Tropospheric Subsidence Research Articles

Updraft Width Modulates Ambient Atmospheric Controls on Convective Cloud Depth

AbstractThe depth of convective clouds affects vertical transport of atmospheric constituents, influencing downstream weather and climate. Atmospheric controls on the maximum depth reached by moist convection are investigated with radar‐tracked convective cells tagged with sounding‐derived atmospheric parameters from a field campaign in central Argentina. Regression analyses show that narrow (<12‐km diameter) and wide (>16‐km diameter) cell depths respond to disparate factors, where cell areas are defined using composite reflectivity signatures. Undiluted lifted parcel indices including convective available potential energy (CAPE) and level of neutral buoyancy (LNB) are top predictors of wide cell maximum depth while mid‐tropospheric relative humidity is the top predictor of narrow cell maximum depth. Because narrow cells are more numerous than wide cells, the overall outcome of the full cell population does not strongly correlate with CAPE and LNB conditions. Tracked cells and atmospheric conditions in a simulation with 3‐km grid spacing covering the field campaign produce similar results to those observed. Narrow cells that are relatively deep have a cooler and moister mid‐troposphere with weaker free tropospheric subsidence, while relatively deep wide cells have much warmer and moister lower tropospheric conditions. These atmospheric differences are present 1 hr before cell initiation at both a fixed observing site and variable cell initiation locations. Simulated narrow cell maximum equivalent potential temperature decreases with height at a rate similar to the ambient vertical gradient, causing these cells to fall short of their LNB and supporting the view that entrainment‐driven dilution is a dominant control on their depth.

Clouds and Radiation in a Mock‐Walker Circulation

AbstractThe Walker circulation connects the regions with deep atmospheric convection in the western tropical Pacific to the shallow‐convection, tropospheric subsidence, and stratocumulus cloud decks of the eastern Pacific. The purpose of this study is to better understand the multi‐scale interactions between the Walker circulation, cloud systems, and interactive radiation. To do this we simulate a mock‐Walker Circulation with a full‐physics general circulation model using idealized boundary conditions. Our experiments use a doubly‐periodic domain with grid‐spacing of 1, 2, 25, and 100 km. We thus span the range from General Circulation Models (GCMs) to Cloud‐system Resolving Models (CRMs). Our model is derived from the Geophysical Fluid Dynamics Laboratory atmospheric GCM (AM4.0). We find substantial differences in the mock‐Walker circulation simulated by our GCM‐like and CRM‐like experiments. The CRM‐like experiments have more upper level clouds, stronger overturning circulations, and less precipitation. The GCM‐like experiments have a low‐level cloud fraction that is up to 20% larger. These differences leads to opposite atmospheric responses to changes in the longwave cloud radiative effect (LWCRE). Active LWCRE leads to increased precipitation for our GCMs, but decreased precipitation for our CRMs. The LWCRE leads to a narrower rising branch of the circulation and substantially increases the fraction of precipitation from the large‐scale cloud parameterization. This work demonstrates that a mock‐Walker circulation is a useful generalization of radiative convective equilibrium that includes a large‐scale circulation.

Large Wildfires in the Western United States Exacerbated by Tropospheric Drying Linked to a Multi‐Decadal Trend in the Expansion of the Hadley Circulation

AbstractAnalyses of wildfire‐climate relationships in North America were conducted using diverse independent observation and reanalysis data sets for the period 1984–2014. Results show that the western United States (WUS) has experienced the most robust increase in burned area, even though Alaska and western‐central Canada have comparable warming trends. In addition to warming, the WUS has been under the influence of multi‐decadal trends in tropospheric relative humidity deficit, reduced cloudiness, increased surface net insolation, and enhanced adiabatic warming and drying from increased tropospheric subsidence, as well as drying from enhanced offshore low‐level flow. These trends are found to be associated with a widening of the descending branch of the Hadley circulation, consistent with climate model projections under greenhouse gases warming. Due to the relative short (~30 years) data record, the aforementioned trend signals are likely also be affected by phase changes of natural interdecadal variability during the data period.

A study of the influence of tropospheric subsidence on spring and summer surface ozone concentrations at the JRC Ispra station in northern Italy

Abstract. The influence of tropospheric ozone on the surface ozone concentrations is investigated at the monitoring station of JRC Ispra, based on 10 years of measurements (2006–2015) of surface ozone data. In situ hourly measurements of ozone and other air pollutants, meteorological parameters, and weekly averaged 7Be (as an indicator of upper-tropospheric–stratospheric influence) and 210Pb measurements (as an indicator of boundary layer influence) have been used for the analysis. In addition, IASI + GOME-2 and IASI ozone satellite data have also been used. It is observed that frequently 7Be and ozone weekly peaks coincide, which might be explained by the impact of deep atmospheric subsidence on surface ozone, particularly during late spring and early summer. Based on this observation, a detailed analysis of selected 7Be and ozone episodes occurring during that period of the year has been performed in order to further elucidate the mechanisms of tropospheric influence on the surface pollutant concentrations. For the analysis, composite NOAA/ESRL reanalysis synoptic meteorological charts in the troposphere have been used as well as IASI satellite ozone measurements and NOAA HYSPLIT back trajectories. The JRC station hourly measurements during subsidence episodes show very low values of local pollution parameters (e.g., NOx, 222Rn, nephelometer data, PM10), close to zero. Conversely, during these periods ozone levels usually reach values around 45–60 ppb during the afternoon hours but also show significantly higher values than the average during the night and morning hours, which is a sign of direct tropospheric influence on the surface ozone concentrations.

Analysis of the Spatial–Temporal Variation of the Surface Ozone Concentration and Its Associated Meteorological Factors in Changchun

Ozone (O3) pollution has become one of the most challenging problems in China, and high O3 concentrations have been a major air quality issue in Changchun. Based on continuous observation data of surface ozone concentrations from ten automatic air monitoring stations and meteorological data from the meteorological bureau in Changchun, the temporal and spatial variations of the O3 concentration and its relationships with meteorological factors were analyzed by correlation analysis during the period of 2013–2017. The results showed the following: A single apex model of the annual mean O3 concentrations of the daily maximum 8 h average (MDA8) was found from the data for 2013 to 2017 in Changchun, with the highest MDA8 O3 concentrations in 2015 and a slight decline from then until 2017. The O3 concentrations in the suburban areas and the south of Changchun were higher than those downtown and north of the city. The seasonal variation of O3 concentrations was obvious, following the order summer > spring > autumn > winter, which was similar to the results of neighboring cities and provinces in Changchun. The days on which O3 concentrations exceeded the standard were concentrated in summer and spring, and the total number of ozone excess days was 91 days; the maximum number of ozone excess days was in 2015. The O3 concentration exceeded the standard in Changchun mainly in March–August, and its monthly mean value curve showed a bimodal type in which the highest values appeared in May and July, while the lowest values appeared in December. The diurnal pattern of ozone showed a single peak mode, and the peak value usually appeared at 14:00–16:00 while the minimum value appeared at 07:00–08:00. O3 concentrations in Changchun and the six selected pollutants CO, NO, NO2, NOx, PM10, and PM2.5 were negatively correlated. Higher temperature is a necessary synoptic condition for ozone pollution in Changchun: when the temperature rose, O3 concentrations increased significantly; further, O3 concentrations were negatively correlated with relative humidity and atmospheric pressure and were positively correlated with temperature and solar radiation. The O3 concentrations were highest when the wind scale approached 14~20 km/h and the wind direction was S. Combined with the research results in the surrounding areas of Changchun, it is indicated that there may be an ozone contribution from south of Changchun through long-range pollution transport and tropospheric subsidence.

Characterizing Atmospheric Transport Pathways to Antarctica and the Remote Southern Ocean Using Radon-222

We discuss remote terrestrial influences on boundary layer air over the Southern Ocean and Antarctica, and the mechanisms by which they arise, using atmospheric radon observations as a proxy. Our primary motivation was to enhance the scientific community’s ability to understand and quantify the potential effects of pollution, nutrient or pollen transport from distant land masses to these remote, sparsely-instrumented regions. Seasonal radon characteristics are discussed at 6 stations (Macquarie Island, King Sejong, Neumayer, Dumont d’Urville, Jang Bogo and Dome Concordia) using 1-4 years of continuous observations. Context is provided for differences observed between these sites by Southern Ocean radon transects between 45-67S made by the Research Vessel Investigator. Synoptic transport of continental air within the marine boundary layer (MBL) dominated radon seasonal cycles in the mid-Southern Ocean site (Macquarie Island). MBL synoptic transport, tropospheric injection, and Antarctic outflow all contributed to the seasonal cycle at the sub-Antarctic site (King Sejong). Tropospheric subsidence and injection events delivered terrestrially-influenced air to the Southern Ocean MBL in the vicinity of the circumpolar trough (or “Polar Front”). Katabatic outflow events from Antarctica were observed to modify trace gas and aerosol characteristics of the MBL 100-200 km off the coast. Radon seasonal cycles at coastal Antarctic sites were dominated by a combination of local radon sources in summer and subsidence of terrestrially-influenced tropospheric air, whereas those on the Antarctic Plateau were primarily controlled by tropospheric subsidence. Separate characterisation of long-term marine and katabatic flow air masses at Dumont d’Urville revealed monthly mean differences in summer of up to 5 ppbv in ozone and 0.3 ng m-3 in gaseous elemental mercury. These differences were largely attributed to chemical processes on the Antarctic Plateau. A comparison of our observations with some Antarctic radon simulations by global climate models over the past two decades indicated that: (i) some models overestimate synoptic transport to Antarctica in the MBL, (ii) the seasonality of the Antarctic ice sheet needs to be better represented in models, (iii) coastal Antarctic radon sources need to be taken into account, and (iv) the underestimation of radon in subsiding tropospheric air needs to be investigated.

An investigation on the origin of regional springtime ozone episodes in the western Mediterranean

Abstract. For the identification of regional springtime ozone episodes, rural European Monitoring and Evaluation Programme (EMEP) ozone measurements from countries surrounding the western Mediterranean (Spain, France, Switzerland, Italy, Malta) have been examined with emphasis on periods of high ozone-mixing ratios, according to the variation of the daily afternoon (12:00–18:00) ozone values. For two selected high ozone episodes in April and May 2008, composite NCEP/NCAR reanalysis maps of various meteorological parameters and/or their anomalies (geopotential height, specific humidity, vertical wind velocity omega, vector wind speed and temperature) at various tropospheric pressure levels have been examined together with the corresponding satellite Infrared Atmospheric Sounding Interferometer (IASI) ozone measurements (at 3 and 10 km), CHIMERE simulations, vertical ozone soundings and HYSPLIT back trajectories. The observations show that high ozone values are detected in several countries simultaneously over several days. Also, the examined spring ozone episodes over the western Mediterranean and in central Europe are linked to synoptic meteorological conditions very similar to those recently observed in summertime ozone episodes over the eastern Mediterranean (Kalabokas et al., 2013, 2015; Doche et al., 2014), where the transport of tropospheric ozone-rich air masses through atmospheric subsidence significantly influences the boundary layer and surface ozone-mixing ratios. In particular, the geographic areas with observed tropospheric subsidence seem to be the transition regions between high-pressure and low-pressure systems. During the surface ozone episodes IASI satellite measurements show extended areas of high ozone in the lower- and upper-troposphere over the low-pressure system areas, adjacent to the anticyclones, which influence significantly the boundary layer and surface ozone-mixing ratios within the anticyclones by subsidence and advection in addition to the photochemically produced ozone there, resulting in exceedances of the 60 ppb standard.

An intensified seasonal transition in the Central U.S. that enhances summer drought

AbstractIn the long term, precipitation in the Central U.S. decreases by 25% during the seasonal transition from June to July. This precipitation decrease has intensified since 1979 and such intensification could have enhanced spring drought occurrences in the Central U.S., in which conditions quickly evolve from being abnormally dry to exceptionally dry. Various atmospheric and land reanalysis data sets were analyzed to examine the trend in the June–July seasonal transition. The intensified deficit in precipitation is accompanied by increased downward shortwave radiation flux, tropospheric subsidence, enhanced evaporative fraction, and elevated planetary boundary layer height, all of which can lead to surface drying. The change in tropospheric circulation was characterized by an anomalous ridge over the western U.S. and a trough on either side—a pattern known to suppress rainfall in the Central U.S. This trending pattern shows similarity with the progression of the 2012 record drought.

A Diagnostic Comparison of Alaskan and Siberian Strong Anticyclones

Abstract Strong anticyclones have a significant impact on the cool season climate over mid- and high-latitude landmasses as they are typically accompanied by arctic air masses that can eventually move into populated midlatitude regions. Composite analyses of Alaskan and Siberian strong anticyclones based on sea level pressure (SLP) thresholds of 1050 and 1060 hPa, respectively, were performed to diagnose large-scale dynamical and thermodynamical parameters associated with the formation of strong anticyclones over these two climatologically favorable regions. The anticyclone composite analyses indicate the presence of moderate-to-high-amplitude ridge–trough patterns associated with anticyclogenesis. These ridge–trough patterns are critical as they lead to dynamically favorable circumstances for rapid anticyclogenesis. The strong Alaskan anticyclone develops downstream of a highly amplified upper-tropospheric ridge and is associated with a region of strong tropospheric subsidence due to differential anticyclonic vorticity advection and cold-air advection over the anticyclone center. The strong Siberian anticyclone is associated with an upper-tropospheric pattern of lesser amplitude, suggesting that these dynamical factors, while still important, are less critical to its development. The relative location of elevated terrain features also appears to contribute greatly to the overall evolution of each of these anticyclones.

Microphysical variability in southeast Pacific Stratocumulus clouds: synoptic conditions and radiative response

Abstract. Synoptic and satellite-derived cloud property variations for the southeast Pacific stratocumulus region associated with changes in coastal satellite-derived cloud droplet number concentrations (Nd) are explored. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd, for the five October months of 2001, 2005, 2006, 2007 and 2008. The ability of the satellite retrievals to capture composite differences is assessed with ship-based data. Nd and ship-based accumulation mode aerosol concentrations (Na) correlate well (r = 0.65), with a best-fit aerosol activation value dln Nddln Na of 0.56 for pixels with Nd>50 cm−3. The adiabatically-derived MODIS cloud depths also correlate well with the ship-based cloud depths (r=0.7), though are consistently higher (mean bias of almost 60 m). The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds both at the surface and the lower free troposphere than the MIN-Nd composite. The MAX-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres (as deduced from radiosondes and NCEP Reanalysis). CloudSat radar reflectivities indicate little near-coastal precipitation. The co-occurrence of more boundary-layer aerosol/higher Nd within a more stable atmosphere suggests a boundary layer source for the aerosol, rather than the free troposphere. The MAX-Nd composite cloud thinning extends offshore to 80° W, with lower cloud top heights out to 95° W. At 85° W, the top-of-atmosphere shortwave fluxes are significantly higher (~50%) for the MAX-Nd composite, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd composite. The change in Nd at this location is small (though positive), suggesting that the MAX-MIN Nd composite differences in radiative properties primarily reflects synoptic changes. Circulation anomalies and a one-point spatial correlation map reveal a weakening of the 850 hPa southerly winds decreases the free tropospheric cold temperature advection. The resulting increase in the static stability along 85° W is highly correlated to the increased cloud fraction, despite accompanying weaker free tropospheric subsidence.

Zonal and Vertical Structure of the Madden–Julian Oscillation

AbstractA statistical study of the three-dimensional structure of the Madden–Julian oscillation (MJO) is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space–time filtered outgoing longwave radiation (OLR) data. MJO convection is generally preceded by low-level convergence and upward motion in the lower troposphere, while subsidence, cooling, and drying prevail aloft. This leads to moistening of the boundary layer and the development of shallow convection, followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection. After the passage of the heaviest rainfall, a westerly wind burst region is accompanied by stratiform precipitation, where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion. The evolution of the heating field leads to a temperature structure that favors the growth of the MJO. The analysis also reveals distinct differences in the vertical structure of the MJO as it evolves, presumably reflecting changes in its vertical heating profile, phase speed, or the basic-state circulation that the MJO propagates through.The dynamical structure and the evolution of cloud morphology within the MJO compares favorably in many respects with other propagating convectively coupled equatorial waves. One implication is that the larger convective envelopes within the Tropics tend to be composed of more shallow convection along their leading edges, a combination of deep convection and stratiform rainfall in their centers, and then a preponderance of stratiform rainfall along their trailing edges, regardless of scale or propagation direction. While this may ultimately be the factor that governs the dynamical similarities across the various wave types, it raises questions about how the smaller-scale, higher-frequency disturbances making up the MJO conspire to produce its heating and dynamical structures. This suggests that the observed cloud morphology is dictated by fundamental interactions with the large-scale circulation.

Modeling springtime shallow frontal clouds with cloud‐resolving and single‐column models

This modeling study compares the performance of eight single‐column models (SCMs) and four cloud‐resolving models (CRMs) in simulating shallow frontal cloud systems observed during a short period of the March 2000 Atmospheric Radiation Measurement (ARM) intensive operational period. Except for the passage of a cold front at the beginning of this period, frontal cloud systems are under the influence of an upper tropospheric ridge and are driven by a persistent frontogenesis over the Southern Great Plains and moisture transport from the northwestern part of the Gulf of Mexico. This study emphasizes quantitative comparisons among the model simulations and with the ARM data, focusing on a 27‐hour period when only shallow frontal clouds were observed. All CRMs and SCMs simulate clouds in the observed shallow cloud layer. Most SCMs also produce clouds in the middle and upper troposphere, while none of the CRMs produce any clouds there. One possible cause for this is the decoupling between cloud condensate and cloud fraction in nearly all SCM parameterizations. Another possible cause is the weak upper tropospheric subsidence that has been averaged over both descending and ascending regions. Significantly different cloud amounts and cloud microphysical properties are found in the model simulations. All CRMs and most SCMs underestimate shallow clouds in the lowest 125 hPa near the surface, but most SCMs overestimate the cloud amount above this layer. These results are related to the detailed formulations of cloud microphysical processes and fractional cloud parameterizations in the SCMs, and possibly to the dynamical framework and two‐dimensional configuration of the CRMs. Although two of the CRMs with anelastic dynamical frameworks simulate the shallow frontal clouds much better than the SCMs, the CRMs do not necessarily perform much better than the SCMs for the entire period when deep and shallow frontal clouds are present.

Upstream influence of numerically simulated squall‐line storms

AbstractThe squall line's impact on its upstream environment is examined using traditional cloud and simplified (parametrized moisture) models. The study was motivated by the need to explain significant differences between the two dynamical frameworks. In both, the first environmental response to the convection consists of a rapidly propagating gravity wave characterized by deep tropospheric subsidence. This gravity wave accelerates the low‐level storm inflow in its wake, with the largest effect seen near the surface.In the more sophisticated model, however, this wave is eventually followed by a shorter vertical wavelength feature, one possessing lower‐tropospheric ascent. This second gravity wave, absent from the simplified model runs, substantially alters the upstream environment yet again. The gentle low‐level uplift establishes the ‘cool/moist tongue’ of air that has been found to stretch well ahead of the storm in simulations. Between the second wave and the main storm updraught, the accelerated inflow is shifted from the ground to the middle troposphere where it helps to push dry air into the convecting region.This subsequent environmental adjustment responds to the establishment of a small yet persistent area of weak cooling during the early mature phase. Located in the middle troposphere at the upstream edge of the main cloud mass, this cooling is the combined effect of cloud‐water evaporation and the ascent of subsaturated air. Though the cooling that excites it is of relatively small magnitude (<2 K), this wave's effect is dramatic and significant. A crude fix to the moisture parametrization served to establish a qualitatively similar tongue‐like feature in the simplified model simulations, bringing the results from the two dynamical frameworks more into line. Copyright © 2002 Royal Meteorological Society.

Development and application of a mesoscale climate model for the tropics: Influence of sea surface temperature anomalies on the West African monsoon

A mesoscale climate model (MCM) is developed from the Pennsylvania State University/National Center for Atmospheric Research mesoscale model 5 to simulate the West African summer monsoon. Results from the MCM are compared to observations, a reanalysis, and global climate model (GCM) output to show that the MCM reasonably simulates the West African monsoon climate and its variability and improves on many shortcomings of the GCM simulations. The MCM's ability to capture correctly processes that cause interannual variability, and thereby allow us to improve our physical understanding of that variability, is investigated by examining the influence of Gulf of Guinea sea surface temperature anomalies (SSTAs) on the West African monsoon. Similar to observations, precipitation decreases over the Sahel and increases along the Guinean coast in response to warm SSTAs in the gulf. The increase in rainfall along the Guinean coast is associated with an increase in lower tropospheric water vapor content due to enhanced evaporation over the warm SSTAs and northward moisture advection in the monsoon flow. This stronger precipitation on the coast occurs despite a decrease in the land/sea temperature gradient, which is the fundamental driver of the monsoon. The decrease in rainfall over the southern Sahel is associated with lower tropospheric subsidence replacing rising vertical motions as the monsoon circulation is shifted equatorward. Dynamically, the subsidence over the southern Sahel is associated with shrinking of both planetary and relative vorticity. The former is related to the equatorward shift of the monsoon, which results in an expansion of the equatorward flow from the Sahel to the northern Guinean coast. The latter occurs because the equatorward flow over the southern Sahel occurs lower in the troposphere (i.e., 850 mbar) than in the control (i.e., 700 mbar), where the meridional relative vorticity gradient is opposite to the gradient at 700 mbar.

Diabatic subsidence in the subtropical upper troposphere derived from SAGE II measurements

Composite diabatic subsidence rates in the subtropical upper troposphere are derived using water vapor measurements of high vertical resolution from SAGE II in conjunction with collocated ECMWF reanalysis. Their relationship with the concentration of upper tropospheric water vapor is examined. The mean vertical profiles of subsidence rate show a monotonic increase in downward mass flux from the tropopause down to at least 400 mb reaching a magnitude of 22 mb day−1. This vertical structure of subsidence suggests horizontal mass convergence and horizontal moisture transport at all levels above 400 mb. Seasonal variation of upper tropospheric subsidence is also presented.

The thermal response of the tropical atmosphere to variations in equatorial Pacific sea surface temperature

For the period 1966–1982, monthly mean temperature anomalies were determined for 30 pressure levels from 1000 mbar (0 km altitude) to 15 mbar (28 km) at five tropical radiosonde stations, four in the western Pacific Ocean (Yap, Majuro, Wake Island, and Pago Pago) and one in the Caribbean (Curacao). Long‐term trends in the data were removed, and the detrended time series were cross correlated with an index of nonseasonal Pacific sea surface temperature (SST) anomalies with lags ranging up to ±30 months. At each station the correlation diagrams show a clear division of the atmosphere into four distinct regions, each having a different response to SST variations. In the main troposphere (850–150 mbar; 2–14 km) the correlation is positive, with a maximum at a lag of 1–4 months, in general agreement with earlier studies. In the lower stratosphere (100–40 mbar; 16–22 km) the correlation is negative, with a similar delay, while the surface layer (1000–850 mbar; 0–2 km) and the upper troposphere (150–100 mbar; 14–16 km) show responses that vary from station to station. The correlation drops essentially to zero above 40 mbar (22 km). Examination of the lags at the different stations indicate that the tropospheric inphase and stratospheric out‐of‐phase responses appear to propagate zonally from an apparent point of origin in the east central equatorial Pacific with a speed of the order of 1 m s−1. The possible mechanisms underlying this propagation are discussed, and it is concluded that simple advection of air from centers of convective activity is unlikely to explain both the tropospheric and the stratospheric responses. An alternative explanation involving variations in the rate of tropospheric subsidence and stratospheric upwelling accompanying variations in convective activity is suggested. The long‐term trends in atmospheric temperature are shown to have a variation with height that parallels the variation in the SST correlation. This may indicate that the atmosphere is responding to a corresponding long‐term trend in Pacific sea surface temperatures.