The Acoustic Nature of “Storm Nose”, “Supercell” Vortices and Tornadoes

The process of cumulonimbus cloud Cb calvus formation in the middle latitudes of real atmosphere is analyzed in this work. Its transformation from initial lifecycle stage to “maturity” undergoes due to the formation of the waveguide called “aerial acoustic channel” in the troposphere near the level of temperature minimum that is close to 2 km altitude. This “aerial acoustic channel” can be considered as analog of “deep sound channel” that corresponds to the minimal sound speed level. Tropospheric “channel” related to the thermal inversion zone is almost unlimited horizontally. Synchronous generation of two compression waves (ascending one above Cb and descending one inside Cb) is caused by Cb calvus dome ascension. The first one can provoke the aerodynamic draft previously unexplained. The second one results in the growth of its “storm nose” and in the axial and peripheral descending mechanisms in Cb. The penetration of Cb into stratosphere results in the destruction of dynamic balance around Cb top and hence in its unloading in the descending decompression wave. Here the air cools down to the “dew point” in the place of conjugation with parental cloud – due to Snellius law it results in the formation of aerosol “vortex” as condensation front; this “vortex” has calculated value of its generatrix against vertical. Due to D. Snow’s criterion, this vortex forms either “supercell” vortex or tornado vortex.

The stability of internal gravity waves in a bounded atmospheric shear layer in the presence of moisture

The stability characteristics of internal gravity waves, generated by an isothermal bounded tangent velocity profile in the presence of a saturated finite layer, are studied. The moist layer with constant thickness the structure of the ness is introduced at different levels in respect to the point of inflection and the variations of moisture content and distance from the origin are examined. The characteristics of the unstable waves are obtained by solving numerically the linearized versions or the full equations of motion, in the inviscid and Boussinesq limit, through the technique of Lalas and Elnaudi (1976). It is shown that the presence of the moist layer can significantly affect the stability characteristics of the waves. Increases in the moisture and distance of the layer from the Inflection point are found to amplify or decay the wave response, because the saturated layer behaves as a solid boundary to the flow. The presence of such effective layer is shown to stabilize short wavelengths and destabilize. Finally, an application of the model's results to the real atmosphere is discussed.

Stratospheric Modulation of Arctic Oscillation Extremes as Represented by Extended-Range Ensemble Forecasts

The Arctic Oscillation (AO) describes a seesaw pattern of variations in atmospheric mass over the polar cap. It is by now well established that the AO pattern is in part determined by the state of the stratosphere. In particular, sudden stratospheric warmings (SSWs) are known to nudge the tropospheric circulation toward a more negative phase of the AO, which is associated with a more equatorward shifted jet and enhanced likelihood for blocking and cold air outbreaks in mid-latitudes. SSWs are also thought to contribute to the occurrence of extreme AO events. However, statistically robust results about such extremes are difficult to obtain from observations or meteorological (re-)analyses due to the limited sample size of SSW events in the observational record (roughly 6 SSWs per decade). Here we exploit a large set of extended-range ensemble forecasts within the subseasonal-to-seasonal (S2S) framework to obtain an improved characterization of the modulation of AO extremes due to stratosphere-troposphere coupling. Specifically, we greatly boost the sample size of stratospheric events by using potential SSWs (p-SSWs), i.e., SSWs that are predicted to occur in individual forecast ensemble members regardless of whether they actually occurred in the real atmosphere. For example, for the ECMWF S2S ensemble this gives us a total of 6101 p-SSW events for the period 1997–2021. A standard lag-composite analysis around these p-SSWs validates our approach, i.e., the associated composite evolution of stratosphere-troposphere coupling matches the known evolution based on reanalyses data around real SSW events. Our statistical analyses further reveal that following p-SSWs, relative to climatology: 1) persistently negative AO states (> 1 week duration) are 16 % more likely, 2) the likelihood for extremely negative AO states (< −3σ) is enhanced by at least 35 %, while that for extremely positive AO states (> +3σ) is reduced to almost zero, 3) a p-SSW preceding an extremely negative AO state within 4 weeks is causal for this AO extreme (in a statistical sense) up to a degree of 27 %. A corresponding analysis relative to strong stratospheric vortex events reveals similar insights into the stratospheric modulation of positive AO extremes.

Field observational constraints on the controllers in glyoxal (CHOCHO) loss to aerosol

Glyoxal (CHOCHO), the simplest dicarbonyl in the troposphere, is an important precursor for secondary organic aerosol (SOA) and brown carbon (BrC) affecting air-quality and climate. The airborne measurement of CHOCHO concentrations during the KORUS-AQ (KORea-US Air Quality study) campaign in 2016 enables detailed quantification of loss mechanisms, pertaining to SOA formation in the real atmosphere. The production of this molecule was mainly from oxidation of aromatics (59 %) initiated by hydroxyl radical (OH), of which glyoxal forming mechanisms are relatively well constrained. CHOCHO loss to aerosol was found to be the most important removal path (69 %) and contributed to roughly ~20 % (3.7 μg sm−3 ppmv−1 hr−1, normalized with excess CO) of SOA growth in the first 6 hours in Seoul Metropolitan Area. To our knowledge, we show the first field observation of aerosol surface-area (Asurf)-dependent CHOCHO uptake, which diverges from the simple surface uptake assumption as Asurf increases in ambient condition. Specifically, under the low (high) aerosol loading, the CHOCHO effective uptake rate coefficient, keff,uptake, linearly increases (levels off) with Asurf, thus, the irreversible surface uptake is a reasonable (unreasonable) approximation for simulating CHOCHO loss to aerosol. Dependency of photochemical impact, as well as aerosol viscosity, are discussed as other possible factors influencing CHOCHO uptake rate. Our inferred Henry's law coefficient of CHOCHO, 7.0 × 108 M atm−1, is ~2 orders of magnitude higher than those estimated from salting-in effects constrained by inorganic salts only, which urges more understanding on CHOCHO solubility under real atmospheric conditions.

Idealized Simulations of Mei-yu Rainfall in Taiwan under Uniform Southwesterly Flow using A Could-Resolving Model

In this study, idealized cloud-resolving simulations are performed for horizontally uniform and steady southwesterly flow at fixed direction/speed combinations to investigate rainfall characteristics and the role of the complex topography in Taiwan during the Mei-yu season, without the influence of a front or other disturbances. Eight directions (180° to 285°, every 15°) and eight speeds (5 to 22.5 m s−1, every 2.5 m s−1) are considered, and near-surface relative humidity (RH) is also altered (from 55–100 %) in a subset of these tests to further investigate the effects of moisture content, yielding a total 109 experiments each having a integration length of 50 h. Three rainfall regimes that correspond to different ranges of the wet Froude number (Frw) are identified from the idealized simulations (with a grid size of 2 km). The low-Frw regime (Frw ≤ ~0.3) where the island circulation from thermodynamic effects during daytime is the main cause of rainfall in local afternoon. The lower the wind speed (and Frw), the more widespread and amount of rainfall. On the other hand, the high-Frw regime (Frw ≥ ~0.4) occurs when the flow at least 12.5 m s−1 impinges on Taiwan terrain at a large angle to favor the flow-over scenario. Thus, topographic rainfall production becomes dominant through mechanical uplift of unstable air. In this scenario, the faster and wetter the flow, the heavier the rainfall on the windward slopes, with the most favorable direction from 240°–255°. Between the two regimes above, a third and mixed regime also exists. The idealized results are discussed for their applicability to the real atmosphere.

Formation kinetics and mechanisms of ozone and secondary organic aerosols from photochemical oxidation of different aromatic hydrocarbons: dependence on NO_<i>x</i> and organic substituents

Aromatic hydrocarbons (AHs) contribute significantly to ozone and secondary organic aerosol (SOA) formation in the atmosphere, but their formation mechanisms are still unclear. Herein, the photochemical oxidation of nine AHs was investigated in a chamber. Only a small amount of ozone was produced from the direct photochemical oxidation of AHs, while a lower number of AH substituents resulted in higher concentrated ozone. Addition of NOx increased ozone and SOA production. The synergetic effect of accelerated NO2 conversion and NO reaction with AHs boosted ozone and volatile intermediate formation. Promoting AH concentration in the VOC / NOx ratio further increased formation rates and concentrations of both ozone and SOA. Additionally, ozone formation was enhanced with increasing AH substituent number but negligibly affected by their substituent position. Differently, SOA yield decreased with an increased substituent number of AHs but increased with ortho-methyl-group-substituted AHs. Model fitting and intermediates consistently confirmed that increasing the substituent number on the phenyl ring inhibited generation of dicarbonyl intermediates, which however were preferentially produced from oxidation of ortho-methyl-group-substituted AHs, resulting in different changing trends of the SOA yield. The restrained oligomerization by increased substituent number was another main cause for decreased SOA yield. These results are helpful to understand the photochemical transformation of AHs to secondary pollutants in the real atmosphere.

Tropospheric response to stratospheric momentum torques

&lt;p&gt;An idealised model is used to examine the tropospheric response to stratospheric momentum torques with an emphasis on the response to high-latitude sudden stratospheric warmings (SSWs). Previous related studies have generally imposed such torques in models that lack a key element of realism; for instance, models that do not have a realistic stratosphere, models without stationary planetary waves (i.e., without topography), and models that do not have a troposphere and so precludes any investigation of a downward impact. The idealised moist model of an atmosphere (MiMA) used here overcomes these three shortcomings and is hence well-suited to study the downward impact of extreme events in the stratosphere in a more realistic setup. In particular, we impose transient zonally-symmetric momentum forcing to various latitude bands in the stratosphere, spun-off from a free-running control run (CTRL). In addition to varying the latitudinal location of the forcing, we vary the depth, duration and magnitude to examine the sensitivity of the tropospheric response. Preliminary results show that in contrast to thermally-forced SSWs for which the initial 'Eliassen adjustment' (i.e., the meridional circulation response during the forcing period) is opposite to that found during free-running SSWs, the momentum-forced events here, produce a meridional circulation that mimics that found in the free-running events. This meridional circulation immediately transfers the imposed momentum forcing to the surface, projecting onto the tropospheric Northern Annular Mode (NAM) and initiating a synoptic-wave feedback, a process that takes much longer to develop in the thermally-forced SSWs. Hence, a sudden and strong enough wave forcing (approximated here by an imposed momentum torque) can induce a meridional circulation that penetrates deep into the troposphere and immediately initiate a tropospheric NAM response. The applicability of these experiments to the real atmosphere will be discussed via comparing the evolution of the forced events to free-running SSWs identified in CTRL.&lt;/p&gt;

Transient aggregation of convection: Observed behavior and underlying processes

Convective self-aggregation is among the most striking features emerging from radiative-convective equilibrium simulations, but its relevance to convective disturbances observed in the real atmosphere remains under debate. This work seeks the observational signals of convective aggregation intrinsic to the life cycle of cloud clusters. To this end, composite time series of the Simple Convective Aggregation Index (SCAI), a metric of aggregation, and other variables from satellite measurements are constructed around the temporal maxima of precipitation. All the parameters analyzed are large-scale means over 10°×10° domains. The composite evolution for heavy precipitation regimes shows that cloud clusters are gathered into fewer members during a period of ±12 h as precipitation picks up. The high-cloud cover per cluster expands as the number of clusters drops, suggesting a transient occurrence of convective aggregation. The sign of the transient aggregation is less evident or entirely absent in light precipitation regimes. An energy budget analysis is performed in search of the physical processes underlying the transient aggregation. The column moist static energy (MSE) accumulates before the precipitation peak and dissipates after, accounted for primarily by the horizontal MSE advection. The domain-averaged column radiative cooling is greater in a more aggregated composite than in a less aggregated one, although the role of radiative-convective feedback behind this remains unclear.