems. The very active summer fire at the end of June 2012 occurred related to warm air mass advection promoted by large westerly meandering. We showed clear fire weather conditions in the SKK from March to October. If a proper fire weather forecast is developed based on our results, more efficient and timely firefighting can be carried out.

A large-scale wildland fire occurred in Sakha in 2021. The results of fire analysis showed that the total number of hotspots in 2021 exceeded 267,000. This is about 5.8 times the average number of fires over the last 19 years since 2002. The largest daily number of hotspots in 2021 was 16,226, detected on 2 August. On 7 August, about half of the daily hotspots (52.6% = 8175/15,537 × 100) were detected in a highest fire density area (HFA, 62.5–65° N, 125–130° E) near Yakutsk under strong southeasterly wind (wind velocity about 12 m/s (43 km/h)). The results of weather analysis using various weather maps are as follows: The large meandering westerlies due to stagnant low-pressure systems in the Barents Sea brought high-pressure systems and warm air masses from the south to high latitudes, creating warm, dry conditions that are favorable conditions for fire. In addition to these, strong southeasterly winds at lower air levels blew which were related to the development of high-pressure systems in the Arctic Ocean. The HFA was located in the strong wind region (>8 m/s) of the v-wind map. The record-breaking Sakha fire season of 2021 is an example of extreme phenomena wrought by rapid climate change.

Fire activity in 288 areas (2.5° N × 10° E) in the Arctic region (50°–70° N, 0°–360° E) was analyzed using about 4.4 million satellite hotspot (HS) data from 2002 to 2021. A total of 21 high fire density areas from eastern Europe to western Canada were selected, and their fire–weather conditions during each active fire period were analyzed using about 1820 various weather maps at the upper and the lower air level. Analysis results showed that the active fires in the Arctic region occurred under the fire–weather conditions associated with the northward movement of cut-off high (COH) and warm air masses detached from the south caused by large westerly meandering (LWM). LWM is a sign of the beginning of an active fire period. Very active fires on HS peak days occurred several days after the start of the northward movement of COHs and under mainly high-pressure conditions in the upper air and strong wind conditions in the lower air. The time lag of these several days suggests that we may be prepared for very active fires. The fire–weather analysis approach described in this paper has shown that future large-scale fire outbreaks are predictable.

Warm air heaters are now widely used in enclosed environments, either as primary or auxiliary heating facilities. However, the influence of these heaters on the indoor air quality has received scant attention, and the currently available data is insufficient. Therefore, this study experimentally investigated the particle concentrations, air velocity, temperature, and relative humidity in a storeroom equipped with a warm air heater. To assess the effects of the heater on the dispersion and deposition of 0.3, 0.5, 1.0, 3.0, and 5.0 µm particles, we analyzed 18 scenarios with various settings for the output power and outlet orientation. The results indicated higher particle deposition rates when the heater was operating. Furthermore, the particles’ decay rate loss coefficients increased with the heater’s output power and the particles’ proximity tothe heater’s air outlet but were also influenced by the direction of the warm air flow.

In the spring period of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, an initiative was in place to increase the radiosounding frequency during warm air intrusions in the Atlantic Arctic sector. Two episodes with increased surface temperatures were captured during April 12–22, 2020, during a targeted observing period (TOP). The large-scale circulation efficiently guided the pulses of warm air into the Arctic and the observed surface temperature increased from −30°C to near melting conditions marking the transition to spring, as the temperatures did not return to values below −20°C. Back-trajectory analysis identifies 3 pathways for the transport. For the first temperature maximum, the circulation guided the airmass over the Atlantic to the northern Norwegian coast and then to the MOSAiC site. The second pathway was from the south, and it passed over the Greenland ice sheet and arrived at the observational site as a warm but dry airmass due to precipitation on the windward side. The third pathway was along the Greenland coast and the arriving airmass was both warm and moist. The back trajectories originating from pressure levels between 700 and 900 hPa line up vertically, which is somewhat surprising in this dynamically active environment. The processes acting along the trajectory originating from 800 hPa at the MOSAIC site are analyzed. Vertical profiles and surface energy exchange are presented to depict the airmass transformation based on ERA5 reanalysis fields. The TOP could be used for model evaluation and Lagrangian model studies to improve the representation of the small-scale physical processes that are important for airmass transformation. A comparison between MOSAiC observations and ERA5 reanalysis demonstrates challenges in the representation of small-scale processes, such as turbulence and the contributions to various terms of the surface energy budget, that are often misrepresented in numerical weather prediction and climate models.

The experimental results of the study focused on the effect of drying processes of warm air drying at the temperature of 65-80°C and warm air-steam mixture drying at the temperature of 105°C of pine and beech wood to the size of sawdust grains created by cutting using RPW 15M frame saw is presented in the paper. Particle size analysis of dry sawdust was performed using two methods - screening method and optical method based on image analysis obtained from a microscope. The results showed that the drying mode did not affect the particle size distribution of the pine sawdust, but sawdust from beech wood dried with steam mixture at 105°C was characterized by finer particles.

Warm air intrusions over Arctic sea ice can change the snow and ice surface conditions rapidly and can alter sea ice concentration (SIC) estimates derived from satellite-based microwave radiometry without altering the true SIC. Here we focus on two warm moist air intrusions during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition that reached the research vessel Polarstern in mid-April 2020. After the events, SIC deviations between different satellite products, including climate data records, were observed to increase. Especially, an underestimation of SIC for algorithms based on polarization difference was found. To examine the causes of this underestimation, we used the extensive MOSAiC snow and ice measurements to model computationally the brightness temperatures of the surface on a local scale. We further investigated the brightness temperatures observed by ground-based radiometers at frequencies 6.9 GHz, 19 GHz, and 89 GHz. We show that the drop in the retrieved SIC of some satellite products can be attributed to large-scale surface glazing, that is, the formation of a thin ice crust at the top of the snowpack, caused by the warming events. Another mechanism affecting satellite products, which are mainly based on gradient ratios of brightness temperatures, is the interplay of the changed temperature gradient in the snow with snow metamorphism. From the two analyzed climate data record products, we found that one was less affected by the warming events. The low frequency channels at 6.9 GHz were less sensitive to these snow surface changes, which could be exploited in future to obtain more accurate retrievals of sea ice concentration. Strong warm air intrusions are expected to become more frequent in future and thus their influence on SIC algorithms will increase. In order to provide consistent SIC datasets, their sensitivity to warm air intrusions needs to be addressed.

The Ross-Amundsen sector is experiencing an accelerating warming trend and a more intensive advective influx of marine air streams. As a result, massive surface melting events of the ice shelf are occurring more frequently, which puts the West Antarctica Ice Sheet at greater risk of degradation. This study shows the connection between surface melting and the prominent intrusion of warm and humid air flows from lower latitudes. By applying the Climate Feedback-Response Analysis Method (CFRAM), the temporal surge of the downward longwave (LW) fluxes over the surface of the Ross Ice Shelf (RIS) and adjacent regions are identified for four historically massive RIS surface melting events. The melting events are decomposed to identify which physical mechanisms are the main contributors. We found that intrusions of warm and humid airflow from lower latitudes are conducive to warm air temperature and water vapor anomalies, as well as cloud development. These changes exert a combined impact on the abnormal enhancement of the downward LW surface radiative fluxes, significantly contributing to surface warming and the resultant massive melting of ice.