Near-surface Moisture Research Articles

Deciphering the seasonal dynamics of multifaceted aerosol-ozone interplay: Implications for air quality management in Eastern China

We employed an enhanced WRF-Chem to investigate the discrete mechanisms of aerosol-radiation-feedback (ARF), extinction-photochemistry (AEP), and heterogeneous-reactions (AHR) across different seasons in eastern China, aiming to assess the synergistic effects arising from the simultaneous operation of multiple processes on O3 and PM2.5. Our findings demonstrated that ARF fostered the accumulation of pollutants and moisture, initiating two distinct feedback mechanisms concerning O3. The elevation in the NO/NO2 ratio amplified O3 consumption. Increased near-surface moisture diminished upper-level cloud formation, thereby enhancing photolysis rates and O3 photochemical production. The pronounced impact of heightened NO/NO2 on O3 led to a decrease of 0.1–2.7 ppb. When decoupled from ARF, AEP led to a more significant reduction in photolysis rates, resulting in declines in both O3 and PM2.5, except for an anomalous increase observed in summer, with O3 increasing by 1.6 ppb and PM2.5 by 2.5 μg m−3. The heterogeneous absorption of hydroxides in spring, autumn, and winter predominantly governed the AHR-induced variation of O3, leading to a decrease in O3 by 0.7–1 ppb. Conversely, O3 variations in summer were primarily dictated by O3-sensitive chemistry, with heterogeneous absorption of NOy catalyzing a decrease of 2.4 ppb in O3. Furthermore, AHR accentuated PM2.5 by facilitating the formation of fine sulfates and ammonium while impeding nitrate formation. In summer, the collective impact of ARF, AEP, and AHR (ALL) led to a substantial reduction of 6.2 ppb in O3, alleviating the secondary oxidation of PM2.5 and leading to a decrease of 0.3 μg m−3 in PM2.5. Conversely, albeit aerosol substantially depleted O3 by 0.4–4 ppb through their interactions except for summer, aerosol feedback on PM2.5 was more pronounced, resulting in a significant increase of 1.7–6.1 μg m−3 in PM2.5. Our study underscored the seasonal disparities in the ramifications of multifaceted aerosol-ozone interplay on air quality.

Updates in the NCEP GFS PBL and Convection Models with Environmental Wind Shear Effect and Modified Entrainment and Detrainment Rates and Their Impacts on the GFS Hurricane and CAPE Forecasts

Abstract To reduce hurricane intensity bias, the NCEP Global Forecast System (GFS) planetary boundary layer (PBL) and convection schemes have been updated with a new parameterization for environmental wind shear and enhanced entrainment and detrainment rates with increasing PBL or subcloud mean turbulent kinetic energy (TKE) in their updraft and downdraft mass-flux schemes. Tests with the GFS show that the updated schemes significantly reduce the hurricane intensity bias by reducing the momentum transport in the mass-flux schemes. Along with the reduced intensity bias, the hurricane intensity and track errors have also been reduced. On the other hand, to reduce the PBL overgrowth over areas with a higher vegetation fraction or larger surface roughness, the entrainment rate in the PBL mass-flux scheme has also been increased with increasing vegetation fraction or increasing surface roughness. This entrainment rate increase has increased near-surface moisture, and as a result, helped to increase the underestimated convective available potential energy (CAPE) forecasts over the continental United States.

Great Salt Lake desert landscape change over multiple temporal scales

This one-day (~260-mile) field trip guide provides an overview of the late Pleistocene to Holocene history of the Great Salt Lake Desert. Stops include Knolls Sand Dunes and areas on or surrounding the Bonneville Salt Flats, such as Juke Box trench, the Bonneville Salt Flats International Speedway, and the saline pan center and edge (Figure 1). We cover the post-Lake Bonneville geomorphic evolution of the Great Salt Lake Desert including changes in land cover over the past century. The Great Salt Lake Desert area provides unique access to saline landscape features including gypsum dunes and a perennial saline pan. We discuss the origin of these features and how they fit within the area’s broader geologic context. The accessibility of sites discussed here depends on surface conditions. In general, late summer to early fall is the most opportune time to visit this area. Vehicular travel to any of the off-road sites is discouraged when there is standing water or high near-surface moisture (wet mud with little traction). Surface conditions can change rapidly, and we recommend researching current conditions before initiating this trip. This desert is hot and dry during the summer and there is no shade and limited access to water; please plan accordingly. Past and current Great Salt Lake Desert depositional changes provide an analog for the modern Great Salt Lake with changing water availability, potential dust production, competing priorities, and rapidly changing land cover. The information presented here impacts understanding natural and geologic heritage, changing management strategies, and landscape dynamism over multiple spatial and temporal scales.

Unraveling the Formation Mechanism of Exceptionally Strong Marine Heatwaves in the Northeast Pacific in 2020

Abstract Throughout 2020, a persistent marine heatwave (MHW) event occurred in the northeast (NE) Pacific, which exhibited a record-breaking intensity and long duration. Three peaks of sea surface temperature (SST) anomaly are identified in April, July, and November 2020, respectively, all of which are caused by the surface latent heat flux (LH) term. A Taylor expansion analysis for the LH term reveals that positive LH anomaly (LHA) played the decisive role in increasing the latent heating effect, whereas the mixed layer depth anomaly was not important in the 2020 MHW. The positive LHA was primarily caused by the southerly wind anomalies, which advected more humid air from lower latitudes to the NE Pacific and thus reduced the difference between the saturated specific humidity at sea surface and actual surface specific humidity. This reduced sea–air humidity difference was conducive to less evaporation, leading to positive LHA. Using an atmospheric general circulation model forced by observed SSTs, we find that although the atmospheric circulation anomalies associated with the 2020 MHW can be partly constrained by both tropical Pacific SST and local SST over the North Pacific, the role of internal atmospheric variability in the 2020 MHW’s formation cannot be overlooked. The prediction skill for the 2020 MHW in the North American Multimodel Ensemble models is limited to one month, indicating the challenge of accurately predicting MHW in the NE Pacific. The finding about the contribution of sea–air humidity difference to the LHA provides a new insight into the formation mechanism of MHW. Significance Statement An exceptionally strong and persistent marine heatwave occurred in the northeast Pacific throughout 2020, with three peaks in April, July, and November, causing considerable damage to local fisheries and ecosystems. This study found that the evolution of this marine heatwave was due to the less-than-normal release of latent heat at the ocean surface. This was mainly caused by the reduced sea–air humidity difference, which was attributed to the increased near-surface moisture transported by southerly wind anomalies. Our model experiments demonstrated that the associated anomaly fields in the atmosphere partly stemmed from stochastic processes. The state-of-the-art dynamic models can only predict this marine heatwave one month in advance, indicating the necessity of improving prediction skill for marine heatwave’s evolution in current models.

Long-Term Trends in Ice Fog Occurrence in the Fairbanks, Alaska, Region Based on Airport Observations

Abstract Ice fog typically occurs at temperatures below approximately −30°C. Ice fog formation and persistence are affected by atmospheric processes at different spatial and temporal scales and can be influenced by anthropogenic activities that add vapor to the near-surface atmosphere. Based on meteorological observations from Fairbanks International Airport and Eielson Air Force Base (Alaska) from 1948/49 to 2021/22, we provide an overview of general ice fog climatology at the sites, changes over time, and synoptic-scale upper-level weather patterns common during ice fog occurrence. On average, ice fog occurrence has decreased by 60%–70% over the study period (median number of ice fog days at Fairbanks airport in the period 1950/51–1979/80: 16.5; median in the period 1990/91–2019/20: 6). The average lengths of ice fog events and of the ice fog season have also decreased. Trends are not linear, and rates of change vary over time. The greatest reduction in ice fog occurred during the 1970s and 1980s. Trends in ice fog hours roughly track decreasing trends in hours with cold temperatures. However, the percentage of cold hours in which ice fog occurs has decreased since approximately the 1980s. This result suggests that local changes in air pollution or near-surface moisture may also play an important role in trends in ice fog occurrence. We use self-organizing maps to assess recurring synoptic-scale weather patterns in the upper atmosphere during ice fog conditions in Fairbanks. Ice fog is typically associated with a northerly flow or low pressure gradients over the study area. Significance Statement We aim to show when and how often ice fog occurs in the Fairbanks region, how this has changed over time, and what kind of larger-scale weather patterns are common during ice fog. Ice fog strongly reduces visibility and represents a hazard to aviation and other traffic. The number of ice fog hours and days per winter has decreased substantially over the 70-yr period of record. Ice fog is, on average, less persistent now than in the past. The reduction is related to fewer days with cold temperatures, but changes in air pollution and other local factors may also play an important role. Further study is needed to fully attribute the causes of the observed changes.

Wind mediates the responses of net ecosystem carbon balance to climatic change in a temperate semiarid steppe of Northern China

As an important carbon sink to mitigate global climate change, the role of arid and semiarid grassland ecosystem has been widely reported. Precipitation and temperature changes have a dramatic impact on the carbon balance. However, the study of wind speed has long been neglected. Intuitively, wind speed regulates the carbon balance of grassland ecosystems by affecting the opening of vegetation stomata as well as near-surface moisture and temperature. It is sufficient that there is a need to conduct field observations to explore the effect of wind speed on the carbon balance in arid and semiarid grassland. Therefore, we conducted observations of carbon fluxes and corresponding climate factors using an eddy covariance system in a typical steppe in Inner Mongolia from 2017 to 2021. The research contents include that, (i) we depicted the changing patterns of carbon fluxes and climate factors at multiple time scales; (ii) we simulated the net ecosystem carbon balance (NECB) based rectangular hyperbolic model and compared it with the observed net ecosystem exchange values; (iii) we quantified the mediated effect of wind speed on NECB by adopting structural equation modeling; (iv) we used the constrained line method to explore what wind speed intervals might have the greatest carbon sequestration capacity of vegetation. The results were as follows, (i) the values of NECB for the five years of the study period were 101.95, −48.21, −52.57, −67.78, and −30.00 g C m−2 yr−1, respectively; (ii) if we exclude the inorganic carbon component of the ecosystem, we would underestimate the annual carbon balance by 41.25, 2.36, 20.59, 22.06 and 43.94 g C m−2 yr−1; (iii) the daytime wind speed during the growing season mainly influenced the NECB of the ecosystem by regulating soil temperature and vapor pressure deficit, with a contribution rate as high as 0.41; (iv) the grassland ecosystem had the most robust carbon sequestration capacity of 4.75 μmol m−2 s−1 when the wind speed was 2–3 m s−1. This study demonstrated the significant implications of wind speed variations on grassland ecosystems.

Historical Arch Bridges-Deterioration and Restoration Techniques

Historic buildings are the most valuable evidence of cultural heritage. They play an essential role in establishing a tangible link between the past and the present by understanding, interpreting, and tracing the epoch of civilization. Unfortunately, the high costs of restoration, vandalism, and arson take their toll. However, new technologies are having a positive impact on the restoration process and are becoming a suitable alternative to labor-intensive, expensive, and unsafe traditional inspections. Therefore, the role of non-destructive testing (NDT) as a new method is becoming more evident. Faro laser scanning, impact echo, impulse sound testing, and geoelectric tomography as non-destructive methods are leading to the inspection of historic structures to preserve their character. These new methods are representative of the development of non-contact techniques for the examination and documentation of structures. Non-destructive testing examines the internal and external structure of complex building components as well as defective areas, quantifies cracks, and detects near-surface moisture. The objective of this work is to identify new adventurous and traditional methods for the reconstruction of the Turkish arch bridges Dara-1 and Halilviran to determine the appropriate rehabilitation methods and their deterioration of construction materials, damage, and failure patterns. Bridge dimensions were measured using a Faro laser scanner, which allows inspectors to capture and evaluate data from bridges and structural components without permanently altering them. The laser captures bridge dimensions by scanning cross-sections of the structure in the horizontal and vertical planes. The data is exported in the form of point clouds that represent all visible aspects and actual dimensions of the bridge in 2D and 3D models. In comparison between traditional and laser scanning methods, the main advantages of the applied method are the time savings on-site and the creation of a three-dimensional model of the structure, which can be used to collect precise and accurate surface data of objects in a non-destructive manner. Doi: 10.28991/CEJ-2023-09-07-010 Full Text: PDF

Factors Affecting the Rapid Recovery of CAPE on 31 March 2016 during VORTEX-Southeast

Abstract In this study, we analyze various sources of CAPE in the environment and their contributions to its time tendency that will complement forecast models and operational analyses that are relatively temporally (∼1 h) coarse. As a case study, the relative roles of direct insolation and near-surface moisture advection in the recovery CAPE on 31 March 2016 in northern Alabama are examined using VORTEX-Southeast (VORTEX-SE) observations and numerical simulations. In between rounds of nontornadic morning storms and tornadic evening storms, CAPE over the VORTEX-SE domain increased from near zero to at least 500 J kg−1. A timeline of the day’s events is provided with a focus on the evolution of the lower levels of the atmosphere. We focus on its responses to solar insolation and moisture advection, which we hypothesize as the main mechanisms behind the recovery of CAPE. Data from the University of Massachusetts S-Band frequency-modulated, continuous-wave (FMCW) radar and NOAA National Severe Storms Laboratory (NSSL) Collaborative Lower Atmospheric Mobile Profiling System (CLAMPS), and high-resolution EnKF analyses from the Advanced Regional Prediction System (ARPS) are used to characterize the boundary layer evolution in the pre-tornadic storm environment. It is found that insolation-driven surface diabatic heating was the primary driver of rapid CAPE recovery on this day. The methodology developed in this case can be applied in other scenarios to diagnose the primary drivers of CAPE development. Significance Statement The mechanisms by which atmospheric instability recovers can vary widely and are often a source of uncertainty in forecasting. We want to understand how and why the environment destabilized enough to produce an evening tornado following morning storms on 31 March 2016. To do this, we used model data and observations from a collocated radar and profiler. It was found that heating from the sun at the surface was the primary cause of destabilization in the environment.

Structures and Mechanisms of Heatwaves Related to Quasi-Biweekly Variability over Southern China

Abstract In the present study, the structures and mechanisms of the heatwaves (HWs) associated with the quasi-biweekly (QBW; 10–20-day period) variability (QBW-HW) over southern China (SC; 106°–120°E, 21°–30°N) are investigated by using observation data from surface stations in China and the related gridded dataset (CN05.1), and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis. We found that the strongest anticyclonic anomaly and subsidence appear over SC during the developing phase of QBW-HW, and then induced excess solar radiation at surface and significant diabatic heating lead to a positive surface air temperature change, thus favoring occurrence of QBW-HW over SC. In addition, we found a wet near-surface atmosphere in the QBW-HW events over SC, and further confirmed that near-surface moisture should play an important role in the occurrence of QBW-HW, via absorptions of longwave and shortwave radiation. This result is quite different from previous studies since they did not pay attention to the near-surface moisture. On the other hand, warmer SAT favors more water vapor evaporated from the moist soil when considering the Clausius–Clapeyron relationship. Then, the positive feedback processes promote the occurrence of QBW-HW over SC. In contrast, during the developing and warm phases of QBW-HW over SC, except for the near-surface level, the troposphere is in a dry condition, even at 850 and 700 hPa. In the QBW-HW events over SC, the factor responsible for the wet near-surface atmosphere is the enhanced surface evaporation, which is attributed to strengthened surface wind speed and background moist soil. Significance Statement Under the background of global warming, heatwaves over Southern China are experiencing an increasing trend. In this study, we want to understand the structures and mechanisms of the heatwaves related to 10–20-day (quasi-biweekly) variability. We that found some structures of heatwaves (e.g., anticyclonic anomalies along with subsidence) are consistent with previous studies. In addition, we also show that the moist soil and increased induced near-surface moisture play a key role in the occurrence of heatwaves over Southern China, via enhanced absorptions of longwave and shortwave radiation. This study is helpful for understanding the processes and prediction of heatwaves over Southern China. Future work should examine the findings by some numerical experiments with a climate model.

Improving Madden–Julian oscillation simulation in atmospheric general circulation models by coupling with a one-dimensional snow–ice–thermocline ocean model

Abstract. A one-column, turbulent, and kinetic-energy-type ocean mixed-layer model (snow–ice–thermocline, SIT), when coupled with three atmospheric general circulation models (AGCMs), yields superior Madden–Julian oscillation (MJO) simulations. SIT is designed to have fine layers similar to those observed near the ocean surface; therefore, it can realistically simulate the diurnal warm layer and cool skin. This refined discretization of the near-surface ocean in SIT provides accurate sea surface temperature (SST) simulation, and thus facilitates realistic air–sea interaction. Coupling SIT with the European Centre/Hamburg Model version 5, the Community Atmosphere Model version 5, and the High-Resolution Atmospheric Model significantly improved MJO simulation in three coupled AGCMs compared to the AGCM driven by a prescribed SST. This study suggests two major improvements to the coupling process. First, during the preconditioning phase of MJO over the Maritime Continent (MC), the often underestimated surface latent heat bias in AGCMs can be corrected. Second, during the phase of strongest convection over the MC, the change in intraseasonal circulation in the meridional circulation enhancing near-surface moisture convergence is the dominant factor in the coupled simulations relative to the uncoupled experiments. The study results show that a fine vertical resolution near the surface, which better captures temperature variations in the upper few meters of the ocean, considerably improves different models with different configurations and physical parameterization schemes; this could be an essential factor for accurate MJO simulation.

Tillage timing to improve soil water storage in Mediterranean long fallow

Creation of a dry soil mulch to maintain near-surface moisture in dry environments is an ancient practice that is still used in many regions of the world for timely establishment of crops after long fallow. Before availability of herbicides, the first tillage during fallow was timed to kill weeds, but now the optimum timing for tillage depends only on the interplay of evaporation and infiltration of rain. The goal is to maximize total root zone water storage and seed-zone water at seeding time. Our objective was to measure the effect of delaying soil mulch creation progressively later in the spring and summer on weed-free fallow for winter wheat (Triticum aestivum L.) in the Mediterranean inland Pacific Northwest drylands of the United States. Over a period of six years and a total of 12 site-years, four to seven tillage timings were compared to an untilled treatment. Soil water was measured at seeding time in the seed zone and total root zone. Seed-zone water and total stored water produced by different tillage timings was surprisingly constant, with few statistically significant or substantial differences. On average, only 2% of stored water was lost in the top 1.0 m of soil over 80 days during the hot, dry period towards the end of fallow. Responses to late spring rain could be seen, and tillage near the end of June (early summer) often produced maximum soil water content.

Climatology and Formation Environments of Severe Convective Windstorms and Tornadoes in the Perm Region (Russia) in 1984–2020

Severe convective windstorms and tornadoes regularly hit the territory of Russia causing substantial damage and fatalities. An analysis of the climatology and formation environments of these events is essential for risk assessments, forecast improvements and identifying of links with the observed climate change. In this paper, we present an analysis of severe convective windstorms, i.e., squalls and tornadoes reported between 1984 and 2020 in the Perm region (northeast of European Russia), where a local maximum in the frequency of such events was previously found. The analysed database consists of 165 events and includes 100 squalls (convective windstorms), 59 tornadoes, and six cases with both tornadoes and squalls. We used various information to compile the database including weather station reports, damage surveys, media reports, previously presented databases, and satellite images for windthrow. We found that the satellite images of damaged forests are the main data source on tornadoes, but their role is substantially lower for windstorm events due to the larger spatial and temporal scale of such events. Synoptic-scale environments and associated values of convective indices were determined for each event with a known date and time. Similarities and differences for the formation conditions of tornadoes and windstorms were revealed. Both squalls and tornadoes occur mostly on rapidly moving cold fronts or on waving quasi-stationary fronts, associated with low-pressure systems. Analyses of 72-h air parcel backward trajectories shows that the Caspian and Aral Seas are important sources of near-surface moisture for the formation of both squalls and tornadoes. Most of these events are formed within high CAPE and high shear environments, but tornadic storms are generally characterised by a higher wind shear and helicity. We also differentiated convective storms that caused forest damage and those did not. We found the composite parameter WMAXSHEAR is the best discriminator between these two groups. In general, storm events causing windthrow mainly occur under conditions more favourable for deep well-organised convection. Thus, forest damage can be considered as an indicator of the storm severity in the Perm region and in adjacent regions with forest-covered area exceeding 50%.

The spatiotemporal characteristics of near-surface water vapor in a coastal region revealed from radar-derived refractivity

AbstractThe radar-retrieved refractivity fields provide detailed depictions of the near-surface moisture distribution at the meso-gamma scale. This study represents a novel application of the refractivity fields by examining the spatiotemporal characteristics of moisture variability in a summertime coastal region in Taiwan over four weeks. The physiography in Taiwan lends itself to a variety of flow features and corresponding moisture behavior, which has not been well-studied. High-resolution of refractivity analyses demonstrate how a highly variable moisture field is related to the complex interaction between the synoptic-scale winds, diurnal local circulations, terrain, storms, and heterogeneous land-use. On average, higher refractivity (water vapor) is observed along the coastline and refractivity decreases inland toward the foothills. Under weak synoptic forcing conditions, the daytime refractivity field develops differently under local surface wind directions determined by the synoptic-scale prevailing wind and the sea breeze fronts. High moisture penetrates inland toward the foothills with southwesterly winds, but it stalls along the coastline with southerly and the northwesterly winds. The moisture distribution may further affect the occurrence of the inland afternoon storms. During the nighttime, the dry downslope wind decreases the moisture from the foothills toward the coast and forms a refractivity gradient perpendicular to the meridionally-oriented mountains. Furthermore, the refractivity fields illustrate higher resolution moisture distribution over surface station point measurements by showing the lagged daytime sea-breeze front between the urban and rural areas and the detailed nighttime heterogeneous moisture distribution related to land-use and rivers.

Ground penetrating radar for soil-water measurement in a semi-arid climate in the Orkhon River basin, central Mongolia

In this paper, we present a velocity analysis technique using ground-penetrating radar (GPR) data to estimate near-surface soil-water content affected by topography and solar radiation on a mountain peak, mountain mid-slope, and alluvial plain sites in a semi-arid climate area in central Asia. By making precise measurements of reflected EM wave velocity, the water content in the near-surface soil was determined. The GPR experiments at several frequencies were carried out in two sequential phases (comprising common-offset and multi-offset methods) to obtain the soil-water content and illustrate the subsurface structure. We then tried to determine the relationship between near-surface moisture content and permafrost thawing. Therefore, we used several different methods including time-domain reflectometry and a resistivity survey. We also confirmed via the GPR data that vegetation cover indicates soil-water content in the near-surface soil. The results evaluated in this study provide meaningful information about soil water as well as subsurface structures. The GPR data acquired at the survey sites indicated a large range of near-surface water content due to the topography of the survey lines.

Observations and simulations of a wind farm modifying a thunderstorm outflow boundary

Abstract. On 18 June 2019, National Weather Service (NWS) radar reflectivity data indicated the presence of thunderstorm-generated outflow propagating east-southeastward near Lubbock, Texas. A section of the outflow boundary encountered a wind farm and then experienced a notable reduction in ground-relative velocity, suggesting that interactions with the wind farm impacted the outflow boundary progression. We use the Weather Research and Forecasting model and its wind farm parameterization to address the extent to which wind farms can modify the near-surface environment of thunderstorm outflow boundaries. We conduct two simulations of the June 2019 outflow event: one containing the wind farm and one without. We specifically investigate the outflow speed of the section of the boundary that encounters the wind farm and the associated impacts on near-surface wind speed, moisture, temperature, and changes to precipitation features as the storm and associated outflow pass over the wind farm domain. The NWS radar and nearby West Texas Mesonet surface stations provide observations for validation of the simulations. The presence of the wind farm in the simulation clearly slows the progress of the outflow boundary by over 20 km h−1, similar to what was observed. Simulated perturbations of surface wind speed, temperature, and moisture associated with outflow passage were delayed by up to 6 min when the wind farm was present in the simulation compared to the simulation without the wind farm. However, impacts on precipitation were localized and transient, with no change to total accumulation across the domain.

The distribution, habitat preference and population dynamics of the pale field-rat (

Abstract Context The pale field-rat (Rattus tunneyi) is a small native rat that formerly had a wide distribution throughout Australia. It has suffered substantial range contraction since European settlement and is now largely absent from arid and semiarid Australia. In this biome, it was known to persist only at two Western Australian locations: Edel Land, on the south-western shore of Shark Bay, and islands off the Pilbara coast. Aims We aimed to establish the extent of the species range at Edel Land, its habitat preference, the temporal stability of its populations with respect to rainfall, and threats to its persistence. Methods We trapped at 54 sites to establish distribution and habitat preference, and re-trapped four of these sites at which R. tunneyi was present in each season for 2.5 years to establish trends in abundance. Key results Trapping resulted in the capture of 45 R. tunneyi individuals across 17 of 54 sites (4104 trap-nights; 1.1% capture success). Rattus tunneyi typically occupied localised areas of dense shrubland, often in habitats with free water or near-surface moisture from drainage from high dunes allowing denser and taller vegetation and, at some sites, year-round growth of grasses or rushes. Regular re-trapping of four sites in each season (2002 – 2004) suggested a declining population, probably owing to a sequence of dry years. Key conclusions Rattus tunneyi at Shark Bay occurred only in localised mesic refuges, apparently dependent on seepage from high dunes generated by major inputs of rainfall from infrequent cyclones or sequences of high-rainfall years. Implications This isolated population is likely to be threatened by browsing by feral goats, opening up otherwise densely vegetated habitats of refuge areas, and their trampling of R. tunneyi burrows; by the depletion of grasses from herbivory by European rabbits; and by the long-term impact of a drying climate. It is unlikely to persist without effective on-going management, particularly of the goat population.

Convective Dynamics and the Response of Precipitation Extremes to Warming in Radiative–Convective Equilibrium

AbstractTropical precipitation extremes are expected to strengthen with warming, but quantitative estimates remain uncertain because of a poor understanding of changes in convective dynamics. This uncertainty is addressed here by analyzing idealized convection-permitting simulations of radiative–convective equilibrium in long-channel geometry. Across a wide range of climates, the thermodynamic contribution to changes in instantaneous precipitation extremes follows near-surface moisture, and the dynamic contribution is positive and small but is sensitive to domain size. The shapes of mass flux profiles associated with precipitation extremes are determined by conditional sampling that favors strong vertical motion at levels where the vertical saturation specific humidity gradient is large, and mass flux profiles collapse to a common shape across climates when plotted in a moisture-based vertical coordinate. The collapse, robust to changes in microphysics and turbulence schemes, implies a thermodynamic contribution that scales with near-surface moisture despite substantial convergence aloft and allows the dynamic contribution to be defined by the pressure velocity at a single level. Linking the simplified dynamic mode to vertical velocities from entraining plume models reveals that the small dynamic mode in channel simulations (2% K−1) is caused by opposing height dependences of vertical velocity and density, together with the buffering influence of cloud-base buoyancies that vary little with surface temperature. These results reinforce an emerging picture of the response of extreme tropical precipitation rates to warming: a thermodynamic mode of about 7% K−1 dominates, with a minor contribution from changes in dynamics.

Brief communication: Rare ambient saturation during drifting snow occurrences at a coastal location of East Antarctica

Abstract. Sublimation of snow particles during transport has been recognized as an important ablation process on the Antarctic ice sheet. The resulting increase in moisture content and cooling of the ambient air are thermodynamic negative feedbacks that both contribute to increase the relative humidity of the air, inhibiting further sublimation when saturation is reached. This self-limiting effect and the associated development of saturated near-surface air layers in drifting snow conditions have mainly been described through modelling studies and a few field observations. A set of meteorological data, including drifting snow mass fluxes and vertical profiles of relative humidity, collected at site D17 in coastal Adélie Land (East Antarctica) during 2013 is used to study the relationship between saturation of the near-surface atmosphere and the occurrence of drifting snow in a katabatic wind region that is among the most prone to snow transport by wind. Atmospheric moistening by the sublimation of the windborne snow particles generally results in a strong increase in relative humidity with the magnitude of drifting snow and a decrease in its vertical gradient, suggesting that windborne-snow sublimation can be an important contributor to the local near-surface moisture budget. Despite a high incidence of drifting snow at the measurement location (60.1 % of the time), saturation, when attained, is however most often limited to a thin air layer below 1 m above ground. The development of a near-surface saturated air layer up to the highest measurement level of 5.5 m is observed in only 8.2 % of the drifting snow occurrences or 6.3 % of the time and mainly occurs in strong wind speed and drift conditions. This relatively rare occurrence of ambient saturation is explained by the likely existence of moisture-removal mechanisms inherent to the katabatic and turbulent nature of the boundary-layer flow that weaken the negative feedback of windborne-snow sublimation. Such mechanisms, potentially quite active in katabatic-generated windborne-snow layers all over Antarctica, may be very important in understanding the surface mass and atmospheric moisture budgets of the ice sheet by enhancing windborne-snow sublimation.

A High-Resolution Modeling Study of the 19 June 2002 Convective Initiation Case during IHOP_2002: Localized Forcing by Horizontal Convective Rolls

The initiation processes of one of the initial convective cells near and on the east side of a dryline on 19 June 2002 during the IHOP_2002 field experiment in the central United States is analyzed in detail based on a high-resolution numerical simulation. Prominent horizontal convective rolls and associated near-surface moisture convergence bands [called roll convergence bands (RCBs) here] develop within the convective boundary layer (CBL) due to surface heating, in the hours leading to convective initiation (CI). The RCBs east of the dryline are advected toward the primary dryline convergence boundary (PDCB) by the southerly moist flow as the CBL deepens with time. Backward trajectories of air parcels forming the initial precipitating updraft of the convective cell are found to primarily originate at about 1-1.5 km above ground, within the upper portion of the shallower CBL earlier on. The representative air parcel is found to follow and stay on top of a surface RCB as the RCB moves toward the PDCB, but the RCB forcing alone is not enough to initiate convection. As this RCB gets close to the PDCB, it moves into a zone of mesoscale convergence and a deeper CBL that exhibits an upward moisture bulge associated with the PDCB. The combined upward forcing of the RCB and the mesoscale PDCB convergence quickly lifts the representative air parcel above its level of free convection to initiate convection. A conceptual model summarizing the CI processes is proposed.

The Influence of a Lake-to-Lake Connection from Lake Huron on the Lake-Effect Snowfall in the Vicinity of Lake Ontario

AbstractLake-effect storms (LES) produce substantial snowfall in the vicinity of the downwind shores of the Great Lakes. These storms may take many forms; one type of LES event, lake to lake (L2L), occurs when LES clouds/snowbands develop over an upstream lake (e.g., Lake Huron), extend across an intervening landmass, and continue over a downstream lake (e.g., Lake Ontario). The current study examined LES snowfall in the vicinity of Lake Ontario and the atmospheric conditions during Lake Huron-to-Lake Ontario L2L days as compared with LES days on which an L2L connection was not present [i.e., only Lake Ontario (OLO)] for the cold seasons (October–March) from 2003/04 through 2013/14. Analyses of snowfall demonstrate that, on average, significantly greater LES snowfall totals occur downstream of Lake Ontario on L2L days than on OLO days. The difference in mean snowfall between L2L and OLO days approaches 200% in some areas near the Tug Hill Plateau and central New York State. Analyses of atmospheric conditions found more-favorable LES environments on L2L days relative to OLO days that included greater instability over the upwind lake, more near-surface moisture available, faster wind speeds, and larger surface heat fluxes over the upstream lake. Last, despite significant snowfalls on L2L days, their average contribution to the annual accumulated LES snowfall in the vicinity of Lake Ontario was found to be small (i.e., 25%–30%) because of the relatively infrequent occurrence of L2L days.