Strong Convective Activity Research Articles

Climate sensitivity and cloud feedback processes imposed by two different external forcings in an aquaplanet GCM

The sensitivity of climate to an increase in sea surface temperature (SST) and CO2, as well as cloud feed- back processes, is analyzed through a series of aquaplanet experiments listed in the Coupled Model Intercomparison Project. Rainfall is strengthened in a +4K anomaly SST experiment due to the enhanced surface evaporation; while in a quadruple CO2 experiment, precipitation and total cloud cover are appreciably weakened. In both the +4K and qua- druple CO2 (4xCO2) experiments, the Hadley cell is im- paired, with an increase in moderate subsidence and a decrease in the frequency of strong convective activity. Regarding cloud radiation forcing (CRF), the analysis tech- nique of Bony et al. (Climate Dynamics, 22:71-86, 2004 )i s used to sort cloud variables by dynamic regimes using the 500-hPa vertical velocity in tropical areas (30°S-30°N). Results show that the tropically averaged CRF change is negative and is dominated mainly by the thermodynamic component. Within convective regimes, the behavior of longwave CRF is different in the +4K and 4xCO2 experi- ments, with positive and negative changes, respectively. The globally averaged CRF also reveals a negative change in both aquaplanet and Earthlike experiments, implying that clouds may play a role in decelerating global warming. The calculated climate sensitivity demonstrates that our results are close to those obtained from other models, with 0.384 and 0.584 Km 2 W ! 1 for aquaplanet and Earthlike experi- ments, respectively.

Climatological aspects of seasonal variation of aerosol vertical distribution over central Indo-Gangetic belt (IGB) inferred by the space-borne lidar CALIOP

Abstract Climatological information on the vertical distribution of aerosol in the lower atmosphere is needed to assess the effects of aerosols on climate. Altitude profiles of the backscatter coefficient (at 532 nm), color ratio (CR) and particle depolarization ratio (PDR) derived from space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were used to investigate the climatological aspects of seasonal variation of aerosol vertical distribution over central Indo-Gangetic belt (IGB). Strong convective activity coupled with capping inversion lead to vertical extension (4–5 km) of aerosol distribution during the pre-monsoon (PrM) season. Trapping of pollution at low altitudes (below 1.5 km) by subsidence have been found during the late post-monsoon (PoM) and winter seasons. Combined results of CR and PDR suggest presence of elevated mineral dust particles during the PrM season, whereas fine mode and spherical (biomass burning/industrial pollution) particles are dominant during the PoM and winter seasons. The observations indicate a seasonal dependency of the aerosol vertical distribution over central IGB.

Relationship between convection over Central America and the intensity of the jet stream bearing on the 1999 December European storms

AbstractDuring 26–28 December 1999, two very powerful and devastating extratropical cyclones hit Western Europe. These two storms were associated with an upper‐level zonal jet, remarkable in its intensity and its large extension over the Atlantic Ocean. In this study, we focus on the relationship between the exceptional features of this jet stream and some heavy precipitation that occurred over Central America during mid‐December. A Rossby‐wave train excited by strong convective activity over this area is likely to play a key role in this teleconnection. To assess this tropical–extratropical interaction, several numerical experiments have been performed with the French global model ARPEGE. Firstly, the effects of convection have been neutralized within a domain localized over Central America. Then, to combine the model with observations, a new strategy has been devised. Within a selected window, the parametrized latent heat release has been deduced from Tropical Rainfall Measuring Mission (TRMM) precipitation data, translated in terms of a potential vorticity (hereafter PV) source and incorporated into the model using PV inversion. These simulations show, in particular, that the convective activity over Central America triggers a wave train that moves poleward and eastward with the group velocity, finally to strengthen the North Atlantic jet stream, leading to windstorms Lothar and Martin. Copyright © 2011 Royal Meteorological Society

Dependence of Model-Simulated Heavy Rainfall on the Horizontal Resolution during the Jakarta Flood Event in January-February 2007

Time-lagged ensemble downscaling experiments with Japan Meteorological Agency-Nonhydrostatic model are performed to study the dependence of heavy precipitation simulated by the model on the horizontal resolution for five days during late January to early February 2007, when the Jakarta Flood event occurred. The model runs with horizontal resolutions of 2, 4, and 5 km downscaled from the model runs with a 20-km resolution demonstrate the ability to reproduce a region of strong convective activity to the north of Java Island during the event. Daily meridional propagation of enhanced precipitation signals is simulated in the model runs with 2- and 4-km resolutions.Cumulative distribution functions of precipitation rate in the model are analyzed for four different regions: ocean, northern coast, mountain, and southern coast. The northern coast region shows the highest contribution of heavy precipitation compared to other regions for all the experiments as well as for satellite-based precipitation estimates. The statistics on the frequency of heavy precipitation show that the diurnal variation of heavy precipitation produced by the model with a 2-km resolution agrees well with that of satellite-based precipitation estimates.

Temporal distribution of weather catastrophes in the USA

The temporal distributions of the nation’s four major storm types during 1950–2005 were assessed, including those for thunderstorms, hurricanes, tornadoes, and winter storms. Storms are labeled as catastrophes, defined as events causing $1 million or more in property losses, based on time-adjusted data provided by the insurance industry. Most catastrophic storms occurred in the eastern half of the nation. Analysis of the regional and national storm frequencies revealed there was little time-related relationship between storm types, reflecting how storm types were reported. That is, when tornadoes occurred with thunderstorms, the type producing the greatest losses was the one identified by the insurance industry, not both. Temporal agreement was found in the timing of relatively high incidences of thunderstorms, hurricanes, and winter storms during 2002–2005. This resulted in upward time trends in the national losses of hurricane and thunderstorm catastrophes, The temporal increase in hurricanes is in agreement with upward trends in population density, wealth, and insurance coverage in Gulf and East coastal areas. The upward trends in thunderstorm catastrophes and losses result from increases in heavy rain days, floods, high winds, and hail days, revealing that atmospheric conditions conducive to strong convective activity have been increasing since the 1960s. Tornado catastrophes and their losses peaked in 1966–1973 and had no upward time trend. Temporal variability in tornado catastrophes was large, whereas the variability in hurricane and thunderstorm catastrophes was only moderate, and that for winter storms was low.

Cloudiness characteristics over Southeast Asia from satellite FY‐2C and their comparison to three other cloud data sets

Fengyun‐2C (FY‐2C), launched in October 2004, is the first operational geostationary meteorological satellite in China. It can provide 1‐h interval cloudiness products with a spatial resolution of 0.04° latitude × 0.04° longitude. The main characteristics of the regional‐scale clouds from a 2‐year FY‐2C data set (from July 2005 to June 2007) are presented, including the spatial distribution and the annual and diurnal cycles of cloudiness. The reliability of FY‐2C cloud products over Southeast Asia is investigated through comparisons to cloud cover from the International Satellite Cloud Climatology Project, the Moderate Resolution Imaging Spectroradiometer on board Terra and Aqua satellites, and conventional ground observations. It is shown that the FY‐2C cloud mask performs consistently with other cloud mask products over Southeast Asia. In the boreal winter, the whole domain is dry with little cloudiness. More extensive cloudiness can be observed over the Sichuan Basin, in the East China Sea and the South China Sea, along the northwestern border of China, and around the ITCZ in the Southern Hemisphere. In the boreal summer, the summer monsoon is the dominant system for the studied domain, which is generally humid with extensive cloudiness, corresponding to zones of strong convective activities. Results also reveal considerable discrepancies among different cloud products over extended areas of north China and Mongolia. The Sichuan Basin is another region of large discrepancies among the four cloud products. Diurnal cycles of FY‐2C cloudiness for the four seasons of a year are analyzed. The diurnal range of cloudiness is generally larger over land than over ocean. Remarkable diurnal variation is found over the Tibetan Plateau, the northern part of the Indian Peninsula, and central Asia where there is generally less precipitation. The peaks of diurnal cycle of cloudiness appear around local noon over the subtropical land, in the morning over the Indian Peninsula, and in the afternoon near the equator.

Numerical forecast and analysis of a tropical-like cyclone in the Ionian Sea

Abstract. A forecasting system has been implemented for operational weather prediction over southern Italy in the framework of the RISKMED (INTERREG IIIB) project, which aims at weather-risk reduction in the central and eastern Mediterranean area. The system comprises two different regional forecasting chains, one based on BOLAM and MOLOCH models, the second based on the Weather Research and Forecasting (WRF) model. On 26 September 2006, a subsynoptic-scale vortex developed over the Mediterranean Sea and affected south-eastern Italy. The cyclone was documented by radar reflectivity maps, Meteosat Second Generation satellite images and surface stations data. The observational analysis showed that the cyclone possessed the typical characteristics of a Mediterranean tropical-like cyclone. All the limited-area models employed in RISKMED were able to properly forecast the formation of the small-scale vortex, although with differences in intensity, trajectory and evolution. The predictability of the event has been evaluated employing a multi-model, multi-analysis approach. The simulation results show strong sensitivity to the specification of the initial and boundary conditions. Sensitivity experiments were performed in order to analyse the mechanisms responsible for the development and the maintenance of the cyclone over the sea. The life cycle of the vortex turned out to be characterized by different phases: the orographic cyclogenesis on the lee side of the Atlas Mountains; the initial phase of development, characterised by the critical role played by the surface heat fluxes; the transition to a tropical-like cyclone mainly as a consequence of the latent-heat release associated with strong convective activity over the Ionian Sea; finally, the maintenance of the vortex strength due to both the surface fluxes and the release of latent heat.

A possible mechanism for the co‐variability of the boreal spring Antarctic Oscillation and the Yangtze River valley summer rainfall

AbstractA significant correlation between the boreal spring Antarctic Oscillation (AAO) and the Yangtze River valley summer rainfall (YRVSR) has been found in previous studies, although the mechanisms that might lead to such far‐reaching teleconnection remain unresolved. In this study, one of possible mechanisms responsible for the co‐variability of the boreal spring AAO and the YRVSR is proposed. It follows that the convection activity over the region of the Maritime Continent serves as a bridge linking the boreal spring AAO and the YRVSR. This physical process can be described schematically as follows: during the boreal spring, a positive‐phase (negative‐phase) AAO is concurrent with a strong (weak) convection activity over the region of the Maritime Continent via anomalous meridional circulations along the central South Pacific and two meridional teleconnection wave train patterns, with one over the southern Indian Ocean at the lower level and the other along the central South Pacific at the upper level. Thereafter, the anomalous convection propagates northward along the seasonal cycle, and then changes the western Pacific subtropical high in the following seasons, consequently impacting on the summer rainfall in the Yangtze River valley. Copyright © 2008 Royal Meteorological Society

Trends in the temperature and water vapor content of the tropical lower stratosphere: Sea surface connection

The tropical lower stratosphere is an important region of the atmosphere, where strong convective activity in the underlying troposphere affects both its chemical and dynamical properties. Temperatures near the tropopause influence the input of water vapor from the troposphere and act as an indicator of the dynamical properties of the region. This paper addresses long‐term trends in the temperature of the tropical lower stratosphere. Correlations with recent changes in tropical stratospheric water vapor are also noted. Special attention is given to the convectively active tropical western Pacific Ocean, where sea surface temperatures (SSTs) are among the highest in the world. The region contains several island radiosonde stations with records reliably extending over several decades. Results show only weak cooling trends occurred before the 1990s, but a strong and rapid cooling of 4° to 6°C took place in the mid‐1990s and has persisted since that time. The properties of the temperature records during and following this cooling event are discussed, and a significant anticorrelation with SST anomalies in the underlying ocean is noted. The rate of ocean warming increased in the early 1990s, coinciding approximately with the mid‐decade cooling event, while individual monthly anomalies in both time series are also anticorrelated. Past work has shown that cooling of the tropical lower stratosphere is a dynamical result of tropospheric convection, which in turn partially depends upon sea surface temperatures. Convection may therefore be the link between the ocean and the stratosphere, and the increased cooling may be an indication of strengthening tropical convection.

The Interaction between Two Separate Propagations of Rossby Waves

Abstract This study deals with two teleconnection patterns and the subsequent wave train propagations during an East Asian summer. Diagnostic results are as follows: 1) a stationary wave ray with zonal wavenumber 5 approximates the arc path linking the correlation centers originating from the Caspian Sea via Lake Baikal to the sea off the southeast coast of Japan (i.e., the OKJ arc path as a focus area) in a pentad correlation map between 500-hPa geopotential height (Z500) and outgoing longwave radiation (OLR) at 30°N, 150°E in June 1979–98. Ray tracing shows that it took 8–10 days for this stationary wave to propagate from an initial position around the Caspian Sea to the focus area, which roughly coincides with the observed case in July 1998. 2) A wave train pattern (P-Ja) observed in the boreal summer propagated along the arc line in the same way as the normal poleward Rossby wave train originating from the Philippines across the North Pacific (P-J), but with a phase shift northeastward of about 90°. 3) Further correlation analyses showed that the P-J-like waves belong mainly to intraseasonal propagating ones while OKJ waves belong mainly to intraseasonal stationary ones. 4) Propagation of the newly observed wave train pattern (P-Ja) occurred following another wave train along the OKJ arc path in mid-July 1998. Both northeastward and southeastward wave propagations merged off the east coast of Japan. 5) The northeastward-propagating wave train observed in mid-July 1998 was triggered by the southeastward-propagating (OKJ) wave train that produced a deep cyclonic circulation and a strong convective activity in the focus area. The link of wave forcing and deep convection was made solely because of a strong upper-level divergence in the focus area.

Global satellite measurements of HDO and implications for understanding the transport of water vapour into the stratosphere

AbstractThe deuterium content of water vapour in a given air mass is sensitive to its temperature and condensation history. Isotopic measurements therefore have the potential to shed light on the transport of air and water vapour into the stratosphere. Previous measurements of the isotopic composition in the upper troposphere and stratosphere have been sparse in terms of both spatial and temporal coverage. Presented here are retrievals of the deuterium content of water vapour (HDO or δD) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite instrument. These retrievals offer the first global scale coverage of the isotopic composition of water vapour in this altitude region and span a time period of almost two years. The spatial coverage and the time span of the dataset offer previously unattainable insight into the mean seasonal and spatial distributions of the isotopic composition of water vapour in the upper troposphere and stratosphere. Measurements of HDO are extremely challenging due to low sensitivity in the spectra at low temperatures and water vapour amounts. Nonetheless, the data show a number of interesting results. Zonal mean profiles show the greatest depletion in δD in the tropical upper troposphere, with decreasing depletion with altitude in the stratosphere due to the influence of methane oxidation. Seasonal zonal means also show a strong depletion in the southern polar spring at around 300 mbar, which is thought to be related to the occurrence of polar stratospheric clouds and dehydration events in the polar vortex. Geographically, the regions and time periods where the greatest depletions are observed in the tropical upper troposphere are those associated with strong convective activity. Results confirm that temporal variability is central to the transport of water vapour into the tropical tropopause layer (TTL) and stratosphere. The data presented here show an annual cycle in δD in the TTL which is tied to temperature and suggests that this signature propagates upwards into the stratosphere. The data also show a number of points where extremely dry air is associated with relatively enriched δD values. We postulate that these points are evidence of ice lofting. Copyright © 2007 Royal Meteorological Society

Modelling suppressed and active convection. Comparing a numerical weather prediction, cloud‐resolving and single‐column model

AbstractThis paper describes the design of and basic results from a case study to compare simulations of convection over the Tropical West Pacific. Simulations are carried out using a cloud‐resolving model (CRM), a global numerical weather prediction (NWP) model and a single‐column version of the NWP model (SCM). The experimental design for each model type is discussed and then results are compared. The periods simulated each include a regime with strong convective activity, a much more suppressed regime with far less convection, as well as the transition between these regimes. The description of the design and basic results from this study are given in some detail, as a study including all these model types is relatively new. Comparing the local forcing due to the dynamics in the NWP model with the observed forcing used to drive the CRM and SCM it is found that there is good agreement for one period chosen but significant differences for another. This is also seen in fields such as rain rate and top‐of‐atmosphere radiation. Using the period with good agreement we are able to identify examples of biases in the NWP model that are also reproduced in the SCM. Also discussed are examples of biases in the NWP simulation that are not reproduced in the SCM. It is suggested that understanding which biases in the SCM are consistent with the full NWP model can help focus the use of an SCM in this framework. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd

Impact of the thermal state of the tropical western Pacific on onset date and process of the South China Sea summer monsoon

Since the early or late onset of the South China Sea summer monsoon (SCSM) has a large impact on summer monsoon rainfall in East Asia, the mechanism and process of early or late onset of the SCSM are an worthy issue to study. In this paper, the results analyzed by using the observed data show that the onset date and process of the SCSM are closely associated with the thermal state of the tropical western Pacific in spring. When the tropical western Pacific is in a warming state in spring, the western Pacific subtropical high shifts eastward, and twin cyclones are early caused over the Bay of Bengal and Sumatra before the SCSM onset. In this case, the cyclonic circulation located over the Bay of Bengal can be early intensified and become into a strong trough. Thus, the westerly flow and convective activity can be intensified over Sumatra, the Indo-China Peninsula and the South China Sea (SCS) in mid-May. This leads to early onset of the SCSM. In contrast, when the tropical western Pacific is in a cooling state, the western Pacific subtropical high anomalously shifts westward, the twin cyclones located over the equatorial eastern Indian Ocean and Sumatra are weakened, and the twin anomaly anticyclones appear over these regions from late April to mid-May. Thus, the westerly flow and convective activity cannot be early intensified over the Indo-China Peninsula and the SCS. Only when the western Pacific subtropical high moves eastward, the weak trough located over the Bay of Bengal can be intensified and become into a strong trough, the strong southwesterly wind and convective activity can be intensified over the Indo-China Peninsula and the SCS in late May. Thus, this leads to late onset of the SCSM. Moreover, in this paper, the influencing mechanism of the thermal state of the tropical western Pacific on the SCSM onset is discussed further from the Walker circulation anomalies in the different thermal states of the tropical western Pacific.

Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season

The level of neutral buoyancy (LNB) around the Japan Islands during the Baiu season in 2001-2005, estimated by lifting a low-level air with the maximum equivalent potential temperature in a vertical direction, is statistically examined from objective analysis data with the horizontal resolution of 20 km, in order to clarify the surrounding atmospheric conditions under which cumulonimbi can form and develop. Three and two peaks are found in the vertical profiles of the appearance rate of the LNB over the sea and on the land, respectively. The high-level (∼ 150 hPa) peak is very weak in June, and the low-level (∼ 900 hPa) peak does not appear on the land. The other peak is found at the middle level (N 700 hPa).The theoretical study shows that the LNB during the Baiu season strongly depends on the lapse rate between the low and middle levels, when the variation of the low-level temperature is within a few degrees. This is because the LNB during the Baiu season rapidly becomes higher as the lapse rate increases. The statistical results show that the lapse rate around the Japan Islands strongly depends on convective activities over upstream regions, especially the southern part of China. Therefore, strong convective activities over the Baiu frontal zone in June produce the middle-level LNB around the Japan Islands to suppress the development of cumulonimbi. In July, the northward shift of the area with strong convective activities considerably releases such suppression, associated with that of the Baiu frontal zone.The middle-level LNB also frequently appears over the other areas around the Baiu frontal zone, although it scarcely appears over the western part of the North Pacific Ocean. Therefore, the middle-level LNB that suggests the existence of cumulonimbi with a cloud top lower than several kilometers is one of the characteristic features around the Baiu frontal zone.

Caracterización espacio temporal del campo de vientos superficiales del Pacífico colombiano y el Golfo de Panamá a partir de sensores remotos y datos in situ

The Colombian Pacific Basin is characterized by the influence of the North and South Trade Winds, which generate the denominated Intertropical Convergence Zone. This presented dynamics in the wind vectors on the region, is associated with a strong convective activity, explaining the high pluviosity indexes over that region. Also, it is considered of great importance to identify the behaviour of the surface winds in the Gulf of Panama, keeping in mind that in this region atmospheric processes of important magnitude are made, as it is it the Panama wind jet. The present work quantifies the horizontal winds field of the Colombian Pacific marine and coastal zone by means of in situ and satellite data, with a 25 Km of spatial scale. In addition, it is obtained the annual, semiannual and multianual behavior of the zonal and southern superficial horizontal wind over the studied areas. It is developed a correlation analysis between the in situ and satellite data in march 2006. The established climatology indicates that Colombian Pacific Basin (CPB) and the Gulf of Panama show a semiannual seasonal behavior. During the first semester the North Trade winds predominate, with average intensities between 5 and 7m.s-1, during this period the Panama wind jet is detected on the CPB, regulating the CPB oceanographic conditions. In the second semester (July-December), the ITCZ and the Panama Wind jet remain absented on the CPB, being presented prevalence of South-west Trade winds of the Choco wind jet on the study areas. Finally it is present the relation between the annual and semiannual magnitudes of the CPB and the Choco wind jet.

Variation of tropical cyclone activity in the South Indian Ocean: El Niño–Southern Oscillation and Madden‐Julian Oscillation effects

The present study examines variation of tropical cyclone (TC) activity in the South Indian Ocean (SIO) during TC seasons (December–March) for the period 1979–2004. The impact of the El Niño–Southern Oscillation and the Madden‐Julian Oscillation (MJO) on the variation is revealed through a composite analysis. During El Niño periods TC genesis was shifted westward, enhancing the formation west of 75°E and reducing it east of 75°E. These changes in the genesis correspond to a westward shift of convection. It may be explained by a remote effect on the SIO; that is, the increase in sea surface temperature in the central eastern Pacific alters the Walker circulation and forms an anomalous anticyclonic circulation in the east SIO during El Niño. The spatial difference in TC passages between El Niño and La Niña shows a significant decrease to the southeast of Madagascar but a moderate increase in the central midlatitude SIO, indicating that TCs move farther east during El Niño. This change is possibly due to the anomalous southwesterlies east of Madagascar. Variation of TC activity also depends on various MJO phases: frequent TC passages for phases 2–4 (strong convective activity straddles along the equatorial Indian Ocean) versus infrequent TC passages for other phases. TC tracks tend to be more south oriented in phase 3 compared with those in phases 2 and 4. This is possibly caused by the increased steering northerlies which are a part of the anticyclonic Rossby wave of Gill type in response to the suppressed MJO‐related convection in the maritime continent.

Study of 11 September 2004 hailstorm event using radar identification of 2-D systems and 3-D cells

Abstract. The most important hail event recorded in the region of the Ebro Valley (NE Spain) in 2004 was the 11 September episode. Large hailstones (some of them with a diameter of over 30 mm) caused important damages in agriculture and properties. The hail event affected an area of 3848 ha and was caused by several multicellular systems. The aim of this paper is the analysis of the associated convective structures using the meteorological radar as well as the MM5 mesoscale model, thermodynamic data and a hailpad network. To achieve this end, the new hailstorm analysis tool RHAP (Rainfall events and Hailstorms Analysis Program) has been applied. It identifies tracks and characterises precipitation systems and convective cells, taking into account 2-D and 3-D structures. The event has also been studied with the TITAN software (Thunderstorm Identification, Tracking, Analysis and Nowcasting) in an attempt to compare both methods. Results show that the episode had a strong convective activity with CAPE values over 4000 J/kg and with hail-forming cells characterised by VIL (Vertical Integrated Liguid) exceeding 40 kg/m2, VILD (VIL density) over 4 g/m3, HP (Hail Probability) of 100% and SHP (Severe Hail Probability) of over 75%. The hail cells evolved into multicellular systems that lasted between 70 and 90 min. Finally, the comparison of RHAP and TITAN has shown significant correlations between methods.

Mesoscale analysis and impact on simulation of IOP14 of the MAP experiment

AbstractThis paper examines the benefit on quantitative precipitation forecasts of assimilating a mesoscale network of surface observations. The analysis is performed with an optimal interpolation system tuned for meso‐β scale (10 km resolution). The case‐study is chosen in the framework of the Mesoscale Alpine Program (MAP). It corresponds to a frontal system crossing France on 2 November 1999 followed by moderate to strong convective activity which developed over the Alps and Italy on 3 November (MAP Intensive Observation Period (IOP) 14).Model sensitivity to the various initial conditions (European Centre for Medium‐Range Weather Forecasts analysis, mesoscale analyses with dense surface observations) and to the resolution (meso‐β versus meso‐γ (2.5 km scale)) is investigated. A comprehensive network of rain‐gauges is used for validation. Conventional objective scores (correlation and equitable threat) are produced for precipitation accumulated over short durations (6 and 1 h) for consistency with the target horizontal scales (meso‐β, meso‐γ). The positive contribution of the surface network to precipitation forecasts is confirmed by a novel and powerful validation method based on the comparison of METEOSAT synthetic radiances (infrared and water vapour channels) computed from the simulations to the corresponding satellite observations. Copyright © 2005 Royal Meteorological Society

Experimental study on Marangoni effect induced by heat and mass transfer

Surface tension gradients along a fluid–fluid interface provoke strong convective activity, such effect is called Marangoni effect. The Marangoni effect might be induced by heat and mass transfer. In both cases, instabilities develop at the interface. The importance of Marangoni effect has led to many investigations over several years [1–6] [L. Scriven and C. Sternling, Nature, 187, pp. 186–188 (1960), H. Lu, Y. Yang and J. Maa, Ind. Eng. Chem. Res., 35, pp. 1921–1928 (1996), V. Kaminsky, A. Vyaz'min, N. Kulov and V. Dil'man, Chem. Eng. Sci., 53, No. 19 (1998), P. Lyford, H. Pratt, F. Greiser and D. Shallcross, Can. J. Chem. Eng., 76 (1998a), P. Lyford, H. Pratt, F. Greiser and D. Shallcross, Can. J. Chem. Eng., 76 (1998b), H. Lu, Y. Yang and J. Maa, Ind. Eng. Chem. Res., 36, pp. 474–482 (1997)]. The aim of this work is to study experimentally the Marangoni effect induced by heat and mass transfer, and to determine the Marangoni number ( Ma) in both cases. Four aliphatic alcohol were studied, from C1 to C4. In the experiments, interfacial instabilities were observed and Ma was calculated for different temperatures and pressures. It was observed that Ma decreases as the carbon number atoms ( n) increase, this behavior can be explained through the changes of some properties (surface tension, diffusivity) as heat and mass transfer occurred. In heat transfer experiments, the highest temperature gradients were reached for methanol, the Bond number ( Bd) was also calculated and it was found that the natural convection predominated. In mass transfer experiments, CO 2 was used as gas phase, and it was observed that as pressure increases the Ma increases, it might be explained by the decrease of CO 2 diffusion in hydrocarbons as pressure increases. In conclusion, the Marangoni effect was observed for aliphatic alcohol, the influence of temperature and pressure was also observed, and finally, the Marangoni number decreases as n increases.

The Interaction of the Madden–Julian Oscillation and the Arctic Oscillation

Abstract Tropical and extratropical interactions on the intraseasonal time scale are studied in the context of the Arctic Oscillation (AO) and the Madden–Julian oscillation (MJO). To simplify the discussion, a high (low) MJO phase is defined as strong (suppressed) convective activity over the Indian Ocean. In the Northern Hemisphere (NH) winter season, a high (low) AO phase is found more likely coupled with a high (low) MJO phase. Based on the regressed patterns and composites of various dynamical fields and quantities, possible mechanisms linking the AO and the MJO are examined. The analysis indicates that the MJO influence on extratropical circulations seems more evident than the AO influence on tropical circulations. The MJO interacts with the AO through meridional dispersion of Rossby waves in the Pacific sector. The geopotential height anomaly center over the North Pacific associated with the MJO can either reinforce or offset the AO Pacific action center. As a result, the AO pattern can be greatly affected by the MJO. When the AO and the MJO are in the same (opposite) phase, the Pacific action center becomes much stronger (weaker) than the Atlantic action center. The eddy momentum transports associated with the MJO in the Pacific sector are closely related to the retraction and extension of tropical Pacific easterlies and the subtropical Asian–Pacific jet. Because of its large scale, this regional effect is also reflected in the zonal mean state of wave transport and wave forcing on zonal wind, which in turn affects the phase of the AO.