The floods in north India in 1988- Forecasting aspects

Severe floods/flash floods ravaged the States of Jammu& Kashmir, Haryana, Himachal Pradesh and Punjab during, (i) 2nd half of July and last half of August 1988. and (ii) 4th week of September 1988. They took place when heavy rains occurred in these States due to the interaction of mid-latitude westerly troughs with the monsoon pulses In the form of equatorial zones of maximum cloudiness (EZMC), located south of 20 N. Satellite pictures and synoptic charts suggest that: (a) The systems which formed in July-August 1988 were fed by moisture mainly from the Arabian Sea and had tropical characteristic, and (b) The systems which formed in SejJtember.198.8 had their moisture supply both from the Arabian Sea and the Bay of Bengal and were extra-tropical in character.. The paper describes two of these events in detail. The satellite picture3 show cloud formation resulting in heavy rains.

QG-DL-Ekman: Dynamics of a diabatic layer in the quasi-geostrophic framework

Quasi-geostrophic (QG) theory describes the dynamics of synoptic scale flows in the troposphere that are balanced with respect to both acoustic and internal gravity waves. Within this framework, effects of (turbulent) friction near the ground are usually represented by Ekman Layer theory. The troposphere covers roughly the lowest ten kilometers of the atmosphere while Ekman layer heights are typically just a few hundred meters. However, this two-layer asymptotic theory does not explicitly account for substantial changes of the potential temperature stratification due to diabatic heating associated with cloud formation or with radiative and turbulent heat fluxes which can be significant in about the lowest three kilometers and in the middle latitudes. To address this deficiency, this paper extends the classical QG–Ekman layer model by introducing an intermediate dynamically and thermodynamically active layer, called the “diabatic layer” (DL) from here on. The flow in this layer is also in acoustic, hydrostatic, and geostrophic balance but, in contrast to QG flow, variations of potential temperature are not restricted to small deviations from a stable and time independent background stratification. Instead, within the DL diabatic processes are allowed to affect the leading-order stratification. As a consequence, this layer modifies the pressure field at the top of the Ekman layer, and with it the intensity of Ekman pumping seen by the quasi-geostrophic bulk flow. The result is the proposed extended quasi-geostrophic three-layer QG-DL-Ekman model for mid-latitude dynamics.

Black Carbon - A Silent Contributor to Climate Change

Black carbon is a potent climate-warming component of particulate matter formed by the incomplete combustion of fossil-fuels, wood and other fuels. Complete combustion would turn all the carbon in the fuel into carbon dioxide, but combustion is never complete, and CO2, CO, volatile organic compounds, organic compounds, and black carbon particles are formed in the process. It contributes to warming by converting incoming solar radiation to heat. When deposited on ice and snow, BC and co-emitted particles reduce surface albedo thereby melting the glaciers. The complex mixture of particulate matter resulting from incomplete combustion is referred as soot. When suspended in the atmosphere, black carbon contributes to warming by converting incoming solar radiations to heat. It also influences cloud formation and impacts regional circulation and rainfall pattern. The Artic and the glaciated regions such as Himalayas are particularly vulnerable to melting as a result. The present paper aims to review the work done on black carbon and its mitigation measure.

Winter thermodynamic vertical structure in the Arctic atmosphere linked to large-scale circulation

Thermodynamic profiles are affected by both the large-scale dynamics and the local processes, such as radiation, cloud formation and turbulence. Based on ERA5 reanalysis, radiosoundings and cloud cover observations from winters 2009–2018, this study demonstrates manifold impacts of large-scale circulation on temperature and specific humidity profiles in the circumpolar Arctic north of 65∘ N. Characteristic wintertime circulation types are allocated using self-organizing maps (SOMs). The study shows that influence of different large-scale flows must be viewed as a progressing set of processes: (1) horizontal advection of heat and moisture, driven by circulation, lead to so-called first-order effects on thermodynamic profiles and turbulent surface fluxes, and (2) the advection is followed by transformation of the air through various physical processes, causing second-order effects. An example of second-order effects is the associated cloud formation, which shifts the strongest radiative cooling from the surface to the cloud top. The temperature and specific humidity profiles are most sensitive to large-scale circulation over the Eurasian land west of 90∘ E and the Arctic Ocean sea ice, whereas impacts over North America and Greenland are more ambiguous. Eurasian land, between 90 and 140∘ E, occasionally receives warm and moist air from the northern North Atlantic, which, with the support of radiative impacts of clouds, weakens the otherwise strong temperature and specific humidity inversions. Altitudes of maximum temperature and specific humidity in a profile and their variability between the circulation types are good indicators of the depth of the layer impacted by surface–atmosphere processes interacting with the large-scale circulation. Different circulation types typically cause variations of a few hundred metres to this altitude, and the layer impacted is deepest over north-eastern Eurasia and North America.

Mechanisms of hydrological responses to volcanic eruptions in the Asian monsoon and westerlies-dominated subregions

Explosive volcanic eruptions affect surface climate especially in monsoon regions, but responses vary in different regions and to volcanic aerosol injection (VAI) in different hemispheres. Here we use six ensemble members from last millennium experiment of the Coupled Model Intercomparison Project Phase 5, to investigate the mechanism of regional hydrological responses to different hemispheric VAI in the Asian monsoon region (AMR). It brings a significant drying effect over the AMR after northern hemisphere VAI (NHVAI), spatially, a distinct “wet get drier, dry gets wetter” response pattern emerges with significant drying effect in the wettest area (RWA) but significant wetting effect in the driest area (RDA) of the AMR. After southern hemisphere VAI (SHVAI), it shows a significant wetting effect over the AMR, but spatial response pattern is not that clear due to small aerosol magnitude. The mechanism of the hydrological impact relates to the indirect change of atmospheric circulation due to the direct radiative effect of volcanic aerosols. The decreased thermal contrast between the land and the ocean after NHVAI results in weakened EASM and SASM. This changes the moisture transport and cloud formation in the monsoon and westerlies-dominated subregions. The subsequent radiative effect and physical feedbacks of local clouds lead to different drying and wetting effects in different areas. Results here indicate that future volcanic eruptions may alleviate the uneven distribution of precipitation in the AMR, which should be considered in the near-term decadal prediction and future strategy of local adaptation to global warming. The local hydrological responses and mechanisms found here can also provide reference to stratospheric aerosol engineering.

Effects of Haze Radiation and Eddy Heat Transport on the Thermal Structure of Pluto’s Lower Atmosphere

The temperature profile of Pluto’s atmosphere has generally been assumed in a radiative–conductive equilibrium. Recent studies further highlighted the importance of radiative heating and cooling effects by haze particles. In this study, we update results from Zhang et al. by taking into account the icy haze composition proposed by Lavvas et al., and find that radiation of such an icy haze could still dominate the energy balance in the middle and upper atmosphere and explain the cold temperature observed by New Horizons. However, additional considerations are needed to explain the rapid decrease in temperature toward the icy surface at altitudes <25 km. We propose that vertical eddy heat transport might help maintain radiative–diffusive equilibrium in the lower atmosphere. In this scenario, our radiative–conductive–diffusive model (including both gas and haze) would match observations if the eddy diffusivity is on the order of 103 cm2 s−1. Alternatively, if eddy heat transport is not effective on Pluto, in order to match observations, haze albedo must increase rapidly with decreasing altitude and approach unity near the surface. This is a plausible result of additional ice condensation and/or cloud formation. In this scenario, haze radiation might still dominate over gas radiation and heat conduction to maintain radiative equilibrium. Better constraints on haze albedo at ultraviolet and visible wavelengths would be a key to distinguish these two scenarios. Future mid-infrared observations from the James Webb Space Telescope could also constrain the thermal emission and haze properties in Pluto’s lower atmosphere.

Dirty thunderstorms caused by volcano explosive eruptions in Kamchatka by the data of electromagnetic radiation

During volcano eruptions, so called dirty thunderstorms are the sources of electromagnetic radiation. They are caused by ash-gas clouds formed during explosive eruptions. Thunderstorm activity in an ash-gas cloud during volcano eruption is monitored by radio equipment. The VLF direction finder, located at Paratunka, monitors thunderstorm activity in the region of Kamchatka Peninsula including dirty thunderstorms accompanying explosive eruptions of Shiveluch and Bezymyanniy volcanoes. In the paper, we analyze records of electromagnetic radiation associated with dirty thunderstorms occurring during volcano eruptions from 2017 to 2020. During that period 24 eruptions of Shiveluch volcano and 5 eruptions of Bezymyanniy volcano occurred. Seventeen and three of them, respectively, caused dirty thunderstorms. Two-stage scenario of development is typical for all the dirty thunderstorms. The first stage lasts for 5–7 minutes and accompanies eruptive column development. However, if the eruption begins according to a smooth scenario, the first stage may be weak. The second stage lasts for 20–80 minutes and is associated with eruptive cloud formation and propagation. The intensity of this dirty thunderstorm stage depends on eruption power as well as on the interaction of an eruptive cloud during its propagation with the clouds of meteorological origin. Based on the obtained data, that is indicated by the increase of cloud-to-cloud stroke number.

Enhanced soot particle ice nucleation ability induced by aggregate compaction and densification

Soot particles, acting as ice nucleating particles (INPs), can contribute to cirrus cloud formation which has an important influence on climate. Aviation activities emitting soot particles in the upper troposphere can potentially impact ice nucleation (IN) in cirrus clouds. Pore condensation and freezing (PCF) is an important ice formation pathway for soot particles in the cirrus regime, which requires the soot INP to have specific morphological properties, i.e. mesopore structures. In this study, the morphology and pore size distribution of two kinds of soot samples were modified by a physical agitation method without any chemical modification, by which more compacted soot sample aggregates could be produced compared to the unmodified sample. The IN activities of both fresh and compacted soot particles with different sizes, 60, 100, 200 and 400 nm, were systematically tested by the Horizontal Ice Nucleation Chamber (HINC) under mixed-phase and cirrus clouds relevant temperatures (T). Our results show that soot particles are unable to form ice crystals at T > 235 K (homogeneous nucleation temperature, HNT) but IN was observed for compacted and larger size soot aggregates (> 200 nm) well below homogeneous freezing relative humidity (RHhom) at T < HNT, demonstrating PCF as the dominating mechanism for soot IN. We also observed that mechanically compacted soot particles can reach a higher particle activation fraction (AF) value for the same T and RH condition, compared to the same aggregate size fresh soot particles. The results also reveal a clear size dependence for the IN activity of soot particles with the same agitation degree, showing that compacted soot particles with large sizes (200 and 400 nm) are more active INPs and can convey the single importance of soot aggregate morphology for the IN ability. In order to understand the role of soot aggregate morphology for its IN activity, both fresh and compacted soot samples were characterized systematically using particle mass and size measurements, comparisons from TEM (transmission electron microscopy) images, soot porosity characteristics from argon (Ar) and nitrogen (N2) physisorption measurements, as well as soot-water interaction results from DVS (dynamic vapor sorption) measurements. Considering the soot particle physical properties along with its IN activities, the enhanced IN abilities of compacted soot particles are attributed to decreasing mesopore width and increasing mesopore occurrence probability due to the compaction process.

Characterizing the hygroscopicty of growing particles in the Canadian Arctic summer

The impact of aerosols on clouds is a well-studied, although still poorly constrained, part of the atmospheric system. New particle formation (NPF) is thought to contribute 40–80 % of the global cloud droplet number concentration, although it is extremely difficult to observe an air mass from NPF to cloud formation. NPF and growth occurs frequently in the Canadian Arctic summer atmosphere, although only a few studies have characterized the source and properties of these aerosols. This study presents cloud condensation nuclei (CCN) concentrations measured on board the CCGS Amundsen in the eastern Canadian Arctic Archipelago from 23 July to 23 August 2016 as part of the Network on Climate and Aerosols: Addressing Uncertainties in Remote Canadian Environments (NETCARE). The study was dominated by frequent ultrafine particle and/or growth events, and particles smaller than 100 nm dominated the size distribution for 92 % of the study period. Using κ-Kohler theory and aerosol size distributions, the mean hygroscopicity parameter (κ) calculated for the entire study was 0.12 (0.06–0.12, 25th–75th percentile), suggesting that the condensable vapours that led to particle growth were primarily non-hygroscopic, which we infer to be organic. Based on past measurement and modelling studies from NETCARE and the Canadian Arctic, it seems likely that the source of these non-hygroscopic, organic, vapours is the ocean. Examining specific growth events suggests that the mode diameter (Dmax) had to exceed 40 nm before CCN concentrations at 0.99 % SS started to increase, although a statistical analysis shows that CCN concentrations increased 13–274 cm−3 during all ultrafine particle and/or growth times (total particle concentrations > 500 cm−3, Dmax < 100 nm) compared to Background times (total concentrations < 500 cm−3) at SS of 0.26–0.99 %. This value increased to 25–425 cm−3 if the growth times were limited to times when Dmax was also larger than 40 nm. These results support past results from NETCARE by showing that the frequently observed ultrafine particle and growth events are dominated by a highly non-hygroscopic fraction, which we interpret to be organic vapours originating from the ocean, and that these growing particles can increase the background CCN concentrations at SS as low as 0.26 %, thus pointing to their potential contribution to cloud properties and thus climate through the radiation balance.