Deep Convective Events Research Articles

Recent changes of the dissolved oxygen distribution in the deep convection cell of the southern Adriatic Sea

The dynamics of dissolved oxygen in the ocean are of crucial importance for understanding marine ecosystems, with influences ranging from exchange with the atmosphere to biological processes and ocean circulation. In this study, we focus on the southern Adriatic Sea, an essential component of the Eastern Mediterranean “conveyor belt”, to investigate long-term oxygen dynamics and its driving factors. We use cross-platform datasets from 2013 to 2020, including remote sensing data, model reanalysis and in-situ observations from Argo floats, ocean gliders and ship-based measurements. Our analysis investigate the interplay of physical, biological and atmospheric forcing that drive oxygen variability. The distribution of dissolved oxygen in the southern Adriatic Sea is influenced by vertical mixing, advection of water masses and ecosystem dynamics. In the surface layer, the variability of dissolved oxygen is triggered by annual primary production and deep convection events. The dynamics in the intermediate and the deep layers are instead primarily influenced by physical processes, such as the vertical mixing and the water masses inflow from the adjacent sub-basins, which is driven by the periodic reversals of northern Ionian Gyre circulation. In particular our study reveals that the water masses advective dynamics driving the increase and decrease of dissolved oxygen have drastically changed in recent years. The highest dissolved oxygen concentrations are currently observed during the northern Ionian Gyre anticyclonic phase, while they have been previously documented during the cyclonic phase. This change appears to be connected with the significant increase in salinity observed in the southern Adriatic Sea in the same period and contributes to a better understanding of the processes that determine oxygen distribution in the Eastern Mediterranean basin.

Convective gravity wave events during summer near 54° N, present in both AIRS and Rayleigh–Mie–Raman (RMR) lidar observations

Abstract. We connect tropospheric deep convective events over western Europe, as measured by the 8.1 µm radiance observations from the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite, to horizontal brightness temperature variance in the 4.3 µm AIRS channel (maximum sensitivity at around 40 km) and temperature perturbations in vertical lidar profiles (between 33-43 km) over Kühlungsborn, Germany (54.12∘ N, 11.77∘ E). Although the lidar and AIRS are sensitive to different parts of the gravity wave spectrum, they both capture the same peaks in gravity wave activity tied to convection. This suggests that a broad range of vertical wavelengths is present in the convective gravity waves. To account for wave propagation conditions from the troposphere to the stratosphere, we also consider the horizontal winds in the troposphere and stratosphere using the ECMWF Integrated Forecasting System (IFS) operational analysis. In this work, we highlight sporadic peaks in gravity wave activity in summer greatly exceeding those typical of summer, which is generally a season with lower wave activity compared to winter. Although these events are present in roughly half of the years (between 2003 and 2019), we focus our study on two case study years (2014 and 2015). These case study years were chosen because of the high cadence of lidar soundings close in time to the convective events. These events, while sporadic, could contribute significantly to the zonal mean momentum budget and are not accounted for in weather and climate models.

Exceptional sea ice loss leading to anomalously deep winter convection north of Svalbard in 2018

AbstractAn important question is will deep convection sites, where deep waters are ventilated and air-gas exchange into the deep ocean occurs, emerge in the Arctic Ocean with the warming climate. As sea ice retreats northward and as Arctic sea ice becomes younger and thinner, air-sea interactions are strengthening in the high-latitude oceans. This includes new and extreme deep convection events. We investigate the associated physical processes and examine impacts and implications. Focusing on a region near the Arctic gateway of Fram Strait, our study confirms a significant sea ice cover reduction north of Svalbard in 2018 compared to the past decade, shown in observations and several numerical studies. We conduct our study using the regional configuration Arctic and North Hemisphere Atlantic of the ocean/sea ice model NEMO, running at 1/12° resolution (ANHA12). Our numerical study shows that the open water condition during the winter of 2018 allows intense winter convection over the Yermak Plateau, as more oceanic heat is lost to the atmosphere without the insulating sea ice cover, causing the mixed layer depth to reach over 600 m. Anomalous wind prior to the deep convection event forces offshore sea ice movement and contributes to the reduced sea ice cover. The sea ice loss is also attributed to the excess heat brought by the Atlantic Water, which reaches its maximum in the preceding winter in Fram Strait. The deep convection event coincides with enhanced mesoscale eddy activity on the boundary of the Yermak Plateau, especially to the east. The resulting substantial heat loss to the atmosphere also leads to a heat content reduction integrated over the Yermak Plateau region. This event can be linked to the minimum southward sea ice volume flux through Fram Strait in 2018, which is a potential negative freshwater anomaly in the subpolar Atlantic.

Seasonal dynamics and annual budget of dissolved inorganic carbon in the northwestern Mediterranean deep-convection region

Abstract. Deep convection plays a key role in the circulation, thermodynamics, and biogeochemical cycles in the Mediterranean Sea, which is considered to be a hotspot of biodiversity and climate change. In the framework of the DEWEX (Dense Water Experiment) project, the seasonal and annual budgets of dissolved inorganic carbon in the deep-convection area of the northwestern Mediterranean Sea are investigated over the period September 2012–September 2013 using a 3D coupled physical–biogeochemical–chemical modeling approach. At the annual scale, we estimate that the northwestern Mediterranean Sea's deep-convection region was a moderate sink of 0.5 mol C m−2 yr−1 of CO2 for the atmosphere. The model results show the reduction of oceanic CO2 uptake during deep convection and its increase during the abrupt spring phytoplankton bloom following the deep-convection events. We highlight the major roles in the annual dissolved inorganic carbon budget of both the biogeochemical and physical fluxes, which amount to −3.7 and 3.3 mol C m−2 yr−1, respectively, and are 1 order of magnitude higher than the air–sea CO2 flux. The upper layer (from the surface to 150 m depth) of the northwestern deep-convection region gained dissolved inorganic carbon through vertical physical transport and, to a lesser extent, oceanic CO2 uptake, and it lost dissolved inorganic carbon through lateral transport and biogeochemical fluxes. The region, covering 2.5 % of the Mediterranean, acted as a source of dissolved inorganic carbon for the surface and intermediate water masses of the Balearic Sea and southwestern Mediterranean Sea and could represent up to 22 % and 11 %, respectively, of the CO2 exchanges with the Atlantic Ocean at the Strait of Gibraltar.

Cascade sensitivity tests to model deep convective systems in complex orography with WRF

The goal of the present study is to improve the analysis of the performance of the Weather Research and Forecasting (WRF) model, Version 4.3.3, under different settings, in the prediction of precipitation in case of deep convective events. This is done through a wide cascade sensitivity test involving parameterizations expected to play a major role in the description of precipitation for deep convective events: cloud microphysics (CM), planetary boundary layer (PBL), surface layer (SL) and land-surface (L-S) model. Four significant precipitation events, which were associated to deep convective systems and occurred between 2019 and 2021 in Lombardia and Liguria regions (Northern Italy), have been simulated according to 45 different WRF settings. High resolution simulations are required when dealing with small scale deep convective systems as well as a suitable verification method. So, traditional statistical indexes have been integrated with new-generation verification methods, based on the identification of precipitation patterns in forecast and observed fields and on the evaluation of their similarity through the calculation of a coupling index. This analysis method has been exploited to evaluate the quality of the high-resolution simulations performed, and to individuate the best-performing WRF setting in simulating intense convective events. From the present work emerges how the unique CM parameterization characterized by a triple-moment treatment for cloud ice significantly outperforms all other CM schemes underlying the main role of cloud ice in the description of the precipitation case studies. Furthermore, PBL and L-S schemes show a leading role, at least as much as CM, in the description of the events.

Sensitivity of Deep Convection and Cross‐Tropopause Water Transport to Microphysical Parameterizations in WRF

AbstractDeep convective storms can overshoot the tropopause, thus altering the composition of the stratosphere by vertically transporting tropospheric air. The transport of water vapor and ice particles into a sub‐saturated environment can hydrate the stratosphere, with implications for radiative forcing and ozone chemistry. Cloud‐resolved models, if employed at high spatial resolutions, are used to probe process‐level questions about cross‐tropopause deep convective hydration and its controls. There is considerable diversity in model representations of processes associated with water transport and transformation, and the choice of a microphysics scheme affects model skill in simulating deep convective events. This motivates our evaluation of state‐of‐the‐art, as well as widely used standard schemes, in a high spatial‐ and temporal‐resolution framework. Six bulk microphysics schemes were employed in a WRF‐LES setup, initialized with a sounding profile representative of a tropopause‐overshooting storm. We used an idealized framework to isolate the effect of microphysics on the dominant processes that control the reach of deep convection and stratospheric hydration. All schemes produced the highest reaching updrafts 8–12 hr into the simulation but the strength and persistence of updrafts varied across the schemes; maximum storm heights ranged 9.1–12.6 km across the schemes. Varying microphysics produced large differences in the vertical extent and horizontal aggregation of convection, and an order of magnitude spread in above‐tropopause water vapor concentrations.

Farmers’ first rain: investigating dry season rainfall characteristics in the Peruvian Andes

In the Peruvian Andes, the first light rainfalls towards the end of the dry season in August-September are known as pushpa. Softening soils and improving sowing conditions, these rains are crucial for planting dates and agricultural planning. Yet pushpa remains to date unexplored in the literature. This study uses observations and convection-permitting model simulations to describe the characteristics of pushpa in the Rio Santa valley (Peru). Comparing an observed pushpa case in August 2018 with a dry and wet event of the same season, we find pushpa to coincide with upper-level westerly winds that are otherwise characteristic for dry periods. These conditions impose an upper-level dry layer that favours small-scale, vertically-capped convection, explaining the low rainfall intensities that are reportedly typical for pushpa. Climatologically, we find 83% of pushpa-type events to occur under westerly winds, dominating in August, when 60% of the modelled spatial rainfall extent is linked to pushpa. Larger, more intense deep-convective events gradually increase alongside more easterly winds in September, causing the relative pushpa cloud coverage to drop to ̃20%. We note high inter-annual and -decadal variability in this balance between pushpa and intense convective rainfall types, with the spatial extent of pushpa rainfall being twice as high during 2000-2009 than for the 2010-2018 decade over the key sowing period. This result may explain farmers’ perception in the Rio Santa valley, who recently reported increased challenges due to delayed but more intense pushpa rains before the rainy season start. We thus conclude that the sowing and germination season is crucially affected by the balance of pushpa-type and deep-convective rain, resulting in a higher probability for late first rains to be more intense.

Seasonal and interannual variability of Mediterranean Sea overturning circulation

The overturning streamfunction is a widely used metric to monitor ocean circulation changes in the North Atlantic Ocean. Analogously, it is known that an overturning circulation develops in the Mediterranean Sea, although substantially weaker and smaller than in the Atlantic. In this work we use monthly mean fields from a high-resolution ocean reanalysis (1/16°×1/16°) to explore seasonal and interannual variability of Mediterranean Sea overturning circulation in depth and water mass parameter spaces (potential density, potential temperature, salinity) from 1987 to 2018 (both included). Results show a clear single zonal clockwise transport cell in all three spaces that extends from the Strait of Gibraltar to the Levantine Basin, in which water masses tend to densify as they move eastward. This densification is mainly controlled by salinity in the Eastern Mediterranean basin (EMED), while in the Western Mediterranean (WMED) both salinity and potential temperature variations should be taken into account. In contrast, the meridional overturning transport, much more complex, is able to better capture smaller-scale variations since it responds faster to perturbations in the circulation.Seasonal variations in overturning transport are reflected in the size and strength of overturning cells in all spaces, being cells remarkably narrower in winter than in summer. Regarding interannual changes, we show that the meridional overturning transport in density space is significantly correlated with the North Atlantic Oscillation, the Mediterranean Oscillation Index and the Scandinavian pattern during wintertime in the WMED, and with the Eastern Atlantic and the East Atlantic–Western Russian in the EMED. The overturning metric in density space captures well the Eastern Mediterranean Transient, that occurred during years 1992–1996. Finally, we explore the usefulness of overturning metric to detect and track water mass changes associated to shorter scale events, using as example the strong deep convection events of years 2004–2006 in the Gulf of Lion, which marked the beginning of the Western Mediterranean Transition.

Spatial Variation in the Synoptic Structure of Convective Systems over the Great Plains

Abstract During the summer, rainfall over the Rocky Mountains peaks in the afternoon. The diurnal time of maximum rain becomes progressively delayed in going eastward from the Rocky Mountains, such that over much of the Great Plains, maximum rain occurs near 0300 local time. We use Rapid Refresh (RAP) reanalysis data from July and August of 2009–19 to show that there is continuous spatial variation in the mean structure of the high-rain events that occur along this anomalous diurnally propagating rainfall feature. High-rain events that occur farther from the Rockies are more likely to be associated with a larger warm anomaly to the south of the rain event center, a smaller cold anomaly, a larger negative surface pressure anomaly, and increased low-level southerly meridional wind. We also use the Integrated Multi-Satellite Retrievals for GPM (IMERG) rainfall dataset to show that, on hourly time scales, afternoon rainfall that occurs over the Rocky Mountains propagates eastward onto the Great Plains but is rapidly attenuated and becomes negligible east of 100°W. This eastward rainfall propagation appears to be mediated in part by the formation of a low-level cold anomaly and increased surface pressure over the Rocky Mountains, a reversal of the upslope wind, and increased low-level zonal mass convergence over the adjacent near plains. Significance Statement Models have difficulty in simulating the diurnal time of nocturnal rainfall over the Great Plains. Deep convection in this region is often associated with severe weather. Rainfall variability over the Great Plains has a significant impact on soil moisture and agricultural productivity. This paper explores how the structure of deep convective rain events over the Great Plains varies with longitude. The results improve our understanding of deep convection development in the Great Plains and have the potential to contribute to improved forecasts of convective rainfall.

Potential Weakening of the June 2012 North American Derecho Under Future Warming Conditions

AbstractDerechos are widespread and long‐lived straight‐line windstorms associated with fast‐moving mesoscale convective systems that cause severe life and property loss. This study investigates how the June 2012 North American derecho could unfold in the future with global warming. Convection‐permitting regional model ensemble simulations of the derecho as observed are compared with simulations under a pseudo‐global warming scenario of the future environments by the end of the 21st century following the Shared Socioeconomic Pathways (SSP5‐8.5). The control simulations can well reproduce the observed derecho features, including storm evolution, surface wind, and precipitation, except for a 2‐hr delay relative to observations. Compared to the control simulations, the derecho shrinks and decays earlier in the future environments, with much less destructive impacts on the Mid‐Atlantic coastal areas. The derecho shrinkage is attributed to lower relative humidity on both sides of the derecho propagation path under global warming, which suppresses updraft and strengthens downdraft, impeding the spatial extent of the derecho. The premature decay is due to the intensification of an isolated deep convection (IDC) event that develops ahead of the derecho. The meridional gradient of future warming strengthens the synoptic environments, enhancing convergence in a corridor of higher instability that facilitates intensification and continuous development of the IDC. By consuming energy and moisture on the derecho propagation path, the IDC hinders the derecho development in the Mid‐Atlantic coastal areas. Focusing on a single derecho, this study reveals how changes in the thermodynamic environment with warming could induce complex changes in the derecho.

Impact of Dense Water Formation on the Transfer of Particles and Trace Metals from the Coast to the Deep in the Northwestern Mediterranean

This study aimed to describe the interannual variability of dense shelf water cascading and open ocean convection in the Gulf of Lions (NW Mediterranean) based on long-term temperature and current records and its impact on particle fluxes and associated metals. These observations highlight the predominant role of the rare intense events of dense shelf water cascading (1999/2000, 2005/2006, 2012/2013) in the basinward export of particles, which are mainly brought by rivers. Measurements of particulate trace metals in 2012 indicate that the monitored intense cascading event may be responsible for a significant fraction (~15%) of the annual input to the shelf. To this first process is added the effect of somehow more recurrent deep convection events (2005, 2009–2013) that remobilize the deep sediments, receptacle of coastal inputs, and disperse them rapidly at the scale of the northern Mediterranean basin, and gradually over the entire western basin. Coastal and oceanic dense water formations are key physical processes in the Mediterranean margins, whose reduction in intensity and recurrence has already been observed and also anticipate in climate scenarios that will likely change the dispersion pathways of chemical particles in this region.

Orographic Controls over Convection in an Inter-Andean Valley in Northern South America

Abstract Convective processes are critical in the northernmost section of the Andes, where highly complex orography combines with tropical dynamics to produce intense precipitating systems. Data from satellite and ground radar have shown that Colombia’s central Magdalena valley has a marked precipitation diurnal cycle, with high rainfall rates produced by deep convective events that intensify on the foothills of the surrounding valley. This study aims to contribute to the understanding of the orographic controls imposed on the convective activity observed in this important tropical valley. The atmospheric environments associated with convective events are assessed using WRF high-resolution simulations for a two-week period that includes intense rainfall events and marked dry spells. In addition, a simulation with modified orography is evaluated to analyze further the effect of topography in controlling the observed convective systems. The results suggest that northward and southward wind regimes channeled by the valley’s topography and cross-valley flow result in a robust nocturnal convergence zone, which is associated with the strong diurnal cycle and the valley configuration. Furthermore, northerly low-level flow is an important source of moisture for convection in the valley. Additionally, results show midlevel tropospheric heating enhancement during days with more rainfall produced by convective systems in the western slope of the valley. The modified topography simulation suggests that circulation and precipitation in the valley are strongly dependent on the actual terrain features.

Control of the Air‐Sea Oxygen to Heat Flux Ratio During Deep Convection Events

AbstractEarth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O2 inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air‐sea O2 flux to heat flux ratio (O2/heat flux ratio) during deep convection. Our theoretical analysis based on a one‐dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O2/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air‐sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss.

Aerosol characteristics and polarimetric signatures for a deep convective storm over the northwestern part of Europe – modeling and observations

Abstract. The Terrestrial Systems Modeling Platform (TSMP) was extended with a chemical transport model and polarimetric radar forward operator to enable detailed studies of aerosol–cloud–precipitation interactions. The model was used at kilometer-scale (convection-permitting) resolution to simulate a deep convective storm event over Germany which produced large hail, high precipitation, and severe damaging winds. The ensemble model simulation was, in general, able to capture the storm structure, its evolution, and the spatial pattern of accumulated precipitation. However, the model was found to underestimate regions of high accumulated precipitation (> 35 mm) and convective area fraction in the early period of the storm. While the model tends to simulate too high reflectivity in the downdraft region of the storm above the melting layer (mostly contributed by graupel), the model also simulates very weak polarimetric signatures in this region, when compared to the radar observations. The above findings remained almost unchanged when using a narrower cloud drop size distribution (CDSD) acknowledging the missing feedback between aerosol physical and chemical properties and CDSD shape parameters. The kilometer-scale simulation showed that the strong updraft in the convective core produces aerosol-tower-like features, increasing the aerosol number concentrations and hence increasing the cloud droplet number concentration and reducing the mean cloud drop size. This could also be a source of discrepancy between the simulated polarimetric features like differential reflectivity (ZDR) and specific differential-phase (KDP) columns along the vicinity of the convective core compared to the X-band radar observations. However, the use of narrow CDSD did improve the simulation of ZDR columns. Besides, the evaluation of simulated trace gases and aerosols was encouraging; however, a low bias was observed for aerosol optical depth (AOD), which could be partly linked to an underestimation of dust mass in the forcing data associated with a Saharan dust event. This study illustrates the importance and the additional complexity associated with the inclusion of chemistry transport model when studying aerosol–cloud–precipitation interactions. But, along with polarimetric radar data for model evaluation, it allows us to identify and better constrain the traditional two-moment bulk cloud microphysical schemes used in the numerical weather prediction models for weather and climate.

Deep convection in the Subpolar Gyre: Do we have enough data to estimate its intensity?

Deep convection in the Subpolar Gyre (SPG) forms a link between the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC). The intensity of convection in ocean studies is usually estimated using mixed layer depth (MLD). Here MLD is derived using vertical profiles of potential density from the gridded ARMOR3D dataset and from in situ observations of the EN4 dataset. Given limited areas of convective chimneys, the robustness of the estimates from an available set of vertical profiles needs to be verified before accessing mechanisms of interannual variability of deep convection. For reaching this goal, we first outlined three convection domains in the SPG with a high frequency of deep convection events: the southwestern Labrador Sea (L-DC), the central Irminger Sea (I-DC), and the area south of Cape Farewell (F-DC). The minimum number of randomly scattered casts, required to be executed from January to April for a robust estimate of the maximum MLD, depends on the typical area of the convective regions within the domain and forms 50 casts for L-DC, 40 casts for I-DC and 10 casts for F-DC. For the investigated convection domains, a sufficient number of casts were collected for several standalone winters of the late 1990s, while continuous time series of the convection intensity can be obtained only since the mid-2000s.

Convective Couplings with Equatorial Rossby Waves and Equatorial Kelvin Waves. Part II: Coupled Precipitation Characteristics

Abstract Detailed precipitation characteristics coupled with equatorial Rossby and Kelvin waves are investigated. We prepare a rainfall event dataset using the Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) level 2 data. Utilizing three indices, area size, maximum echo-top height, and stratiform precipitation ratio, rainfall events are classified into mesoscale convective system (MCS), deep, congestus, and shallow convective events, and “other” type. We perform composite analyses based on the wave phase defined in Part I. Precipitation amount in Rossby waves is in phase with column water vapor (CWV) anomalies and is mainly contributed by MCSs, which are simultaneously activated with deep convection. The large CWV can support deep development and organization of convection. Shallow and congestus convective events indicate their peaks just before the active phase on 10°N or in the later part of the convectively suppressed phase on the equator. A five-step evolution is shown in Kelvin waves. In the first stage, shallow convective events are triggered by high SST, followed by a dominance of congestus convective events. Then, in the developing stage, deep convective events become dominant. In the mature stage, heavy precipitation is contributed by MCSs, and mostly stratiform rain is maximized at later stages. Kelvin waves indicate relatively weak connection to CWV fluctuation. Although contrasted evolutions are indicated, large contributions by MCSs to precipitation amount are common among the two coupled waves. This is considered to result in the commonality of the equivalent depths with their top-heavy heating. Significance Statement A coupling mechanism between equatorial waves and convective activity is a key issue in the tropical meteorology. While many previous idealized studies suggested some instability mechanisms, detailed precipitation characteristics is not enough investigated. We prepare rainfall-event dataset observed from a spaceborne Precipitation Radar on board the TRMM satellite to quantify detailed precipitation characteristics statistically and compare equatorial Rossby waves and equatorial Kelvin waves. We found that organized convective systems and deep convection are simultaneously activated in the Rossby waves and a clear transition of convective activity is shown in the Kelvin waves. These characteristics highly correspond to waves’ synoptic-scale structures. Our observational results of detailed evolutions of rainfall events will improve understanding of coupling processes.

Impact of Intermittent Convection in the Northwestern Mediterranean Sea on Oxygen Content, Nutrients, and the Carbonate System

AbstractUsing Argo profiling floats, cruises and mooring data, we reconstructed the dissolved oxygen (O2) dynamics in the Gulf of Lion and the Ligurian Sea, with a focus on the intermediate waters. By applying the CANYON‐MED neural network‐based method on the large network of O2‐equipped Argo floats we derived nutrients and carbonate system variables in the Gulf of Lion and the Ligurian Sea at different depths in the water column and derived trends over the 2012–2020 period. In these waters, the O2 minimum is strongly affected by the intermittent convection process, and the two areas show dissimilar responses to the mixing events. In the absence of deep convection events, the O2‐depleted layer tends to spread vertically and intensify even more so in the Ligurian than in the Gulf of Lion. In both areas, over the 2012–2020 period, nutrients increase overall in deep layers, with a concomitant impact on nutrient molar ratios tending toward an increase in P‐limitation. Acidification estimates derived in different layers of the water column show an overall increase in dissolved inorganic carbon and a concurrent pH decrease. These trends were strongly affected by convection events slowing down the overall acidification trend.

Bacterioplankton response to physical stratification following deep convection

Dissolved organic carbon (DOC) produced by primary production in the sunlit ocean can be physically transported to the mesopelagic zone. The majority of DOC exported to this zone is remineralized by heterotrophic microbes over a range of timescales. Capturing a deep convective mixing event is rare, as is observing how microbes respond in situ to the exported DOC. Here, we report ship and Argo float observations from hydrostation North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) 2 Station 4 (N2S4; 47.46°N, 38.72°W), a retentive anticyclonic eddy in the subtropical region of the western North Atlantic. Changes in biogeochemistry and bacterioplankton responses were tracked as the water column mixed to approximately 230 m and restratified over the subsequent 3 days. Over this period, rapid changes in bacterioplankton production (BP) and cell abundance were observed throughout the water column. BP increased by 91% in the euphotic zone (0–100 m) and 55% in the upper mesopelagic zone (100–200 m), corresponding to 33% and 103% increases in cell abundance, respectively. Within the upper mesopelagic, BP upon the occupation of N2S4 (20 ± 4.7 nmol C L–1 d–1) was significantly greater than the average upper mesopelagic BP rate (2.0 ± 1.6 nmol C L–1 h–1) at other stations that had been stratified for longer periods of time. BP continued to increase to 31 ± 3.0 nmol C L–1 d–1 over the 3-day occupation of N2S4. The rapid changes in BP in the upper mesopelagic did not coincide with rapid changes in community composition, but the taxa that increased in their relative contribution included those typically observed in the epipelagic zone. We interpret the subtle but significant community structure dynamics at N2S4 to reflect how injection of labile organic matter into the upper mesopelagic zone by physical mixing supports continued growth of euphotic zone-associated bacterioplankton lineages on a timescale of days.

Cross-Tropopause Transport of Surface Pollutants during the Beijing 21 July Deep Convection Event

Abstract Air transport from the troposphere to the stratosphere plays an important role in altering the vertical distribution of pollutants in the upper troposphere and lower stratosphere (UTLS). On 21 July 2012, Beijing was hit by an unprecedented extreme rainfall event. In the present study, the Community Multiscale Air Quality Modeling System (CMAQ) is used to simulate the change in vertical profiles of pollutants during this event. The integrated process rate (IPR) method was applied to quantify the relative contributions from different atmospheric processes to the changes in the vertical profile of pollutants and to estimate the vertical transport flux across the tropopause. The results revealed that, in the tropopause layer, during the torrential rainfall event, the values of O3 decreased by 35% and that of CO increased by 98%, while those of SO2, NO2, and PM2.5 increased slightly. Atmospheric transport was the main cause for the change in O3 values, contributing 32% of the net increase and 99% of the net decrease of O3. The calculations showed that the transport masses of CO, O3, PM2.5, NO2, and SO2 to the stratosphere by this deep convection in 25 h were 6.0 × 107, 2.4 × 107, 7.9 × 105, 2.2 × 105, and 2.7 × 103 kg, respectively, within the ∼300 km × 300 km domain. In the midlatitudes of the Northern Hemisphere, penetrating deep convective activities can transport boundary layer pollutants into the UTLS layer, which will have a significant impact on the climate of this layer.

Deep Convection as Inferred From the C2OMODO Concept of a Tandem of Microwave Radiometers

Probing the atmosphere from space using radiometers is a challenging way to improve our knowledge of deep convection. Exploration of water absorption bands in the 183, 325, and 448 GHz range is promising because of the radiation scattered by icy hydrometeors produced by deep convection. We investigate what properties of deep convection could be inferred from the Convective Core Observations through MicrOwave Derivatives in the trOpics (C2OMODO) concept of a tandem of microwave radiometers separated by several tens of seconds. Two tropical deep convective events (Hector the Convector and a radiative-convective equilibrium case) are simulated with the Meso-NH non-hydrostatic numerical model, the outputs of which are used to compute brightness temperatures (Tbs) using the Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV) code. We find different relationships between the ice water path, vertical ice momentum, vertical ice velocity, and the time derivative dTb/dt. They depend on where they are probed after separating the growing convective cores from their surrounding environment with a radiometric point of view. Tb and dTb/dt are highly dependent on the ice water path that depends on horizontal and vertical ice advection and microphysical processes. Looking at deep convection in general, we find that the ice water path increases linearly with decreasing dTb/dt. In the specific case of the core of growing convective cells, the vertical ice momentum and the vertical ice velocity are related to dTb/dt. However, such a relationship breaks down in the anvil because horizontal ice advection can dominate microphysical processes. These results are robust to horizontal resolution and time delay.