Adiabatic Lifting Research Articles

Extratropical Stratospheric Air Intrusions Over the Western North Pacific and the Genesis of Downstream Monsoon Low‐Pressure Systems

AbstractLow‐pressure systems (LPS) are convectively coupled vortices that contribute nearly half of the summer monsoon rainfall over the Indian subcontinent. About one third of the boreal summer monsoon LPS are caused by downstream amplification of westward propagating disturbances from the western North Pacific (WNP). Analysis of downstream LPS events from 1979 to 2017 reveals that 43% of them are caused by extratropical stratospheric air intrusions over the WNP. Stratospheric air intrusions lead to high tropospheric potential vorticity (PV), and the downstream vortex seeds are observed to initiate and intensify to the southwest of the PV anomalies. The PV anomalies can deform the temperature in its neighborhood and cause adiabatic lifting, which in turn can induce and intensify low‐level cyclonic vortices. The subsequent intensification of the low‐level vortex is aided by deep convection, observed to the southwest of the PV anomaly, through vortex stretching and low‐level PV generation by diabatic heating.

The Role of Large-Scale Moistening by Adiabatic Lifting in the Madden–Julian Oscillation Convective Onset

AbstractThe initiation of the Madden–Julian oscillation over the Indian Ocean is examined through the use of a moisture budget that applies a version of the weak temperature gradient (WTG) approximation that does not neglect dry adiabatic vertical motions. Examination of this budget in ERA-Interim reveals that horizontal moisture advection and vertical advection by dry adiabatic lifting govern the moistening of the troposphere for both primary and successive MJO initiation events. For both types of initiation events, horizontal moisture advection peaks prior to the maximum moisture tendency, while dry adiabatic lifting peaks after the maximum moisture tendency. Once convection initiates, moisture is maintained by anomalous radiative and adiabatic lifting. The dry adiabatic lifting during successive MJO initiation is attributed to the return of the circumnavigating circulation from a previous MJO event, while in primary events the planetary-scale circulation appears to originate over South America. Examination of the same budget with data from the DYNAMO northern sounding array shows that adiabatic lifting contributes significantly to MJO maintenance, with a contribution that is comparable to that of surface heat fluxes. However, results from the DYNAMO data disagree with those from ERA-Interim over the importance of adiabatic lifting to the moistening of the troposphere prior to the onset of convection. In spite of these differences, the results from the two datasets show that small departures from WTG balance in the form of dry adiabatic motions cannot be neglected when considering MJO convective onset.

Is a more physical representation of aerosol activation needed for simulations of fog?

Abstract. Aerosols play a crucial role in the fog life cycle, as they determine the droplet number concentration and hence droplet size, which in turn controls both the fog's optical thickness and lifespan. Detailed aerosol-microphysics schemes which accurately represent droplet formation and growth are unsuitable for weather forecasting and climate models, as the computational power required to calculate droplet formation would dominate the treatment of the rest of the physics in the model. A simple method to account for droplet formation is the use of an aerosol activation scheme, which parameterises the droplet number concentration based on a change in supersaturation at a given time. Traditionally, aerosol activation parameterisation schemes were designed for convective clouds and assume that supersaturation is reached through adiabatic lifting, with many imposing a minimum vertical velocity (e.g. 0.1 m s−1) to account for the unresolved subgrid ascent. In radiation fog, the measured updraughts during initial formation are often insignificant, with radiative cooling being the dominant process leading to saturation. As a result, there is a risk that many aerosol activation schemes will overpredict the initial fog droplet number concentration, which in turn may result in the fog transitioning to an optically thick layer too rapidly. This paper presents a more physically based aerosol activation scheme that can account for a change in saturation due to non-adiabatic processes. Using an offline model, our results show that the equivalent cooling rate associated with the minimum updraught velocity threshold assumption can overpredict the droplet number by up to 70 % in comparison to a typical cooling rate found in fog formation. The new scheme has been implemented in the Met Office Natural Environment Research Council (NERC) Cloud (MONC) large eddy simulation (LES) model and tested using observations of a radiation fog case study based in Cardington, UK. The results in this work show that using a more physically based method of aerosol activation leads to the calculation of a more appropriate cloud droplet number. As a result, there is a slower transition to an optically thick (well-mixed) fog that is more in line with observations. The results shown in this paper demonstrate the importance of aerosol activation representation in fog modelling and the impact that the cloud droplet number has on processes linked to the formation and development of radiation fog. Unlike the previous parameterisation for aerosol activation, the revised scheme is suitable to simulate aerosol activation in both fog and convective cloud regimes.

Atmospheric River Sectors: Definition and Characteristics Observed Using Dropsondes from 2014–20 CalWater and AR Recon

AbstractAtmospheric rivers (ARs) are responsible for intense winter rainfall events impacting the U.S. West Coast, and have been studied extensively during CalWater and AR Recon field programs (2014–20). A unique set of 858 dropsondes deployed in lines transecting 33 ARs are analyzed, and integrated vapor transport (IVT) is used to define five regions: core, cold and warm sectors, and non-AR cold and warm sides. The core is defined as having at least 80% of the maximum IVT in the transect. Remaining dropsondes with IVT > 250 kg m−1 s−1 are assigned to cold or warm sectors, and those outside of this threshold form non-AR sides. The mean widths of the three AR sectors are approximately 280 km. However, the core contains roughly 50% of all the water vapor transport (i.e., the total IVT), while the others each contain roughly 25%. A low-level jet occurs most often in the core and warm sector with mean maximum wind speeds of 28.3 and 21.7 m s−1, comparable to previous studies, although with heights approximately 300 m lower than previously reported. The core exhibits characteristics most favorable for adiabatic lifting to saturation by the California coastal range. On average, stability in the core is moist neutral, with considerable variability around the mean. A relaxed squared moist Brunt–Väisälä frequency threshold shows ~8%–12% of core profiles exhibiting near-moist neutrality. The vertical distribution of IVT, which modulates orographic precipitation, varied across AR sectors, with 75% of IVT residing below 3115 m in the core.

Can stable isotopes ride out the storms? The role of convection for water isotopes in models, records, and paleoaltimetry studies in the central Andes

Globally, changes in stable isotope ratios of oxygen and hydrogen (δO18 and δD) in the meteoric water cycle result from distillation and evaporation processes. Isotope fractionation occurs when air masses rise in elevation, cool, and reduce their water-vapor holding capacity with decreasing temperature. As such, δO18 and δD values from a variety of sedimentary archives are often used to reconstruct changes in continental paleohydrology as well as paleoaltimetry of mountain ranges. Based on 234 stream-water samples, we demonstrate that areas experiencing deep convective storms in the eastern south–central Andes (22–28° S) do not show the commonly observed relationship between δO18 and δD with elevation. These convective storms arise from intermontane basins, where diurnal heating forces warm air masses upward, resulting in cloudbursts and raindrop evaporation. Especially at the boundary between the tropical and extra-tropical atmospheric circulation regimes where deep-convective storms are very common (∼26° to 32° N and S), the impact of such storms may yield non-systematic stable isotope-elevation relationships as convection dominates over adiabatic lifting of air masses. Because convective storms can reduce or mask the depletion of heavy isotopes in precipitation as a function of elevation, linking modern or past topography to patterns of stable isotope proxy records can be compromised in mountainous regions, and atmospheric circulation models attempting to predict stable isotope patterns must have sufficiently high spatial resolution to capture the fractionation dynamics of convective cells.

Weak Pressure Gradient Approximation and Its Analytical Solutions

Abstract A weak pressure gradient (WPG) approximation is introduced for parameterizing supradomain-scale (SDS) dynamics, and this method is compared to the relaxed form of the weak temperature gradient (WTG) approximation in the context of 3D, linearized, damped, Boussinesq equations. It is found that neither method is able to capture the two different time scales present in the full 3D equations. Nevertheless, WPG is argued to have several advantages over WTG. First, WPG correctly predicts the magnitude of the steady-state buoyancy anomalies generated by an applied heating, but WTG underestimates these buoyancy anomalies. It is conjectured that this underestimation may short-circuit the natural feedbacks between convective mass fluxes and local temperature anomalies. Second, WPG correctly predicts the adiabatic lifting of air below an initial buoyancy perturbation; WTG is unable to capture this nonlocal effect. It is hypothesized that this may be relevant to moist convection, where adiabatic lifting can reduce convective inhibition. Third, WPG agrees with the full 3D equations on the counterintuitive fact that an isolated heating applied to a column of Boussinesq fluid leads to a steady ascent with zero column-integrated buoyancy. This falsifies the premise of the relaxed form of WTG, which assumes that vertical velocity is proportional to buoyancy.

Numerical Tests of the Weak Pressure Gradient Approximation

Abstract Cloud-resolving simulations of convection over a surface temperature hot spot are used to evaluate the weak pressure gradient (WPG) and weak temperature gradient (WTG) approximations. The premise of the relaxed form of WTG—that vertical velocity is equal to buoyancy times a positive time scale—is found to be violated by thick layers of negative buoyancy in steady-state ascent. The premise of WPG—that horizontal divergence and pressure anomalies are collocated—is validated by these simulations. When implemented in a cloud-resolving model, WPG replicates buoyancy transients exceptionally well, including the adiabatic lifting of air below buoyancy anomalies. WTG captures neither this effect nor the associated triggering of moist convection. For steady states, WTG produces vertical velocity profiles that are too top heavy. On the other hand, WPG generates velocity profiles that closely match fully resolved hot-spot simulations. Taken together, the evidence suggests that WPG is a relatively accurate method for parameterizing supradomain-scale (SDS) dynamics.

Cirrus clouds triggered by radiation, a multiscale phenomenon

Abstract. In this study, the influence of radiative cooling and small eddies on cirrus formation is investigated. For this purpose the non-hydrostatic, anelastic model EULAG is used with a recently developed and validated ice microphysics scheme (Spichtinger and Gierens, 2009a). Additionally, we implemented a fast radiative transfer code (Fu et al., 1998). Using idealized profiles with high ice supersaturations up to 144% and weakly stable stratifications with Brunt-Vaisala frequencies down to 0.0018 s−1 within a supersaturated layer, the influence of radiation on the formation of cirrus clouds is remarkable. Due to the radiative cooling at the top of the ice supersaturated layer with cooling rates down to −3.5 K/d, the stability inside the ice supersaturated layer decreases with time. During destabilization, small eddies induced by Gaussian temperature fluctuations start to grow and trigger first nucleation. These first nucleation events then induce the growth of convective cells due to the radiative destabilization. The effects of increasing the local relative humidity by cooling due to radiation and adiabatic lifting lead to the formation of a cirrus cloud with IWC up to 33 mg/m3 and mean optical depths up to 0.36. In a more stable environment, radiative cooling is not strong enough to destabilize the supersaturated layer within 8 h; no nucleation occurs in this case. Overall triggering of cirrus clouds via radiation works only if the supersaturated layer is destabilized by radiative cooling such that small eddies can grow in amplitude and finally initialize ice nucleation. Both processes on different scales, small-scale eddies and large-scale radiative cooling are necessary.

Dynamical reconstruction of the record low column ozone over Europe on 30 November 1999

The assembly of the record‐low total column ozone observed over Europe on 30 November 1999 is investigated using a three‐dimensional reconstruction based on POAM, SAGE, and HALOE data and a passive tracer advected by UKMO winds on isentropic surfaces. The reconstruction reproduces TOMS observation to within 10–20 DU over the northern hemisphere and agrees well with select ozonesonde profiles in the mid‐ to high latitudes, although values in the vicinity of the ozone minimum are systematically higher than TOMS. Relative role is quantified for the disparate dynamical processes that contributed constructively to the low‐ozone episode: poleward intrusion of ozone‐poor subtropical air in the lowermost stratosphere, adiabatic lifting of ozone maximum, and displacement of ozone‐poor Arctic vortex air aloft. Of these, the first two are found to account for about three quarters of the reconstructed column anomaly between 320 and 1900 K whereas the last for about one quarter.

The Kleiner Feldberg Cloud Experiment 1990. An overview

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

The Origin of a Severe Thunderstorm in Kansas on 10 May 1985

Abstract This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection. Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of ...

Parameterization of the Condensation Process in Small Nonprecipitating Cumuli

Measurements of the droplet size distribution collected in a field of small nonprecipitating cumuli are analyzed. It appears that despite the high variability of the measured droplet concentration, the droplet spectral shape at given levels is almost invariable. An original method, based on the coincidence effects in the droplet counter, is used to show that most of the low concentration samples are in fact heterogeneous and that the actual droplet concentration in the cloudy parts of these samples is comparable to the concentration in regions of the cloud which are uniform. Observed spectra are much broader than provided by adiabatic lifting. The frequency distribution of the spectral shapes is compared to the results of a one-dimensional cloud model for various values of the entrainment rate. The limitations of the previous models are discussed and the inhomogeneous mixing formulation of Baker et al. is then modified to allow nucleus activation in the entrained air before its fine scale mixing with the cloudy droplets. Finally the proposed scheme is simplified to derive a parameterization of the broad spectra. The range of spectral shapes reproduced at any level by the parameterization is in agreement with their observed frequency distribution.

Absorption of NH 3 and SO 2 during activation of atmospheric cloud condensation nuclei

A numerical model is presented that considers the micro-physics and chemistry of cloud condensation nuclei (CCN) as the nuclei are transported vertically from the base of a cloud. The CCN are initially composed of mixtures of (NH 4) 2SO 4, H 2SO 4 and H 2O and are in equilibrium with gaseous SO 2 and NH 3 concentrations. The model incorporates liquid phase oxidation of S(IV) to S(VI) during adiabatic lifting of the CCN. Simultaneous absorption of SO 2 and NH 3 between the cloud droplets and gaseous dispersion medium is also considered. The model also evaluates whether the droplets are in chemical equilibrium with respect to gaseous SO 2 and NH 3 concentrations. Results from the model indicate that oxidation of S(IV) increases cloud droplet acidity during activation of the CCN. Large cloud droplets also exhibit gas phase mass transfer limitations with respect to SO 2 and NH 3. pH values of the resulting cloud droplet size distribution range over 3 pH units within the cloud at typical atmospheric conditions.

Atmospheric boundary layer modification in the marginal ice zone

A case study of the Andreas et al. (1984) data on atmospheric boundary layer modification in the marginal ice zone is made. Our model is a two‐dimensional, multilevel, linear model with turbulence, lateral and vertical advection, and radiation. Good agreement between observed and modeled temperature cross sections is obtained. In contrast to the hypothesis of Andreas et al., we find the air flow is stable to secondary circulations. Adiabatic lifting and, at long fetches, cloud top longwave cooling, not an air‐to‐surface heat flux, dominate the cooling of the boundary layer. The accumulation with fetch over the ice of changes in the surface wind field is shown to have a large effect on estimates of the surface wind stress. We speculate that the Andreas et al. estimates of the drag coefficient over the compact sea ice are too high.

SUPERADIABATIC LAPSE RATES OF TEMPERATURE IN RADIOSONDE OBSERVATIONS

Some instances of superadiabatic lapse rates in temperature observed in radiosonde observations may be produced or accelerated by adiabatic lifting. Examples are shown which suggest this possibility.