Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

Numerical study of convection observed during the Winter Monsoon Experiment using a mesoscale two-dimensional model [presentation]

Black carbon in soot is an efficient absorbing agent of solar irradiation that is preferentially emitted in the tropics and can form atmospheric brown clouds in mixture with other aerosols. These factors combine to make black carbon emissions the second most important contribution to anthropogenic climate warming, after carbon dioxide emissions.

Global and regional climate changes due to black carbon

There is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere. Hence, storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones, supplied with increased moisture, produce more intense precipitation events. Such events are observed to be widely occurring, even where total precipitation is decreasing: 'it never rains but it pours!' This increases the risk of flooding. The atmo- spheric and surface energy budget plays a critical role in the hydrological cycle, and also in the slower rate of change that occurs in total precipitation than total column water vapor. With modest changes in winds, patterns of precipitation do not change much, but result in dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in the mid- to high latitudes: the 'rich get richer and the poor get poorer'. This pattern is simulated by climate mod- els and is projected to continue into the future. Because, with warming, more precipitation occurs as rain instead of snow and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in summer, especially over continental areas. However, with more precipitation per unit of upward motion in the atmosphere, i.e. 'more bang for the buck', atmo- spheric circulation weakens, causing monsoons to falter. In the tropics and subtropics, precipitation patterns are dominated by shifts as sea surface temperatures change, with El Nino a good example. The volcanic eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and to widespread drought, as precipitation shifted from land to oceans and evaporation faltered, providing lessons for possible geoengineering. Most models simulate precipitation that occurs prematurely and too often, and with insufficient intensity, resulting in recycling that is too large and a lifetime of moisture in the atmosphere that is too short, which affects runoff and soil moisture.

/pdf/changes-in-precipitation-with-climate-change-3ofcyuiy76.pdf

Changes in precipitation with climate change

Climate change projections for the Mediterranean region

Anthropogenic aerosols are intricately linked to the climate system and to the hydrologic cycle. The net effect of aerosols is to cool the climate system by reflecting sunlight. Depending on their composition, aerosols can also absorb sunlight in the atmosphere, further cooling the surface but warming the atmosphere in the process. These effects of aerosols on the temperature profile, along with the role of aerosols as cloud condensation nuclei, impact the hydrologic cycle, through changes in cloud cover, cloud properties and precipitation. Unravelling these feedbacks is particularly difficult because aerosols take a multitude of shapes and forms, ranging from desert dust to urban pollution, and because aerosol concentrations vary strongly over time and space. To accurately study aerosol distribution and composition therefore requires continuous observations from satellites, networks of ground-based instruments and dedicated field experiments. Increases in aerosol concentration and changes in their composition, driven by industrialization and an expanding population, may adversely affect the Earth's climate and water supply.

A satellite view of aerosols in the climate system

Wave development, using Rayleigh's 1880 model of barotropic, inviscid and incompressible flow, with a mean zonal wind which is linearly sheared between two edges, is described in terms of the interaction between two counter-propagating Rossby waves (CRWs). Although the solutions described by this approach could also be described by a sum of the normal modes originally obtained by Rayleigh, we offer a CRW description of Rayleigh's model because it provides a useful pedagogical framework for illustrating, in a precise and quantifiable manner, the interacting Rossby wave view of instability. A CRW interpretation of a modified version of the Rayleigh model, consisting of a jet-like flow featuring two strips of vorticity with opposite signs, is also given.

/pdf/counter-propagating-rossby-waves-in-the-barotropic-rayleigh-38hw69nhpb.pdf

Counter-propagating rossby waves in the barotropic rayleigh model of shear instability

Numerical investigation of the nature of one of the most typical Eastern Mediterranean atmospheric circulation phenomena — the Red Sea Trough is undertaken. The role of interaction of typical atmospheric flow systems with the local topography of the North African region is analyzed with the help of idealized numerical simulations employing the Penn State and the National Center for Atmospheric Research (NCAR) MM4 modeling system. The simulations are designed, based on results of a climatological evaluation of the 250 hPa wind field. Idealized initial data sets corresponding to typical transient and winter period positions of the upper tropospheric westerly jet stream are constructed. The data for the analysis are from the National Center for Environmental Prediction (NCEP) 25 y (1965–1989) objective analysis archive. It was found that the primary factor in the Red Sea trough generation is the interaction of the mid-tropospheric westerlies with the terrain in the area of the Red Sea.

/pdf/interaction-of-topography-and-tropospheric-flow-a-possible-192rqr8erq.pdf

Interaction of Topography and Tropospheric Flow - A Possible Generator for the Red Sea Trough?

Aims Different aspects of soaring-bird migration are influenced by weather. However, the relationship between weather and the onset of soaring-bird migration, particularly in autumn, is not clear. Although long-term migration counts are often unavailable near the breeding areas of many soaring birds in the western Palaearctic, soaring-bird migration has been systematically monitored in Israel, a region where populations from large geographical areas converge. This study tests several fundamental hypotheses regarding the onset of migration and explores the connection between weather, migration onset and arrival at a distant site. Location Globally gridded meteorological data from the breeding areas in north-eastern Europe were used as predictive variables in relation to the arrival of soaring migrants in Israel. Methods Inverse modelling was used to study the temporal and spatial influence of weather on initiation of migration based on autumn soaring-bird migration counts in Israel. Numerous combinations of migration duration and temporal influence of meteorological variables (temperature, sea-level pressure and precipitable water) were tested with different models for meteorological sensitivity. Results The day of arrival in Israel of white storks, honey buzzards, Levant sparrowhawks and lesser spotted eagles was significantly and strongly related to meteorological conditions in the breeding area days or even weeks before arrival in Israel. The cumulative number of days or cumulative value above or below a meteorological threshold performed significantly better than other models tested. Models provided reliable estimates of migration duration for each species. Main conclusions The meteorological triggers of migration at the breeding grounds differed between species and were related to deteriorating living conditions and deteriorating migratory flight conditions. Soaring birds are sensitive to meteorological triggers at the same period every year and their temporal response to weather appears to be constrained by their annual routine.

/pdf/is-there-a-connection-between-weather-at-departure-sites-4ph2lem8eo.pdf

Is there a connection between weather at departure sites, onset of migration and timing of soaring‐bird autumn migration in Israel?

Investigation of the role of moist dynamics in the atmospheric processes during the abnormally intensive stormy period of November 1–5, 1994, over the Mediterranean region is presented. Analysis focuses on the eastern Mediterranean area employing 96-hour model predictions over the Mediterranean region with the Florida State University (FSU) global spectral model. Sensitivity simulations differ only by prescribing the initial moisture fields in the selected regions located in the Arabian Sea, Red Sea, and over large areas in the low and high latitudes. Both the tropospheric trajectories and the model tests suggest that the heavy rains over the eastern Mediterranean were associated with unusually intensive propagation of air masses from the Arabian Sea region, which took place before and during the first 24 hours of the period. Air masses from the Arabian Sea participated also in the intensive developments in Italy and southern France on November 5, 1994. According to the model simulations the convective processes over the equatorial Africa play a major role in the developments. Also, the intensification of the upper tropospheric westerly winds due to the convection over the equatorial Africa stimulates the northward extension of the Red Sea trough which consequently diminishes the moisture influx for the tropical convection and therefore seems to serve as a negative feedback.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD01710

Pinhas Alpert

Papers

Counter-propagating rossby waves in the barotropic rayleigh model of shear instability

Interaction of Topography and Tropospheric Flow - A Possible Generator for the Red Sea Trough?

Is there a connection between weather at departure sites, onset of migration and timing of soaring‐bird autumn migration in Israel?

Role of large scale moist dynamics in November 1-5, 1994, hazardous Mediterranean weather

A method to determine the effect of mineral dust aerosols on air quality