Midlatitude Atmosphere Research Articles

Dual-channel infrared OPO lidar optical system for remote sensing of greenhouse gases in the atmosphere: Design and characteristics

Current global warming and climate change under the greenhouse effect call for a thorough understanding of the spatial distribution of greenhouse gases in different atmospheric layers. Lidar systems are the most effective for remote greenhouse gas monitoring. We design a two-channel infrared OPO lidar optical system for remote DIAL/DOAS sensing of carbon dioxide and water vapor in the atmosphere. In this work, optimal geometric parameters of the transceiving channel of the lidar optical system are chosen, a need to focus laser radiation at a distance of 1 km from an observation point is demonstrated, and numerical simulations confirm the possibility of detecting lidar signals ranging from 10−7 to 10−10 W in the informative spectral range 4800–5100 cm−1 (1960–2083 nm). Laboratory experiments with the main components of the lidar system with experimentally confirmed parameters, which simulate atmospheric measurements of CO2 absorption at a calibrated sensing wavelength of 2005 nm (4987 cm−1) (pressure of 1 atm; CO2 concentration corresponding to the midlatitude summer background atmosphere) in the informative spectral range of the lidar system, enable selecting a pair of wavelengths with resonant absorption of the target gas near 2005 nm to study the background state of the atmosphere in the surface layer. Efficiency of the lidar optical system is confirmed by in situ test experiments, where backscattering signals from a topographic target with an albedo of ∼0.15 spaced 168 m apart from an observer are recorded at 60 mV when operating along a horizontal atmospheric path. The lidar system we design can be used in measuring complexes at carbon test sites. It can also be used for atmospheric monitoring in industrial centers, at background measuring stations, and in swamp areas.

The Role of Diabatic Heating in the Midlatitude Atmospheric Circulation Response to Climate Change

Abstract Climate models generally predict a poleward shift of the midlatitude circulation in response to climate change induced by increased greenhouse gas concentration, but the intermodel spread of the eddy-driven jet shift is large and poorly understood. Recent studies point to the significance of midlatitude midtropospheric diabatic heating for the intermodel spread in the jet latitude. To examine the role of diabatic heating in the jet response to climate change, a series of simulations are performed using an idealized aquaplanet model. It is found that both increased CO2 concentration and increased saturation vapor pressure induce a similar warming response, leading to a poleward and upward shift of the midlatitude circulation. An exception to this poleward shift is found for a certain range of temperatures, where the eddy-driven jet shifts equatorward, while the latitude of the eddy heat flux remains essentially unchanged. This equatorward jet shift is explained by the connection between the zonal-mean momentum and heat budgets: increased diabatic heating in the midlatitude midtroposphere balances the cooling by the Ferrel cell ascending branch, enabling an equatorward shift of the Ferrel cell streamfunction and eddy-driven jet, while the latitude of the eddy heat flux remains unchanged. The equatorward jet shift and the strengthening of the midlatitude diabatic heating are found to be sensitive to the model resolution. The implications of these results for a potential reduction in the jet shift uncertainty through the improvement of convective parameterizations are discussed. Significance Statement The latitude of the eddy-driven jet displays considerable variation in climate models, and the factors influencing this variability are poorly understood. This work connects the strength of midlatitude diabatic heating to the structure of the midlatitude circulation and the eddy-driven jet latitude. The direction of the eddy-driven jet shift in response to climate change is found to depend on the diabatic heating response, which in turn depends on the parameterized convective heating. These results highlight the role of convective parameterizations in the representation of the midlatitude circulation in climate models. Additionally, the results imply that the eddy-driven jet shift cannot be explained solely based on the storm-track response to climate change, in contrast with previously suggested explanations.

Storm surge changes around the UK under a weakened Atlantic meridional overturning circulation

Climate model projections of future North Atlantic storm track changes under global warming are very uncertain, with models showing a variety of responses. Atmospheric storms force storm surges which are a major contributor to coastal flooding hazard in the UK, and so it is important to know how this process might be influenced by climate change—not only what future is probable, but what is possible? As a contribution to answering that question, we drive a simplified model of the north-west European coastal shelf waters with atmospheric forcing taken from climate simulations with HadGEM3-GC3-MM (1/4 degree ocean, approx. 60 km atmosphere in mid-latitudes) which exhibit a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC). The first is a ‘hosing’ simulation in which a rapid shut-down of the AMOC is induced by modelling the addition of freshwater to the North Atlantic. The second is the HadGEM3 GC3.05 perturbed parameter ensemble simulation under Representative Concentration Pathway 8.5 (RCP 8.5) which was used to inform the UK Climate Projections 2018 (UKCP18). This model has a high climate sensitivity and exhibits substantial weakening of the AMOC. We find substantial simulated increases at some sites: up to about 25% increase in the expected annual maximum meteorological component of the storm surge. In both the hosing simulation and the ensemble simulation, the greatest projected increases are seen at some west coast sites, consistent with strengthening of the strongest westerly winds. On the south-east coast, projected changes are smaller in the hosing simulation and generally negative in the ensemble simulation. The ensemble simulation shows a decrease in the strongest northerly winds as well as the growth in the westerlies. Overall, these low-likelihood increases over the 21st century associated with storminess are smaller than the likely contribution from mean sea-level rise over the same period, but, importantly, larger than the so-called “high-end” changes associated with storminess that were reported in UKCP18.

Building transport models from baroclinic wave experimental data

In this paper, we study baroclinic waves from both the experimental and the theoretical perspective. We obtain data from a rotating annulus experiment capable of producing a series of baroclinic eddies similar to those found in the mid-latitude atmosphere. We analyze the experimental outputs using two methods. First, we apply a technique that involves filtering data using the empirical orthogonal function (EOF) analysis, which is applied to both velocity and surface temperature fields. The second method relies on the construction of a simple kinematic model based on key parameters derived from the experimental data. To analyze eddy-driven fluid transport, we apply the method of Lagrangian descriptors to the underlying velocity field, revealing the attracting material curves that act as transport barriers in the system. These structures effectively capture the essential characteristics of the baroclinic flow and the associated transport phenomena. Our results show that these barriers are in good agreement with the transport patterns observed in the rotating annulus experiment. In particular, we observe that the structures obtained from the kinematic model, or the one derived in terms of filtered velocities, perform well in this regard.

Atmospheric impact of 2-methylpentanal emissions: kinetics, photochemistry, and formation of secondary pollutants

Abstract. The tropospheric fate of 2-methylpentanal (2MP) has been investigated in this work. First, the photochemistry of 2MP under simulated solar conditions was investigated by determining the UV absorption cross sections (220–360 nm) and the effective photolysis quantum yield in the UV solar actinic region (λ > 290 nm). The photolysis rate coefficient in that region was estimated using a radiative transfer model. Photolysis products were identified by Fourier transform infrared (FTIR) spectroscopy. Secondly, a kinetic study of the chlorine (Cl) and hydroxyl (OH) reactions of 2MP was also performed at 298 K and as a function of temperature (263–353 K), respectively. For the Cl reaction, a relative kinetic method was used in a smog chamber coupled to FTIR spectroscopy, whereas for the OH reaction, the pulsed laser photolysis (PLP) with laser-induced fluorescence (LIF) technique was employed. The estimated lifetime of 2MP depends on the location, the season, and the time of the day. Under mild–strong irradiation conditions, UV photolysis of 2MP may compete with its OH reaction in a mid-latitude inland urban atmosphere, while Cl reaction dominates in mid-latitude coastal urban areas at dawn. Finally, the gaseous product distribution of the Cl and OH reactions was measured in a smog chamber as well as the formation of secondary organic aerosols (SOAs) in the Cl reaction and its size distribution (diameter between 5.6 and 560 nm). The implications for air quality are discussed based on the observed products.

Results of Remote Monitoring of Methane Concentration in the Air of Western Siberia Using the On-board Infrared Lidar Complex

The description of the developed infrared on-board differential absorption lidar for measuring methane content in the air was presented. The lidar was installed on board of aircraft-laboratory Tu-134 "Optic". Flight tests of the developed lidar and experimental measurements of methane concentration along the vertical routing were carried out in the summer atmosphere of mid-latitudes. Lidar measurements of methane content in the air were analyzed. They were compared with local measurements from the gas analyser installed on board of aircraftlaboratory and the results of preliminary numerical modelling. It was concluded that the on-board lidar can measure methane concentration within background values in the mid-latitude summer atmosphere.

Latitudinal storm track shift in a reduced two-level model of the atmosphere

The eddy-driven jet stream and storm tracks in the mid-latitude atmosphere are known to shift in latitude on various timescales, but the physical processes that cause these shifts are still unclear. In this study, we introduce a minimal dynamical system derived from the classical Phillips two-level model with the goal of elucidating the essential mechanisms responsible for the interaction between eddies and mean flow. Specifically, we aim to understand the link between the structure of the eddies and the shift of the latitudinal maximum of the zonal flow. By varying the horizontal shape of the eddies, we find three distinct dynamical regimes whose occurrence depends on the intensity of the external baroclinic forcing: a purely zonal flow, a barotropic eddy regime with net poleward momentum flux, and a baroclinic eddy regime with both net poleward momentum and heat flux. For weak baroclinic forcing, the classical zonal flow solution with latitudinal maximum at the centre of the beta-channel is found. For strong forcing, if eddies are southwest-northeast tilted and zonally elongated, the system is in the baroclinic eddy regime, resulting in a poleward shift of the jet. The intermediate barotropic eddy regime also features a poleward shifted jet, yet with eddies structurally distinct from the baroclinic regime. Changing the parameters yields transitions between the regimes that can be either continuous or discontinuous in terms of the properties of the atmosphere. The findings of this study also provide insights into the properties of the storm track change between the jet entrance and jet exit regions of the North Atlantic.

The Role of Coupled North Pacific Atmosphere–Ocean Interactions in Impacts of Tibetan Plateau Snow Anomalies on the Northern Hemisphere Winter Atmospheric Circulation

Abstract Previous studies indicate observed influences of autumn and winter Tibetan Plateau (TP) snow-cover anomalies on the winter Pacific–North American (PNA) teleconnection. This study simulates atmospheric and oceanic responses to persistent autumn–winter TP snow forcing using an atmospheric general circulation model (AGCM) and a coupled atmospheric-oceanic general circulation model (AOGCM), and quantifies the role of atmosphere–ocean interactions over North Pacific in TP snow effects. The AOGCM experiment induces a stronger and more realistic remote PNA response to heavy TP snow anomalies, and also a significant winter horseshoe-like North Pacific sea surface temperature (SST) pattern resulting from an anomalous equivalent barotropic cyclone, or a strengthened Aleutian low, with associated cyclonic wind stress anomalies. The horseshoe-like SST anomaly pattern is used as boundary forcing (without prescribed heavy TP snow) in another AOGCM experiment, which simulates an enhanced winter Aleutian low and a PNA-like response similar to the original AOGCM responses, indicating that that the direct Pacific–North American atmospheric response to persistent TP snow forcing in the AGCM is amplified in the AOGCM by the North Pacific midlatitude atmosphere–ocean interactions. This suggests that the mechanisms of the winter PNA responses to TP snow forcing involve dynamical atmospheric processes such as horizontal propagation of Rossby wave energy and transient eddy feedbacks, and also North Pacific atmosphere–ocean interactions, which provide a positive feedback on the development of the remote PNA teleconnection. Significance Statement The Tibetan Plateau has a 2.5 million km2 area and a 4000 m average elevation, and is often called “the third pole” because of its polar-like climate in midlatitude Eurasia. A heavy Tibetan Plateau snow cover increases the albedo effect and causes tropospheric cold temperature and low height changes that can be carried or advected to the North Pacific by the prevailing westerlies. The purpose of this study is to better understand how Tibetan Plateau snow anomalies influence the North Pacific and the remote atmospheric circulation through numerical simulations. Our results find that those North Pacific responses amply the direct atmospheric response to Tibetan Plateau snow-cover anomalies. This greatly improves our understanding of physical mechanisms of Tibetan Plateau snow impacts.

A regime shift in North Pacific annual mean sea surface temperature in 2013/14

This study revealed a new regime shift in the North Pacific annual mean sea surface temperature (SST) in 2013/14, robust in multiple SST data sets. This regime shift shows a horseshoe pattern with warming SST along the North American western coast and toward the Central Pacific. It suggests a phase reversal of the Victoria Mode (VM) and it seems unrelated to the shift in the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO). Associated with this regime shift, the annual surface air temperature significantly enhanced over the high-latitude landmass after 2014, whereas it declined over northeastern Canada and southwestern Greenland. The annual precipitation generally increased (decreased) in the Northern (Southern) Hemisphere landmass monsoon regions. There are no regime shifts in Equatorial Central-East Pacific SST and the intensity and locations of the Aleutian Low around 2013/14. These facts suggest the regime shift of North Pacific SST in 2013/14 may not originate from the decadal variations of the midlatitude atmosphere and tropical ocean. The extensive influence of this regime shift in 2013/14 on the atmospheric, oceanic and ecological systems warrants further investigation.

Influence of Reduced Winter Land–Sea Contrast on the Midlatitude Atmospheric Circulation

Abstract Even though winter land–sea thermal contrast (LSC) is expected to undergo a strong weakening in the future warmer climate, its effects have been poorly investigated. Here we run a set of idealized winter simulations featuring reduced LSC in the Northern Hemisphere (NH) extratropics, or in individual extratropical sectors of the NH (Atlantic and Pacific), using an intermediate-complexity atmospheric general circulation model. Reduced LSC is obtained by imposing a warming of surface land temperatures in East Asia and North America. For similar warming intensities over the two regions, the response of the model to East Asia forcing is significantly stronger and dominates the response to the sum of the two forcing patterns. We find that the LSC reduction causes a weakening and poleward shift of the midlatitude jet streams, and a strong interference with zonal wavenumbers 1 and 2. In particular, East Asian warming reduces the amplitude of waves 1 and 2, producing a strengthening of the stratospheric vortex, while a weaker vortex due to a moderate amplification of wave 1 is detected when warming North America. Eventually, stratospheric signals propagate downward in the troposphere affecting the midlatitude winter NH even remotely from the forcing. In this work we pinpoint some mechanisms by which weakened winter LSC influences the NH extratropical circulation: the results may become useful to interpret the response to long-term projections displaying reduced LSC along with other climate change forcing patterns.

A Study on TGF Detectability at 2165 m Altitude: Estimates for the Mountain-Based Gamma-Flash Experiment

Gamma-Flash is an Italian program devoted to the realization of both a ground-based and an airborne gamma-ray and neutron detection system, for in situ measurements of high-energy phenomena correlated to thunderstorm activity, such as Terrestrial Gamma-ray Flashes (TGFs), gamma-ray glows, and associated neutron emissions. The ground-based Gamma-Flash experiment is currently under installation at the Osservatorio Climatico “Ottavio Vittori” (CNR-ISAC) on Mt. Cimone, in Northern-Central Italy (2165 m a.s.l.), and it will be operational starting in Summer 2022. We studied the detectability of TGFs in the surroundings of the ground-based Gamma-Flash experiment, to identify an investigable spatial region around the detectors from which typical TGFs can survive and be revealed onground. We carried out numerical simulations of gamma-ray propagation in the mid-latitude atmosphere, and we developed a qualitative analytical model to integrate the results. This analysis allows one to identify a spatial region extending up to 4 km distance on ground and up to 10 km altitude a.s.l., considering typical TGFs emitting ∼1018 gamma-ray photons at the source. Lightning sferics data acquired by the LINET network demonstrate that such a region is interested by frequent cloud-to-ground and intra-cloud lightning, pointing out the suitability of the location for the purposes of the Gamma-Flash program.

No Change Detected on Earth’s Mid-Latitude Atmospheric Ozone by the 2017 Total Solar Eclipse

Six high altitude balloon flights were completed during the summer of 2017 to measure the effect of the total solar eclipse on Earth’s ozone over the eastern Snake River Plain in Idaho. The stratospheric ozone layer undergoes a noisy diurnal pattern driven primarily by photochemistry above 30 km and by atmospheric dynamics for altitudes below 30 km. The flights for this project rarely exceeded that boundary and were an attempt to detect photochemistry effects in the lower stratosphere. The first five flights determined a baseline for the distribution of ozone from ground level to the mid-stratosphere. The sixth flight was done during the total solar eclipse and was compared to the baseline. These data were also compared to multiple years’ data taken in the Uintah Basin in northeast Utah. All measurements were consistent with each other and show spatial and temporal variations in the ozone column that are expected. The balloon’s instrumentation payload was at the tropopause during eclipse totality and no change in either temperature or ozone was detected that was above the normal noise level in the previous data sets. The conclusion is that no photochemical processes are strong enough to clearly modify Earth’s ozone in the lower stratosphere on timescales shorter than three hours. This is consistent with previously reported satellite data of total column ozone.

The Midlatitude Response to Polar Sea Ice Loss: Idealized Slab-Ocean Aquaplanet Experiments with Thermodynamic Sea Ice

Abstract Slab-ocean aquaplanet simulations with thermodynamic sea ice are used to study the zonally symmetric mechanisms whereby polar sea ice loss impacts the midlatitude atmosphere. Imposed sea ice loss (difference without and with sea ice with historical CO2 concentration) leads to global warming, polar amplified warming, and a weakening of poleward atmospheric energy transport and the midlatitude storm-track intensity. The simulations confirm an energetic mechanism that predicts a weakening of storm-track intensity in response to sea ice loss, given the change of surface albedo and assuming a passive ocean. Namely, sea ice loss increases the absorption of shortwave radiation by the surface (following the decrease of surface albedo), which increases surface turbulent fluxes into the atmosphere thereby weakening poleward atmospheric energy transport. The storm-track intensity weakens because it dominates poleward energy transport. The quantitative prediction underlying the mechanism captures the weakening but underestimates its amplitude. The weakening is also consistent with weaker mean available potential energy (polar amplified warming) and scales with sea ice extent, which is controlled by the slab-ocean depth. The energetic mechanism also operates in response to sea ice loss due to melting (difference of the response to quadrupled CO2 with and without sea ice). Finally, the midlatitude response to sea ice loss in the aquaplanet agrees qualitatively with the response in more complex climate models. Namely, the storm-track intensity weakens and the energetic mechanism operates, but the method used to impose sea ice loss in coupled models impacts the surface response.

Towards using state-of-the-art climate models to help constrain estimates of unprecedented UK storm surges

Abstract. Our ability to quantify the likelihood of present-day extreme sea level (ESL) events is limited by the length of tide gauge records around the UK, and this results in substantial uncertainties in return level curves at many sites. In this work, we explore the potential for a state-of-the-art climate model, HadGEM3-GC3, to help refine our understanding of present-day coastal flood risk associated with extreme storm surges, which are the dominant driver of ESL events for the UK and wider European shelf seas. We use a 483-year present-day control simulation from HadGEM3-GC3-MM (1/4∘ ocean, approx. 60 km atmosphere in mid-latitudes) to drive a north-west European shelf seas model and generate a new dataset of simulated UK storm surges. The variable analysed is the skew surge (the difference between the high water level and the predicted astronomical high tide), which is widely used in analysis of storm surge events. The modelling system can simulate skew surge events comparable to the catastrophic 1953 North Sea storm surge, which resulted in widespread flooding, evacuation of 32 000 people, and hundreds of fatalities across the UK alone, along with many hundreds more in mainland Europe. Our model simulations show good agreement with an independent re-analysis of the 1953 surge event at the mouth of the river Thames. For that site, we also revisit the assumption of skew surge and tide independence. Our model results suggest that at that site for the most extreme surges, tide–surge interaction significantly attenuates extreme skew surges on a spring tide compared to a neap tide. Around the UK coastline, the extreme tail shape parameters diagnosed from our simulation correlate very well (Pearson's r greater than 0.85), in terms of spatial variability, with those used in the UK government's current guidance (which are diagnosed from tide gauge observations), but ours have smaller uncertainties. Despite the strong correlation, our diagnosed shape parameters are biased low relative to the current guidance. This bias is also seen when we replace HadGEM3-GC3-MM with a reanalysis, so we conclude that the bias is likely associated with limitations in the shelf sea model used here. Overall, the work suggests that climate model simulations may prove useful as an additional line of evidence to inform assessments of present-day coastal flood risk.

Occurrence of heatwave in Korea by the displacement of South Asian high

The South Asian high (SAH) index was defined using the 200 hPa geopotential height for 1973–2019. Of the movements of the SAH center in the north–south, east–west, northwest-southeast, and southwest-northeast directions, the movements in the northwest-southeast direction showed the highest positive correlation with heatwave days (HWDs) in South Korea. Thirteen years with the highest values in the northwestward shift of the SAH (positive SAH years) and 13 years with the highest values in the southeastward shift of the SAH (negative SAH years) were selected from a time series of SAH indices from which the linear trend was removed, and the differences between these two groups were analyzed. An analysis of vertical meridional circulation averaged along 120°–130° E showed that in the latitude zones containing Korea, anomalous downward flows with anomalous high pressures formed in the entire troposphere and coincided with a positive anomaly of air temperature and specific humidity. An analysis of stream flows and geopotential heights showed that in the positive SAH years, anomalous anticyclones developed in Korea, the North Pacific, North America, Western Europe, and the Iranian Plateau. These anticyclones had the wavenumber-5 pattern and showed more distinct barotropic vertical structures at higher altitudes, which resembled the circumglobal teleconnection (CGT) pattern. The maintenance of CGT depends on the interaction between the CGT circulation and the Indian summer monsoon (ISM), which has a major influence on the mid-latitude atmosphere. Strengthening of the ISM results in the formation of upper-level anomalous anticyclones in the northwestern Iranian Plateau and produces continuous downstream cells along the waveguide due to the Rossby wave dispersion. When diabatic heating by Indian summer monsoon precipitation is strengthened, the SAH is strengthened to the northwest of India, and a positive CGT pattern is formed. As a result, anomalous anticyclones formed in all layers of the Korean troposphere, resulting in heatwaves, tropical nights, and droughts exacerbated in South Korea.

Local and remote forcing effects of oceanic eddies in the subtropical front zone on the mid-latitude atmosphere in Winter

Multiple oceanic eddies coexist in the North Pacific subtropical front zone (STFZ) in winter, which can be classified into the isolated single eddies, the combined double isotropic eddies and pairs of anisotropic eddies. The forcings of these eddies on the mid-latitude atmosphere are investigated using Climate Forecast System Reanalysis (CFSR) data from year 1979 to 2009, which are divided into the remote and local effects in this research. In the years with a stronger subtropical front, there are more cyclonic isolated and double eddies to the north, more anticyclonic isolated and double eddies to the south of the STFZ, and more eddy pairs with cold to the north and warm to the south concentrated around the main axis of the STFZ. These eddy distributions enhance the strength of the subtropical front, intensify the propagation of upwards baroclinic waves in the lower atmosphere, and finally enhance the zonal wind at upper atmosphere, which is defined as the remote effects of the eddies. However, distinct from this basin-scale remote forcings, three types of oceanic eddies also have different local forcings on the marine atmospheric boundary layer (MABL) above and on the middle atmosphere as expressed in local precipitation difference. The local effects of the isolated single eddies and combined double isotropic eddies take place near the eddy center, whereas that of the pairs of anisotropic eddies at the boundary of the two eddies.

Improved Simulation of Midlatitude Climate in a New Channel Model Compared to Contemporary Global Climate Models

AbstractA midlatitude channel model (MCM), zonally global but meridionally bounded, is constructed for the Northern Hemisphere (0°–360°, 26°N–60°N), based on the Weather Research and Forecasting model. The MCM simulates the midlatitude at a higher resolution than typically possible in a global climate model (GCM). On the other hand, compared to four lateral boundaries in a standard regional or limited‐area model, MCM is bounded on two meridional sides only allowing the simulated atmosphere to develop more freely. Based on a 4‐year simulation at ∼0.33° horizontal grid‐spacing, the MCM realistically captures the annual mean and seasonal cycle of the midlatitude atmosphere and the meridional heat transport by the stationary and transient eddies that dominate the winter weather. Moreover, a comparison of MCM with the ensemble mean of 20 contemporary atmospheric GCMs reveals that the MCM performs better than the GCM ensemble mean. Possible applications of this new modeling configuration are discussed.

The effect of the differences in near-infrared water vapour continuum models on the absorption of solar radiation

There are currently significant disagreements in the strength of the water vapour continuum in the near-infrared region. To understand the effects of these disagreements on the absorption of solar radiation, line-by-line radiative transfer calculations were performed from 2000 to 10,000 cm−1 (1–5 μm) for three standard atmospheres; tropical, mid-latitude summer and sub-arctic winter atmospheres. These calculations were carried out at a solar zenith angle of 60° using line parameters from HITRAN (HIgh-resolution TRANsmission). Three currently available water vapour continuum models were selected for this study; versions 2.5 and 3.2 of the semi-empirical MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) model and the laboratory-measured CAVIAR (Continuum Absorption at Visible and Infrared Wavelengths and its Atmospheric Relevance) model. The differences between the contributions of both MT_CKD models to near-infrared absorption and heating are modest for all three atmospheres. The additional absorption due the CAVIAR model more than doubles those due to both MT_CKD models for the tropical and mid-latitude summer atmospheres. For both atmospheres, the extra heating of the CAVIAR model is up to a factor of 5 more than those of the MT_CKD models. For the sub-arctic winter atmosphere, the differences between the extra absorption and heating of the CAVIAR and those of both MT_CKD models are relatively less. Thus, an update of the MT_CKD model from version 2.5 to 3.2 has a relatively small impact on near-infrared spectrally integrated absorbed solar fluxes and heating rates. But their contributions to the calculations of these quantities differ significantly from that of the much stronger CAVIAR model.

Evidence for Coupling Between the Subseasonal Oscillations in the Southern Hemisphere Midlatitude Ocean and Atmosphere

AbstractA regional perspective is taken to investigate the intriguing ∼25‐day periodicity in the Southern Hemisphere (SH) baroclinic annular mode (BAM) and the possible underlying mechanisms. Results show that the 25‐day periodicity exists not only in the zonal average but also in the regional average of eddy activity as long as the domain of the average is wide enough to accommodate a wave packet. The favorable domain for the ∼25 days periodicity is located downstream of the largest baroclinicity in midlatitudes. The wave activity developed there can travel circumglobally several times with a typical group velocity of 25 m s−1, and reenter the domain every 12.5 days, setting the base periodicity. A slower oceanic oscillation at a time scale of ∼100 days in the same region appears to select and reinforce the multiples of 12.5 days, giving rise to a pronounced ∼25‐day periodicity in the wave activity.

Examination of Global Midlatitude Atmospheric River Lifecycles Using an Object‐Oriented Methodology

AbstractTracking atmospheric rivers (ARs) across their lifecycles is a field of recent interest with a multitude of emerging methodologies. The CONNected‐objECT (CONNECT) algorithm is adapted for the tracking of global midlatitude AR lifecycles and associated precipitation by implementing a seeded region growing segmentation algorithm, creating the AR‐CONNECT algorithm. To facilitate the permissiveness of the methodology, AR‐CONNECT is without hard‐coded geometric criteria yet is still shown to extract synoptic‐scale elongated objects >99.99% of the time. One of the consequences of the methodology is the ability to occasionally track atmospheric water vapor anomalies before evolving into AR geometries, effectively tracking AR genesis further back than other studies. With the aid of subdaily satellite‐derived rain data, we investigate the climatology, trends, and patterns of AR lifecycles from 1983–2016 and compare with other AR tracking studies. We find that AR frequency, genesis, and terminus locations are in generally good agreement with other AR tracking methodologies, though with key differences, and that AR frequencies in each hemisphere are determined by the number of AR hotspots. Furthermore, we uncover evidence that certain AR characteristics, such as frequency, areal extent, and duration, show evidence of increasing trends. Midlatitude precipitation uncovered by AR‐CONNECT shows contributions up to 50% over land and 65% over the ocean. Trend analysis of AR precipitation shows an increase in precipitation associated with ARs propagated by the Southern Jet Stream and ARs that traverse over the Sahara Desert, among others, but is determined not to be a driver of changes in global precipitation.