Yoav Yair

Lightning activity over land and sea on the eastern coast of the Mediterranean

This paper presents a study of the characteristics of lightning activity during the Cyprus low winter storms over the eastern coast of the Mediterranean. The focus is on changes in the nature of thunderstorms crossing the coastline from the sea into the northern and central parts of Israel, as manifested in their electrical activity. It is based on the Lightning Position and Tracking System (LPATS) measurements of lightning ground strikes during four winter seasons between 1995 and 1999. The spatial distribution shows a maximum of lightning ground strikes over Mount Carmel, possibly due to its topographical forcing. The annual variation shows a major maximum in January with two minor peaks, one in November and another in March, which can be explained by changes in the static instability of the atmosphere throughout the rainy period. The average fraction of positive ground flashes was found to be 6% and their average peak current 141 kA. The average peak current of negative ground flashes was 227 kA. Larger frequencies of ground flashes were detected over the sea than over land during the study period. This is probably due to the large heat and humidity fluxes from the sea surface, which destabilize the colder air above and drive cloud convection. The annual distribution shows that during midwinter (December‐January‐ February) there is higher flash density over the sea, while during autumn and spring the flash density is similar above the two regions. The diurnal variation shows that the maximum in maritime lightning activity was at 0500 LST and over land at 1300 LST. The mean peak current of positive ground flashes was higher over land and of negative ground flashes, over the sea.

Sprite observations from the space shuttle during the Mediterranean Israeli dust experiment (MEIDEX)

Abstract The Mediterranean Israeli dust experiment (MEIDEX) flew on-board the space shuttle in winter 2003, in a 39°-inclination orbit for 16 days, passing over the major thunderstorm regions on Earth. The primary science instrument of the MEIDEX payload is a Xybion IMC-201 image-intensified radiometric camera with six narrow band filters, boresighted with a wide-FOV color video camera. During the nightside of the orbit there will be dedicated observations toward the Earths limb above areas of active thunderstorms, in an effort to image transient luminous events (TLEs) from space. Optical observations from space will be conducted with the 665 nm filter that matches the observed wide peak centered at 670 nm that typifies red sprites, and also with the 380 and 470 nm filters for recording blue jets. Observations will consist of a continuous recording of the Earths limb, from the direction of the dusk terminator towards the nightside. Areas of high convective activity will be forecasted and uplinked to the crew before the observation. The astronaut will direct the camera toward areas with lightning activity, observed visually through the windows and on monitors in the crew cabin. Simultaneously with the optical observations from space, dedicated ground measurements will be conducted on a global scale. Two field sites in the Negev Desert in Israel will be used to collect electromagnetic data in the ELF and VLF frequency range. Additional ground stations in Germany, Hungary, USA, Antarctica, Chile, South Africa, Australia, Taiwan and Japan will also record Schumann resonance and VLF signals. The coordinated measurements from various locations on Earth and from space will enable us to triangulate the location and determine the polarity and charge moment of the parent lightning of the optically observed TLEs. The success of the campaign will further clarify the geographical distribution of Sprites, Elves and Jets.

Predicting Cloud-to-Ground and Intracloud Lightning in Weather Forecast Models

A new prognostic, spatially and temporally dependent variable is introduced to the Weather Research and Forecasting Model (WRF). This variable is called the potential electrical energy (Ep). It was used to predict the dynamic contribution of the grid-scale-resolved microphysical and vertical velocity fields to the production of cloud-to-ground and intracloud lightning in convection-allowing forecasts. The source of Ep is assumed to be the noninductive charge separation process involving collisions of graupel and ice particles in the presence of supercooled liquid water. The Ep dissipates when it exceeds preassigned threshold values and lightning is generated. An analysis of four case studies is presented and analyzed. On the 4-km simulation grid, a single cloud-to-ground lightning event was forecast with about equal values of probability of detection (POD) and false alarm ratio (FAR). However, when lighting was integrated onto 12-km and then 36-km grid overlays, there was a large improvement in the forecast skill, and as many as 10 cloud-to-ground lighting events were well forecast on the 36-km grid. The impact of initial conditions on forecast accuracy is briefly discussed, including an evaluation of the scheme in wintertime, when lightning activity is weaker. The dynamic algorithm forecasts are also contrasted with statistical lightning forecasts and differences are noted. The scheme is being used operationally with the Rapid Refresh (13 km) data; the skill scores in these operational runs were very good in clearly defined convective situations.

Lightning phenomenology in the Tel Aviv area from 1989 to 1996

We present the results of a continuing survey of lightning characteristics in Tel-Aviv, Israel, for the period 1989–1996, based on daily registrations of a CGR3 lightning flash counter [Mackerras, 1985]. The lightning season in Israel lasts from October to April, and the long-term average of the annual flash density in the Tel-Aviv area was found to be 4.7±2.3 km−2 y−1. The mean intracloud/cloud-to-ground flash ratio was found to be 2.5±1.3, with maxima in the autumn months. This may be attributed to the higher altitudes of the −10°C and −25°C isotherms (which signify the locations of charge centers) and to the weaker wind shears that occur in these months. The average fraction of positive ground flashes (PGF) in Tel-Aviv thunderstorms was F = 0.16±0.08. Storms that exhibited larger than average PGF fraction were found to be subjected to a strong shear of the horizontal wind. The observed empirical relation between the PGF fraction and the intensity of the wind shear W (in m s−1 km−1) was log F = 0.0305W + 0.073.

Transient luminous events in the vicinity of Taiwan

Abstract Sprites were observed over Asian continent and over oceans around Taiwan in the summer of 2001 (Geophys. Res. Lett. 29(4) 2002). In this article, we report some characteristic differences between the oceanic and the land sprites. Qualitatively, the oceanic sprites are tended to be brighter than the land sprites. Also some of the oceanic sprites have very peculiar forms, which do not match any of the existing types. In two of the recorded sprites, the diffuse hair region contains a distinct short streak. We suspect that they probably are tracks left behind by micrometeorites, which might also have helped in lowering the threshold of the electric field needed for sprite generation and produced exceptionally bright sprites. For the optical survey, one of the observation sites was situated in the campus of the National Cheng Kung University, which is located at the center of the Tainan metropolitan area with more than one million residents. However, on June 7, 2001, we were able to recorded 6 oceanic sprites from this highly light-polluted site. Our experience exemplifies the exceptional brightness of the oceanic sprites. The Asian TLEs recorded in this survey were observed to occur above thunderstorms at stationary fronts and over localized thunderstorms. The sole attempt to observe TLEs over typhoon Chebi failed, hence, whether they exist over tropical storms in the Asian region is yet to be confirmed.

Positive cloud‐to‐ground flashes and wind shear in Tel‐Aviv thunderstorms

The occurrence of negative and positive ground flashes in Tel-Aviv, Israel (32.05 N, 34.45 E) has been monitored through daily registrations of a CGR3-SN5 lightning flash counter (Mackerras, 1985). Measurements were conducted from 1987 to 1995, with the most continuous data sets available for the period 1992–1995. The lightning data was augmented by radar measurements of the thunderclouds (starting from 1994) and by radiosonde data. It was found that the fraction R of positive lightning from the total ground flash count is highly variable, with a long-term average of 0.25. It is shown that storms having a relatively large value of R coincide with strong vertical shear of the horizontal wind component. The dependence of R on the intensity of the wind shear S in the cloud layer between the 0°C and −25°C isotherms, may be expressed by: log R = aS - b, where a=0.1504 and b=1.1471.

A "Thinking Journey" to the Planets Using Scientific Visualization Technologies: Implications to Astronomy Education

We present a novel approach to teaching astronomy and planetary sciences, centered on visual images and simulations of planetary objects. The basic idea of the “Thinking Journey” concept is to take the students to other celestial objects as tourists, and to teach science through the observatio of various natural phenomena in these new environments. The power of scientific visualization, through still and dynamic images, makes such a journey an exciting learning experience. The introduction of new technologies (3D animations, virtual reality) greatly enhances the visualization capabilities the teacher can use, allowing him to simulate actual flights over the terrain of other planets and to study them as if observing from a spaceship in orbit. The present program focuses on the study of the Moon and of the planet Mars, by means of observation, interpretation, and comparison to planet Earth. Students learn to recognize geological and atmospheric processes, discuss astronomic phenomena, and discover that the same basic physical laws govern all objects in the solar system.

Water-cumulus in Jupiter's atmosphere : numerical experiments with an axisymmetric cloud model

Abstract The formation and evolution of convective H 2 O clouds in the deep Jovian troposphere are investigated using an axisymmetric model with parameterized microphysics to calculate the vertical and radial wind components and the liquid and ice contents in the clouds. A constant upwelling velocity from within the deep atmosphere was assumed, and the model was initiated with a slightly superadiabatic lapse rate below the lifting condensation level (LCL), which was located at 5160 mbar. The abundance of water in Jupiters atmosphere was assumed to be solar, and several relative humidity profiles were evaluated. The initial temperature profile above the LCL was assumed to be dry-adiabatic. The role of condensate mass-loading in the dynamic evolution of the cloud was considered. Results show that for plausible values of superadiabatic deviations and upwelling velocities, the Jovian water clouds are 25 km deep and extend up to the 3-bar pressure level. These cumulus clouds contain 2.5–5 g/m 3 of liquid water and ice, with updrafts at the axis reaching 30 m/sec and a cloud top temperature of 235 K. Clouds could retain their vertical structure for long periods of time. According to the model the water clouds reach the ammonia condensation level only when the temperature and relative humidity profiles are coupled in such a way that instability (or at least neutrality) to moist convection is ensured over a large depth of the troposphere. Under intermediate conditions, at which the upper layers were assumed to be stable, clouds were 40–50 km deep with updrafts of 60–70 m/sec, but did not reach the ammonia cloud deck. Thus, they were probably not connected to the observed equatorial “plumes”.

Model interpretation of Jovian lightning activity and the Galileo Probe results

An axisymmetric numerical cloud model with detailed microphysics is used to evaluate the development of water clouds under various Jovian conditions. A formulation of the noninductive ice-ice charge separation mechanism is used to calculate the electrical structure of the clouds and to identify the charged regions at the altitudes of the Galileo entry probe trajectory. The results show that for midlatitude conditions, with reasonable assumptions, the water clouds in the deep Jovian troposphere have an electrical dipole structure, with a large positive charge center located at the 3 bar pressure level and a lower negative charge center at the 4.5 bar level. This structure is modified as the cloud matures and the breakdown electric field is exceeded and lightning occurs. Our model predicts that the weak convection, which occurs at the stable equatorial region, is capable of producing water clouds with 10 m s -1 updrafts and water and ice content of 5 g kg -1 . Charge separation and lightning generation in these clouds is less intensive and lightning frequency is lower than at midlatitudes. For a low oxygen abundance of 0.2 times solar, we show that ice clouds form at temperatures where charge separation is weak, and no lightning can occur. The results suggest a very low probability that there was lightning activity near the path of the descending probe.

Winter Thunderstorms in Israel: A Study with Lightning Location Systems and Weather Radar

Abstract A study of the morphology and evolution of winter thunderstorms in Israel and over the eastern Mediterranean was conducted during the 1995–96 winter season. Electrically active cells were analyzed by combining data from weather radar and an operational lightning positioning and tracking system. This enabled the identification of reflectivity features of electrically active cells, and tracing of the spatial and temporal evolution of thunderstorms. The results show that, in winter, rain clouds became thunderclouds if their echo top was higher than 6500 m (at a temperature level colder than −30°C), provided that the reflectivity at the level of the −10°C isotherm was larger than 35 dBZ. The period between the first radar echo and the first detected lightning flash (probably a ground flash) was found to be 10–15 min, a period at which the top of the 40-dBZ echo was located higher than the −8°C level.