Aircraft observed diurnal variations of the planetary boundary layer under heatwaves

Impact of planetary boundary layer structure on the formation and evolution of air-pollution episodes in Shenyang, Northeast China

Summary: The structure and evolution of Titan’s planetary boundary layer (PBL) are investigated through large eddy simulation (LES) modeling.  The PBL is the interface between the surface and the free atmosphere through which energy, mass, and momentum are exchanged.  Results from LES studies forced by diurnally-varying radiation are presented.  The development of three distinct PBL layers are noted: 1) a near-surface layer dominated by frictional dissipation; 2) a mixed-layer of near neutral stability; and 3) an entrainment layer capping the top of the PBL.  The three layers are similar in character to what is often observed in the Earth’s convective PBL. The depth and evolution of these layers is strongly affected by the initial thermal conditions, the surface thermophysical properties, and soil moisture.Background and Motivation:  The PBL is the lowest atmospheric layer and is critical to the exchange of mass, momentum, and energy between the surface and the atmosphere. Thus, the structure, evolution, and dynamics of Titan’s PBL is relevant to understanding numerous physical processes on Titan, including but not limited to:Volatile evaporation from damp regolith and lakes, and contributions to the global volatile cycle; Dunes and other aeolian formations, and waves upon lakes driven by turbulent surface wind stress; Conversion of surface insolation to atmospheric heating through turbulent fluxes; In addition, missions that descend through and operate on the surface or in the atmosphere have in interest in predictions of the turbulent structure and evolution of the PBL in both the horizontal and vertical.  These predictions are relevant to the engineering design and operational planning of Dragonfly, and the predictions can be largely verified through the measurements that will be made by Dragonfly when it explores Titan.The sounding from the Huygens probe provides the only direct, vertically resolved measurement of the structure of the PBL at a single moment in time. How the observed structures develop and evolve remain uncertain, and the turbulent exchange processes are challenging to constrain from the single profile. LES techniques provide a mechanism for understanding the observed structure and dynamics of the PBL.Methodology: The PBL structure and evolution are investigated through LES using a version of the mesoscale Titan Weather, Research, and Forecasting (mtWRF) model (Rafkin and Soto, 2020). mtWRF has been undergoing rapid development with the addition of radiative transfer (Chatain et al., 2022), topography (Moisan et al., 2024), and realistic 3-D lake geometries (Chatain et al., 2024). In this study, we add the capability for soil moisture and prognostic latent heat fluxes and we configure mtWRF with LES grid spacing of 10 to 40 m.  The baseline simulation uses a dry regolith (no latent heat flux). Simulations with the addition of a moist surface are compared against the dry scenario. Since LES of Titan is a mostly unexplored area of research, the simulations are conducted in 2-D in order to speed analysis and discovery.  The simulations will be extended to 3-D once the PBL dynamics are better understood in 2-D.The model is homogeneously initialized with a Huygens HASI sounding (Fulchignoni et al., 2002) over a vertical domain of 6 km and a horizontal domain of 40 km with 20 m spacing.  Starting at ~0945 local time (LT) when the HASI sounding is valid, the model is integrated until sunrise the following day.  Restarting from that point, the model is integrated again until sunset and the results of the daytime PBL evolution are analyzed.Results: In the baseline, dry regolith case, the daytime boundary layer grows to a depth of ~1 km.  The lowest ~100 m are slightly superadiabatic with the bulk of the PBL showing a near neutral profile (Fig. 1). The top of the PBL exhibits an entrainment layer where the free atmosphere above the PBL is mixed downward. The peak eddy heat flux is found just above the shallow superadiabatic layer and then decreases rapidly toward zero (Fig. 1). Narrow, thermally buoyant plumes with perturbations of ~0.1 K over ambient  are evident (Fig. 2). The maximum variance near the surface is explained by the plumes that originate in this layer.  The maximum aloft is the result of gravity waves triggered by the convection below and which are trapped in the stable region at 1 km.  The subgrid-scale turbulent kinetic energy (TKE) is similar to the thermal perturbations but shows a maximum value in the near-surface layer with a secondary maximum in the middle of the PBL (Fig. 3).  The 1 km level has yet another local maximum of TKE, but is subdued compared to the thermal variance (Fig. 2).  

Impact of thermal structure of planetary boundary layer on aerosol pollution over urban regions in Northeast China

The Weather Research and Forecasting model was applied to analyze variations in the planetary boundary layer (PBL) structure over Southeast England including central and suburban London. The parameterizations and predictive skills of two nonlocal mixing PBL schemes, YSU and ACM2, and two local mixing PBL schemes, MYJ and MYNN2, were evaluated over a variety of stability conditions, with model predictions at a 3 km grid spacing. The PBL height predictions, which are critical for scaling turbulence and diffusion in meteorological and air quality models, show significant intra‐scheme variance (> 20%), and the reasons are presented. ACM2 diagnoses the PBL height thermodynamically using the bulk Richardson number method, which leads to a good agreement with the lidar data for both unstable and stable conditions. The modeled vertical profiles in the PBL, such as wind speed, turbulent kinetic energy (TKE), and heat flux, exhibit large spreads across the PBL schemes. The TKE predicted by MYJ were found to be too small and show much less diurnal variation as compared with observations over London. MYNN2 produces better TKE predictions at low levels than MYJ, but its turbulent length scale increases with height in the upper part of the strongly convective PBL, where it should decrease. The local PBL schemes considerably underestimate the entrainment heat fluxes for convective cases. The nonlocal PBL schemes exhibit stronger mixing in the mean wind fields under convective conditions than the local PBL schemes and agree better with large‐eddy simulation (LES) studies.

BackgroundDevastating health effects from recent heat waves, and projected increases in frequency, duration, and severity of heat waves from climate change, highlight the importance of understanding health consequences of heat waves.ObjectivesWe analyzed mortality risk for heat waves in 43 U.S. cities (1987–2005) and investigated how effects relate to heat waves’ intensity, duration, or timing in season.MethodsHeat waves were defined as ≥ 2 days with temperature ≥ 95th percentile for the community for 1 May through 30 September. Heat waves were characterized by their intensity, duration, and timing in season. Within each community, we estimated mortality risk during each heat wave compared with non-heat wave days, controlling for potential confounders. We combined individual heat wave effect estimates using Bayesian hierarchical modeling to generate overall effects at the community, regional, and national levels. We estimated how heat wave mortality effects were modified by heat wave characteristics (intensity, duration, timing in season).ResultsNationally, mortality increased 3.74% [95% posterior interval (PI), 2.29–5.22%] during heat waves compared with non-heat wave days. Heat wave mortality risk increased 2.49% for every 1°F increase in heat wave intensity and 0.38% for every 1-day increase in heat wave duration. Mortality increased 5.04% (95% PI, 3.06–7.06%) during the first heat wave of the summer versus 2.65% (95% PI, 1.14–4.18%) during later heat waves, compared with non-heat wave days. Heat wave mortality impacts and effect modification by heat wave characteristics were more pronounced in the Northeast and Midwest compared with the South.ConclusionsWe found higher mortality risk from heat waves that were more intense or longer, or those occurring earlier in summer. These findings have implications for decision makers and researchers estimating health effects from climate change.

This study investigates a summer heat wave (HW) associated with downslope winds (DWs) affecting the central region of the state of Rio Grande do Sul (RS), Brazil. The temporal evolution of both phenomena is analyzed in the atmospheric boundary layer (ABL) using a combination of micrometeorological and rawinsonde data. For spatial characterization, ERA-5 reanalysis data are used. The HW covered a large area in southern Brazil, Argentina, and Paraguay. The main features of the HW were locally enhanced in the central region of RS by the development of DWs. The establishment of DWs near the surface depends on the dynamics of the ABL and local topographic features. The results showed that DWs that occurred during the HW contributed to the extreme temperatures and were associated with strong northerly winds, low relative humidity, and a drop in the dew points. Together, these extreme events influenced the turbulent and mean flow patterns of the ABL. The increase in turbulent activity associated with the warming of the ABL favored enhanced growth of morning ABL, while at night the simultaneous effects of radiative and turbulence cooling inhibited the formation of a strong stable ABL. The analysis highlights the complex interplay of synoptic and local factors associated with DWs and HW.

Neglected High-frequency Variability events of the sea surface Wind Coupled with Precipitation (HV-WCP) events are revealed over the Kuroshio Sea Surface Temperature (SST) front in the East China Sea (ECS) during wintertime. This phenomenon is characterized as modulation events of the surface wind directions, whereby the southerly anomaly changes into a northwest anomaly, accompanied with increasing wind speed, and coupled with decreasing precipitation from morning to night on the day with the peak variability. The events are associated with significant climate effects on the rainfall anomaly over southeastern China and adjacent ocean areas of the ECS. To sustain these events, the large-scale circulation needs to exhibit declines in the upstream monsoon and weakening vertical mixing in the planetary boundary layer (PBL). The Kuroshio thermal effect on the underlying atmosphere becomes prominent due to weakening vertical mixing, which results in pronounced PBL height gradient over the Kuroshio SST front. More plentiful moisture transported from PBL to the free atmosphere, generates the initialization of rainfall and the cyclone anomaly. Meanwhile, the latent heat releasing caused by this precipitation forces positive vorticity anomaly within the PBL, which again strengthens the cyclone and the upward moisture transport. Therefore, a complement between local circulation and rainfall is developed over the Kuroshio region. Due to the significant effect of the Kuroshio SST front on the PBL structure, the HV-WCP intensity is able to maintain its strength over the Kuroshio region. However, because of the northwestern background monsoon, the HV-WCP events vanish quickly when the coupled system moves out of Kuroshio SST frontal region.

Impacts of PBL schemes on PM2.5 simulation and their responses to aerosol-radiation feedback in GRAPES_CUACE model during severe haze episodes in Jing-Jin-Ji, China

Legacy of aerosol radiative effect predominates daytime dust loading evolution

The frequency, intensity and duration of heat waves are all expected to increase as the climate warms in response to increasing greenhouse gas concentrations. The focus of this study is on another dimension of heat waves, their spatial extent, something that has not been studied systematically by researchers but has important implications for associated impacts. Of particular interest are spatially contiguous heat wave regions, examined here over the conterminous US for the May–September season in both the current climate and climate model projections from the CMIP5 archive (11 models total) using the RCP4.5 and RCP8.5 radiative forcing scenarios. Given their myriad impacts, heat waves are defined using multiple temperature variables, one which includes atmospheric moisture. In addition to their spatial extent, several other physical attributes are computed across contiguous heat wave regions, including a proxy for energy use. An estimate of the human population exposed to current and future heat waves is also evaluated. We find that historical climate model simulations, in aggregate, show good fidelity in capturing key characteristics of heat waves in the current climate while projections show a substantial increase in spatial extent and other attributes by mid-century under both scenarios, though generally less for RCP4.5, as expected. Overall, the study presents a framework for examining the behavior, and associated impacts, of a frequently overlooked aspect of heat waves. The projected increases in the spatial extent and other attributes of heat waves reported here provides a new perspective on some of the potential consequences of the continued increase in atmospheric greenhouse gas concentrations.

A Heat-wave-Intensity-Duration-Frequency (HWIDF) curve is employed to establish the relationship between the intensity, duration, and frequency of heat wave incidents. This approach offers a more comprehensive understanding of heat waves by considering their intensity, duration, and frequency. The HWIDF curves for India's six climate zones, namely, The Arid zone, Semiarid zone, Montane, Humid subtropical zone, Tropical wet, and Tropical wet & dry zone, are considered for the study. These curves are used to evaluate the probability of encountering heat waves with varying levels of intensity and duration. They also help measure changes in heat wave intensities for different return periods in relation to the evolving climate. Subsequently, the MRI-ESM2-0 models were employed to assess the disparities in HWIDF (Heat Wave Intensity-Duration-Frequency) by comparing scenarios with and without human emissions. This analysis aimed to determine the specific impact of human activities on heat waves. The analysis shows that heat waves in arid zones, lasting from one to ten days, with maximum and average intensities of 44.32℃ and 36.56 ℃ or less, occur with a frequency of once every two years (probability=0.5). These heat waves have higher intensities compared to other zones. On the other hand, montane zones experience lower intensities compared to other zones. Our findings indicate that heat wave intensity poses a greater danger under historical conditions when compared to natural conditions, particularly over a span of five and ten consecutive days. The likelihood of experiencing severe heat waves in the Humid Subtropical and Montane zones is more or less compared to other zones. Hence, our study indicates that the rising probability of severe heat waves, in terms of their severity and duration, could be attributed to anthropogenic warming.

ABSTRACTIn the past few decades, severe heat waves were representative of the weather conditions in various parts of the world. They had distinctly negative impacts on humans, plants, animals and different economic sectors. Particularly in cities, where most humans are living, their well‐being, efficiency and health were affected, which even caused a relatively high mortality rate. This is why a large interest still exists to analyse heat waves in the past by use of statistical methods.As an investigation on heat waves is not available for the territory of Ukraine up to now, a retrospective analysis was conducted. Its main aims were (1) to determine the frequency of heat wave episodes for this country and (2) to analyse their spatiotemporal distribution, duration and intensity. On the basis of 13 selected stations of the meteorological network of the Ukrainian Hydrometeorological Centre, daily values of maximum air temperature (Ta,max) in the summer months June to August were used to determine heat wave episodes according to the definition recommended by the IPCC. For all stations, complete and reliable Ta,max time series were available for the period 1951–2011. With respect to some stations, the Ta,max time series already started some years earlier, e.g. in 1936 for five stations and even in 1911 for four stations. However, the Ta,max time series of almost all stations had gaps in the summer months before 1951, particularly in the 1940s of the 20th century. As they could not be filled in a satisfying way, this limitation was considered in the data analysis.The results indicate that most of the heat wave episodes occurred at stations located in Eastern Ukraine. In contrast to other decades, the number of heat wave episodes was highest for almost all stations in the decade 2001–2010. The fewest heat wave episodes were determined for the two decades 1961 to 1980. The longest heat wave duration varied between 7 d in Henichesk (southern seashore) and 24 d in Lugansk (Eastern Ukraine). For many stations, the longest heat wave duration occurred in the first two decades of August 2010, i.e. in the period of the extremely severe heat wave in Western Russia.The intensity of heat waves was analysed by use of the cumulative Ta,max excess. The results show that heat waves of the longest duration mostly represented the strongest heat waves. The 24‐d heat wave 2010 in Lugansk was the strongest in the investigation period quantified by a cumulative Ta,max excess of 128 °C.

<p>Based on the data of meteorological observations, executed in 2013-2019 at Research Station “Ice base Cape Baranova” (RS) and original algorithm, taken into account accuracy of measurements and footprints, the components of surface heat budget are calculated. It is shown that in winter due to radiation cooling turbulent sensible heat flux (H) directs to underlying surface. In summer H due to radiation heating of surface with low albedo directs to atmosphere and reaches 25% of the incoming short-wave radiation. The turbulent latent heat flux (LE) in winter directs to atmosphere. Its value is not more than 10% of H. During summer LE has no predominant direction. </p><p>Comprehensive monitoring carried out at RS since 2013 allowed to examine the role of large-scale processes in the polar atmosphere and hydrosphere on the formation of local climate in the region. In 2016, 2018 and 2019 sea ice cover of the Barents and Kara Seas in October, the month of active freezing of active soil layer, occupied the minimal area starting 1978 year (http://wdc.aari.ru/datasets/d0042/). This circumstance along with peculiarities of circulation processes in the atmosphere had led to anomalous of temperature and humidity regimes of lower troposphere. These years monthly mean air temperature up to 700 hPa was about -4 °C compared to -7 - -11 °C in 2013 - 2015 and 2017. In 2016 the lower troposphere was warmer by 2 - 3 °C  and specific humidity in atmospheric boundary layer was 30–60% higher its values in 2013–2015 and 2017. Even in 2018, when the area of open water adjacent to the Severnaya Zemlya archipelago was significantly larger than in 2016, specific humidity at altitudes up to 3 km was 4-12 percents less.</p><p>In 2016 monthly mean wind speed, mainly of southwestern direction, reached maximum value, more than 7 m/s. It led to weakening of atmospheric surface layer stratification (z/L <0.2). The air specific humidity  significantly increased also, up to 3.0 and 2.7 g /kg at 2 meters and at z<sub>0</sub> . Long-wave radiation fluxes increased by more than 15 – 20 W/m<sup>2</sup>. Same time due to increase of underlying surface temperature, its long-wave radiation cooling, which was not compensated by the increase of incoming long-wave radiation increased up to -27 W/m<sup>2</sup>. H, directed to the underlying surface, increased to 10 W/m<sup>2  </sup>and LE, directed to atmosphere, increased almost 2 times, up to 12 W/m<sup>2</sup>. As a result of multidirectional changes of heat fluxes, defining surface heat balance, its value in October 2016 (-31.6 W/m<sup>2</sup>) was comparable to calculated for other years.</p><p>The most probable explanation of the revealed features of atmospheric boundary and surface layers in October 2016 are the absence of sea ice cover in the waters, adjacent to the archipelago, prevented cooling of atmosphere, and strong zonal component of the wind velocity, caused the transfer of warm and moist air masses of Atlantic origin into the study area.</p><p>The work had been done under financial support of the Ministry of Science and Higher Education of the Russian Federation (project no. RFMEFI61619X0108).</p>

A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration (Na) reveals a high aerosol number density in this region with average surface level number concentration (N0) of about 6600 cm-3 (0.12–3.0 μm). The average vertical profile of Na approximately satisfies an exponential decline function with a scale height of 1419 m. The Na profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of Na profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.

Vertical distributions of black carbon aerosols over rural areas of the Yangtze River Delta in winter