Impressions of the turbulence variability in a weakly stratified,flat-bottom deep-sea ‘boundary layer’

The character of turbulent overturns in a weakly stratified deep-sea is investigated in some detail using 144 high-resolution temperature sensors at 0.7 m intervals, starting 5 m above the bottom. A 9-day, 1 Hz sampled record from the 912 m depth flat-bottom (<0.5% bottom-slope) mooring site in the central-north Alboran Sea (W-Mediterranean) demonstrates an overall conservative temperature range of only 0.05 °C, a typical mean buoyancy period as large as 3 h and a 1 Hz-profile-vertically-averaged turbulence dissipation rate maximum of only 10⁻⁸ m² s⁻³. Nonetheless, this ‘boundary layer’ varies in height between <6 and >104 m above the bottom and is thus not homogeneous throughout; the temperature variations are seldom quiescent and are generally turbulent in appearance, well exceeding noise levels. The turbulence character is associated with small-scale internal waves; examples are found of both shear- and convection-driven turbulence; particular association, although not phase-locked, is found between turbulence variations and tidal rather than with inertial motions; the mean buoyancy frequency of a few times the inertial frequency implies the importance of ‘slantwise convection’ in the direction of the earth rotational vector rather than in the direction of gravity. Such convection is observed both in near-homogeneous and weakly stratified form.     

Instabilities of the tidally induced bottom boundary layer in the rotating frame and their mixing effect

To investigate the stability of the bottom boundary layer induced by tidal flow (oscillating flow) in a rotating frame, numerical experiments have been carried out with a two-dimensional non-hydrostatic model. Under homogeneous conditions three types of instability are found depending on the temporal Rossby number Ro_t, the ratio of the inertial and tidal periods. When Ro_t < 0.9 (subinertial range), the Ekman type I instability occurs because the effect of rotation is dominant though the flow becomes more stable than the steady Ekman flow with increasing Ro_t. When Ro_t > 1.1 (superinertial range), the Stokes layer instability is excited as in the absence of rotation. When 0.9 < Ro_t < 1.1 (near-inertial range), the Ekman type I or type II instability appears as in the steady Ekman layer. Being much thickened (100 m), the boundary layer becomes unstable even if tidal flow is weak (5 cm/s). The large vertical scale enhances the contribution of the Coriolis effect to destabilization, so that the type II instability tends to appear when Ro_t > 1.0. However, when Ro_t < 1.0, the type I instability rather than the type II instability appears because the downward phase change of tidal flow acts to suppress the latter. To evaluate the mixing effect of these instabilities, some experiments have been executed under a weak stratification peculiar to polar oceans (the buoyancy frequency N²  10⁻⁶ s⁻²). Strong mixing occurs in the subinertial and near-inertial ranges such that tracer is well mixed in the boundary layer and an apparent diffusivity there is evaluated at 150–300 cm²/s. This suggests that effective mixing due to these instabilities may play an important role in determining the properties of dense shelf water in the polar regions.     

Pic 2005, a field campaign to investigate low-tropospheric ozone variability in the Pyrenees

The Pic 2005 field campaign took place from 13 June to 7 July 2005 close to the high-altitude permanent atmospheric observatory Pic-du-Midi (PDM), situated at 2875 m asl in the French Pyrenees. The experimental set-up combined in situ ground-based observations at PDM with ozone lidar measurements at two lower sites in close vicinity (600 m asl/28 km away, and 2380 m asl/500 m away). Such an experimental configuration is appropriate to address the question of the vertical layering of the chemical atmosphere in a mountain area and above the plain nearby, and how this influences measurements conducted on a mountain summit under the influence of horizontal transport at regional scale, and vertical transport at local scale. Forecast tools made it possible to plan and carry out 6 one-day Intensive Observation Periods (IOPs), mostly in anticyclonic conditions favoring local thermally induced circulations, with and without local pollution in the lower troposphere.It was thus possible to document i) ozone diurnal variations at PDM; ii) correlation between ozone measurements at PDM and their counterparts at the same altitude in the free troposphere; iii) ozone variability in the vicinity of PDM.The field campaign provided direct experimental evidence that at daytime in the encountered conditions (mostly anticyclonic), PDM failed in a large extent to be representative of the troposphere above the surrounding flat areas at similar altitude. First, ozone daily averages at PDM were found lower than their free-tropospheric counterpart. Thermally induced circulations and convection pumping clean air from the rural boundary layer can account qualitatively for ozone depletion observed at PDM during daytime. However the surface measurements do not support the hypothesis of direct lifting of near-surface air masses up to PDM. Thus, mixing with free-tropospheric air, photochemistry and surface deposition in the valleys appear to be needed ingredients to account quantitatively for the observed variations (in proportions that further studies should determine). Second, ozone variability was found to be much lower at PDM than in the free troposphere—again an indication of atmospheric mixing. In particular at daytime, the PDM observatory did not allow for detection of ozone-rich layers simultaneously visible above the plain. Beyond these first results, the data set presented here paves way to detailed studies of the IOPs.     

Dynamics of the Leeuwin Current: Part 2. Impacts of mixing,friction, and advection on a buoyancy-driven eastern boundary current over a shelf

The boundary currents over the Western Australian continental shelf and slope consist of the poleward flowing Leeuwin Current (LC) and the equatorward flowing Leeuwin Undercurrent (LUC). Key properties of the LC are its poleward strengthening, deepening to the south, and shelfbreak intensification. The alongshore flow reverses direction below about 300 m, forming the LUC at greater depths. To investigate the processes that cause these features, we obtain solutions to an idealized, regional ocean model of the South Indian Ocean. Solutions are forced by relaxing surface density to a prescribed, meridionally varying density profile ρ^*(y) with a timescale of δt. In addition, vertical diffusion is intensified near the ocean surface. This diffusion establishes the minimum thickness over which density is well-mixed. We define this thickness as the “upper layer”. Solutions are obtained with and without a continental shelf and slope off Western Australia and for a range of values of δt and mixing parameters. Within this upper layer, there is a meridional density gradient that balances a near-surface, eastward geostrophic flow. The eastward current downwells near the eastern boundary, leading to westward flow at depth. The upper layer's meridional structure and zonal currents crucially depend on coastal processes, including the presence of topography near the eastern boundary. Kelvin waves inhibit the upper layer from deepening at the coast. Rossby waves propagate the coastal density structure offshore, hence modifying the interior currents. A comparison of the solutions with or without a continental shelf and slope demonstrate that topographic trapping of Rossby waves is a necessary process for maintaining realistic eastern boundary current speeds. Significant poleward speeds occur only onshore of where the upper layer intersects the slope, that is, at a grounding line. Its poleward transport increases when surface-enhanced vertical mixing is applied over a greater depth. When the timescale δt is sufficiently short, the poleward current is nearly barotropic. The current's spatial structure over the shelf is controlled by horizontal mixing, having the structure of a Munk layer. Increasing vertical diffusion deepens the upper layer thickness and strengthens the alongshore current speed. Bottom drag leads to an offshore flow along the bottom, reducing the net onshore transport and weakening the current's poleward acceleration. When δt is long, poleward advection of buoyancy forms a density front near the shelf break, intensifying poleward speeds near the surface. With bottom drag, a bottom Ekman flow advects density offshore, shifting the jet core offshore of the shelf break. The resulting cross-shelf density gradient reverses the meridional current's direction at depth, leading to an equatorward undercurrent.     

The origin of sub-surface source waters define the sea–air flux of methane in the Mauritanian Upwelling,NW Africa

Concentrations and flux densities of methane were determined during a Lagrangian study of an advective filament in the permanent upwelling region off western Mauritania. Newly upwelled waters were dominated by the presence of North Atlantic Central Water and surface CH₄ concentrations of 2.2 ± 0.3 nmol L⁻¹ were largely in equilibrium with atmospheric values, with surface saturations of 101.7 ± 14%. As the upwelling filament aged and was advected offshore, CH₄ enriched South Atlantic Central Water from intermediate depths of 100–350 m was entrained into the surface mixed layer of the filament following intense mixing associated with the shelf break. Surface saturations increased to 198.9 ± 15% and flux densities increased from a mean value over the shelf of 2.0 ± 1.1 μmol m⁻² d⁻¹ to a maximum of 22.6 μmol m⁻² d⁻¹. Annual CH₄ emissions for this persistent filament were estimated at 0.77 ± 0.64 Gg which equates to a maximum of 0.35% of the global oceanic budget. This raises the known outgassing intensity of this area and highlights the importance of advecting filaments from upwelling waters as efficient vehicles for air-sea exchange.     

MJO prediction skill,predictability, and teleconnection impacts in the Beijing Climate Center Atmospheric General Circulation Model

This study evaluates performance of Madden–Julian oscillation (MJO) prediction in the Beijing Climate Center Atmospheric General Circulation Model (BCC_AGCM2.2). By using the real-time multivariate MJO (RMM) indices, it is shown that the MJO prediction skill of BCC_AGCM2.2 extends to about 16–17 days before the bivariate anomaly correlation coefficient drops to 0.5 and the root-mean-square error increases to the level of the climatological prediction. The prediction skill showed a seasonal dependence, with the highest skill occurring in boreal autumn, and a phase dependence with higher skill for predictions initiated from phases 2–4. The results of the MJO predictability analysis showed that the upper bounds of the prediction skill can be extended to 26 days by using a single-member estimate, and to 42 days by using the ensemble-mean estimate, which also exhibited an initial amplitude and phase dependence. The observed relationship between the MJO and the North Atlantic Oscillation was accurately reproduced by BCC_AGCM2.2 for most initial phases of the MJO, accompanied with the Rossby wave trains in the Northern Hemisphere extratropics driven by MJO convection forcing. Overall, BCC_AGCM2.2 displayed a significant ability to predict the MJO and its teleconnections without interacting with the ocean, which provided a useful tool for fully extracting the predictability source of subseasonal prediction.     

Assessing 4D-VAR for dynamical mapping of coastal high-frequency radar in San Diego

The problem of dynamically mapping high-frequency (HF) radar radial velocity observations is investigated using a three-dimensional hydrodynamic model of the San Diego coastal region and an adjoint-based assimilation method. The HF radar provides near-real-time radial velocities from three sites covering the region offshore of San Diego Bay. The hydrodynamical model is the Massachusetts Institute of Technology general circulation model (MITgcm) with 1 km horizontal resolution and 40 vertical layers. The domain is centered on Point Loma, extending 117 km offshore and 120 km alongshore. The reference run (before adjustment) is initialized from a single profile of T and S and is forced with wind data from a single shore station and with zero heat and fresh water fluxes. The adjoint of the model is used to adjust initial temperature, salinity, and velocity, hourly temperature, salinity and horizontal velocities at the open boundaries, and hourly surface fluxes of momentum, heat and freshwater so that the model reproduces hourly HF radar radial velocity observations. Results from a small number of experiments suggest that the adjoint method can be successfully used over 10-day windows at coastal model resolution. It produces a dynamically consistent model run that fits HF radar data with errors near the specified uncertainties. In a test of the forecasting capability of the San Diego model after adjustment, the forecast skill was shown to exceed persistence for up to 20 h.     

Vertical profiles of aerosol black carbon in the atmospheric boundary layer over a tropical coastal station: Perturbations during an annular solar eclipse

Altitude profiles of aerosol black carbon (BC) in the atmospheric boundary layer (ABL) over a tropical coastal station, Trivandrum have been examined on two days using an aethalometer attached to a tethered balloon. One of these days (15th January, 2010) coincided with a (annular) solar eclipse, the longest of this century at this location, commenced at 11:05 local time and ended by 15:05, lasting for 7 min and 15 s (from 13:10:42), with its maximum contact occurring at ~ 13:14 IST with ~ 92% annularity, thereby providing an opportunity to understand the eclipse induced perturbations. Concurrent measurements of the ABL parameters such as air temperature, relative humidity and pressure were also made on these days to describe the response of the ABL to the eclipse. BC profiles, in general, depicted similar features up to an altitude of ~ 200 m on the eclipse day and control day, above which it differed conspicuously with profiles on eclipse day showing increasingly lower concentration as we moved to higher altitudes. Examination of the meteorological profiles showed that the altitude of maximum convection rapidly fell down during the eclipse period compared to that on control day indicating a rather shallow convection on eclipse day. Comparison of diurnal variations of BC at the surface level showed that the rate of decrease in BC during daytime on the eclipse day was smaller than that on the control day due to the reduced convection, shallow ABL and consequent reduction in the ventilation coefficient. Moreover the time of the nocturnal increase has advanced by ~ 1:30 h on the eclipse day, occurred at around 19:30 IST in contrast to all the other days of January 2010, where this increase usually occur well after 20:30 IST, with a mean value of 21:00 IST. This is attributed to the weak sea-breeze penetration during the eclipse day, which led to an early onset of the land breeze.     

Analysis of quantitative precipitation forecasts using the Dynamic State Index

The German Weather Service (DWD) has two non-hydrostatic operational weather prediction models with different spatial resolution and precipitation parametrisations. The coarser COSMO-EU model has a spatial resolution of 7 km, whereas the higher-resolution COSMO-DE model has a gridspace of 2.8 km and explicitly resolves deep convection. To improve the numerical weather prediction (NWP) models it is necessary to understand precipitation processes. A central goal is the statistical evaluation of precipitation forecasts with dynamic parameters. Here, the Dynamic State Index (DSI) is used as a dynamic threshold parameter. The DSI theoretically describes the change of atmospheric flow fields as deviations from a stationary adiabatic solution of the primitive equations (Névir, 2004). For seasonal area means the DSI shows a remarkably high correlation with the precipitation forecasts provided by the COSMO-DE model. This is especially the case for the summer of 2007. The same analysis has been performed with the COSMO-EU forecast data and the results were compared with those from the COSMO-DE model. Moreover, an independent precipitation analysis, with a resolution corresponding to 7 km and 2.8 km, has been compared with respect to modelled precipitation and the DSI. In addition, correlations between the DSI and modelled as well as observed precipitation as a function of the forecast time for the different grid resolutions are also presented. The results show, that after 12 h, the correlation of the persistence forecast with the DSI reaches two thirds of the initial value. Thus, the DSI offers itself as a new dynamic forecast tool for precipitation events.     

Severe weather reports and proximity to deep convection over Northern Argentina

Conventional surface data and quantitative estimations of precipitation are used to document the occurrence and spatial distribution of severe weather phenomena associated with deep moist convection over southeastern South America.Data used in this paper are 24-hour rainfall, maximum hourly gusts and present weather reports from the surface station network for Argentina to the north of 40°S and cover the period 2000–2005. Hourly rainfall estimated with the CMORPH technique (CPC MORPHing technique, R. J. Joyce et al., 2004) is included in the analysis in order to increase the density of the precipitation database from January 2003 to December 2005. Extreme events are detected by means of a 95th-percentile analysis of the 24-hour rainfall and wind; values greater than 30 mm and 25 m s^?1 respectively are considered extreme in the study area. These results are related to the presence of deep convection by considering the 235 K and 218 K cloud shield evolution in Geostationary Operational Environmental Satellite-12 Infrared (GOES-IR) imagery evaluated by the Forecasting and Tracking of Cloud Cluster (FORTRACC) technique. Rainfall above 30 mm day^?1 and present convection-related weather events tend to occur in the northeast of the country.Finally, an analysis is made of the relationship between severe phenomena and the location and lifecycle of Mesoscale Convective Systems (MCSs) defined by the 218 K or 235 K levels. According to the reports, favorable locations for severe weather concentrate to the northeast of the cloud shield anvil centroid although most of the cases are found in the northwest. This feature can be seen in systems with anvil areas larger than 250,000 km² in association to the predominant mid-level wind shear direction from the northwest over the area. Moreover, systems with centers located north of 30°S present a more circular shape while those to the south are more elongated with a NW–SE main axis clearly related to the presence and interaction with frontal zones over the area. Most of the events occur previous to the moment when the systems reach their maximum extension, between 2 and 10 h after the initiation of the system depending on the size of the MCSs.     

Neutrally Stratified Turbulent Ekman Boundary Layer: Universal Similarity for a Transitional Rough Surface

The geostrophic Ekman boundary layer for large Rossby number (Ro) has been investigated by exploring the role played by the mesolayer (intermediate layer) lying between the traditional inner and outer layers. It is shown that the velocity and Reynolds shear stress components in the inner layer (including the overlap region) are universal relations, explicitly independent of surface roughness. This universality of predictions has been supported by observations from experiment, field and direct numerical simulation (DNS) data for fully smooth, transitionally rough and fully rough surfaces. The maxima of Reynolds shear stresses have been shown to be located in the mesolayer of the Ekman boundary layer, whose scale corresponds to the inverse square root of the friction Rossby number. The composite wall-wake universal relations for geostrophic velocity profiles have been proposed, and the two wake functions of the outer layer have been estimated by an eddy viscosity closure model. The geostrophic drag and cross-isobaric angle predictions yield universal relations, which are also supported by extensive field, laboratory and DNS data. The proposed predictions for the geostrophic drag and the cross-isobaric angle compare well with data for Rossby number Ro ≥ 10⁵. The data show low Rossby number effects for Ro < 10⁵ and higher-order effects due to the mesolayer compare well with the data for Ro ≥ 10³.     

Analysis of rainfall characteristics of the Madden–Julian oscillation using TRMM satellite data

Rainfall characteristics of the Madden–Julian oscillation (MJO) are analyzed primarily using tropical rainfall measuring mission (TRMM) precipitation radar (PR), TRMM microwave imager (TMI) and lighting imaging sensor (LIS) data. Latent heating structure is also examined using latent heating data estimated with the spectral latent heating (SLH) algorithm.The zonal structure, time evolution, and characteristic stages of the MJO precipitation system are described. Stratiform rain fraction increases with the cloud activity, and the amplitude of stratiform rain variation associated with the MJO is larger than that of convective rain by a factor of 1.7. Maximum peaks of both convective rain and stratiform rain precede the minimum peak of the outgoing longwave radiation (OLR) anomaly which is often used as a proxy for the MJO convection. Stratiform rain remains longer than convective rain until ∼4000 km behind the peak of the mature phase. The stratiform rain contribution results in the top-heavy heating profile of the MJO.Associated with the MJO, there are tri-pole convective rain top heights (RTH) at 10–11, ∼7 and ∼3 km, corresponding to the dominance of afternoon showers, organized systems, and shallow convections, respectively. The stratiform rain is basically organized with convective rain, having similar but slightly lower RTH and slightly lags the convective rain maximum. It is notable that relatively moderate (∼7 km) RTH is dominant in the mature phase of the MJO, while very tall rainfall with RTH over 10 km and lightning frequency increase in the suppressed phase. The rain-yield-per flash (RPF) varies about 20–100% of the mean value of ∼2–10 × 10⁹ kg fl⁻¹ over the tropical warm ocean and that of ∼2–5 × 10⁹ kg fl⁻¹ over the equatorial Islands, between the convectively suppressed phase and the active phase of MJO, in the manner that RPF is smaller in the suppressed phase and larger in the active phase.     

Nitrate in groundwater of China: Sources and driving forces

Identifying the sources of reactive nitrogen (N) and quantifying their contributions to groundwater nitrate concentrations are critical to understanding the dynamics of groundwater nitrate contamination. Here we assessed groundwater nitrate contamination in China using literature analysis and N balance calculation in coupled human and natural systems. The source appointment via N balance was well validated by field data via literature analysis. Nitrate was detected in 96% of groundwater samples based on a common detection threshold of 0.2 mg N L^?1, and 28% of groundwater samples exceeded WHO's maximum contaminant level (10 mg N L^?1). Groundwater nitrate concentrations were the highest beneath industrial land (median: 34.6 mg N L^?1), followed by urban land (10.2 mg N L^?1), cropland (4.8 mg N L^?1), and rural human settlement (4.0 mg N L^?1), with the lowest found beneath natural land (0.8 mg N L^?1). During the period 1980–2008, total reactive N leakage to groundwater increased about 1.5 times, from 2.0 to 5.0 Tg N year^?1, in China. Despite that the contribution of cropland to the total amount of reactive N leakage to groundwater was reduced from 50 to 40% during the past three decades, cropland still was the single largest source, while the contribution from landfill rapidly increased from 10 to 34%. High reactive N leakage mainly occurred in relatively developed agricultural or urbanized regions with a large population. The amount of reactive N leakage to groundwater was mainly driven by anthropogenic factors (population, gross domestic product, urbanization rate and land use type). We constructed a high resolution map of reactive N source appointment and this could be the basis for future modeling of groundwater nitrate dynamics and for policy development on mitigation of groundwater contamination.     

A Mediterranean nocturnal heavy rainfall and tornadic event. Part I: Overview,damage survey and radar analysis

This study presents an analysis of a severe weather case that took place during the early morning of the 2nd of November 2008, when intense convective activity associated with a rapidly evolving low pressure system affected the southern coast of Catalonia (NE Spain). The synoptic framework was dominated by an upper level trough and an associated cold front extending from Gibraltar along the Mediterranean coast of the Iberian Peninsula to SE France, which moved north-eastward. South easterly winds in the north of the Balearic Islands and the coast of Catalonia favoured high values of 0–3 km storm relative helicity which combined with moderate MLCAPE values and high shear favoured the conditions for organized convection. A number of multicell storms and others exhibiting supercell features, as indicated by Doppler radar observations, clustered later in a mesoscale convective system, and moved north-eastwards across Catalonia. They produced ground-level strong damaging wind gusts, an F2 tornado, hail and heavy rainfall. Total lightning activity (intra-cloud and cloud to ground flashes) was also relevant, exhibiting several classical features such as a sudden increased rate before ground level severe damage, as discussed in a companion study. Remarkable surface observations of this event include 24 h precipitation accumulations exceeding 100 mm in four different observatories and 30 minute rainfall amounts up to 40 mm which caused local flash floods. As the convective system evolved northward later that day it also affected SE France causing large hail, ground level damaging wind gusts and heavy rainfall.     

Social networks and transitions to co-management in Jamaican marine reserves and small-scale fisheries

How social networks support or constrain the transition to co-management of small-scale fisheries and marine reserves is poorly understood. In this paper, we undertake a comparative analysis of the social network structures associated with the transition to co-management in three Jamaican marine reserves. Data from quantitative social relational surveys (n = 380) are integrated with data from semi-structured interviews (n = 63) and focus groups (n = 10) to assess how patterns of relational ties and interactions between and among fishermen and other local level actors (e.g., managers, wardens, NGO staff) support and constrain the transition to co-management. Our research suggests that the transitions to co-management were supported by a combination of three network structure and relational attributes: (i) the presence and position of institutional entrepreneurs; (ii) a dense central core of network actors; and (iii) the prevalence of horizontal ties and vertical linkages held by the community-based organizations formally responsible for the management of the marine reserves. Our findings also show that overall low network cohesion in the three reserves and limited social influence among the wardens may be problematic for sustained collective action that extends beyond the core set of network actors. These findings suggest the importance of strategies to enhance collective action, specifically through attention to the attributes of the corresponding social networks, as a means to contribute to successful transitions to co-management of marine reserves and small-scale fisheries. Our results provide more precise guidance, through social network analysis, on where in the respective networks social capital and leadership may require support or enhancement, and thus on how to target interventions for greatest effect.     

A method for determining thermophysical properties of organic material in aqueous solutions: Succinic acid

A method for determining evaporation rates and thermodynamic properties of aqueous solution droplets is introduced. The method combines evaporation rate measurements using modified TDMA technique with data evaluation using an accurate evaporation model. The first set of data has been collected and evaluated for succinic acid aqueous solution droplets.Evaporation rates of succinic acid solution droplets have been measured using a TDMA system at controlled relative humidity (65%) and temperature (298 K). A temperature-dependent expression for the saturation vapour pressure of pure liquid phase succinic acid at atmospheric temperatures has been derived by analysing the evaporation rate data with a numerical model. The obtained saturation vapour pressure of liquid phase succinic acid is ln(p) = 118.41 − 16204.8/T − 12.452ln(T). The vapour pressure is in unit of Pascal and the temperature in Kelvin. A linear expression for the enthalpy of vaporization for liquid state succinic acid is also presented.According to the results presented in the following, a literature expression for the vapour pressure of liquid phase succinic acid defined for temperatures higher than 461 K [Yaws, C.L., 2003. Yaws' Handbook of Thermodynamic and Physical Properties of Chemical Compounds, Knovel] can be extrapolated to atmospheric temperatures with very good accuracy. The results also suggest that at 298 K the mass accommodation coefficient of succinic acid is unity or very close to unity.     

Westerly wind events,diurnal cycle and central Pacific El Niño warming

In the past three decades, the strongest central Pacific (CP) El Niño event was observed in 2009–2010 by satellites. When intensity of this CP El Niño reached its maximum, large diurnal variations of sea surface temperature (SST) were also observed from tropical atmosphere ocean moorings in the central equatorial Pacific. Solar radiation in the equatorial central Pacific is larger than 140 W/m², which leads to the amplitude of diurnal cycle of SST primarily determined by large-scale wind patterns. Intraseasonal westerly wind events (WWEs) can lead to an eastward displacement of the warm pool and also can weaken the trade winds in central Pacific. When the occurrence of equatorial WWEs is more than 20 days in a month, monthly mean wind speed in central equatorial Pacific has high possibility of wind speed less than 3 m/s, thus has pronounced diurnal cycle of SST. The diurnal cycle of SST will rectify daily mean SST. Reduced mixing at the base of the mixed layer and suppression of entrainment due to the accumulated effect of diurnal cycle may lead to warmer SST in the following month. This study suggests the occurrence of more diurnal SST events may contribute to the increasing intensity of the CP El Niño events.     

Variability of rainfall and effective onset and length of the monsoon season over a sub-humid climatic environment

The study has analyzed the variability and trends in monthly, seasonal and annual rainfall and rainy days of four locations over different agro-ecological zones of Bihar, namely Samastipur (zone-I), Madhepura (zone-II), Sabour (zone-IIIA) and Patna (zone-IIIB). The Mann–Kendall nonparametric test was employed for detection of statistical significance and slopes of the trend lines were determined using the method of least square linear fitting. The variability and trends of onset of effective monsoon and length of monsoon period were also analyzed using the same method. The mean annual rainfall varies from 1137 mm at Patna to 1219 mm at Sabour. July is the rainiest month in all the zones followed by August. Maximum increase in annual rainfall was found at Sabour (40.1% of mean/30 years at 95% confidence level) and minimum for Patna (10.1% of mean/30 years). Significant increasing trend of rainfall during July, August and September at rates of 41.9, 83.2, and 112.7% of the mean/30 years, respectively has been noticed at Madhepura. Analysis of rainy days indicates that rainy days increased during winter and annually for all the sites. The mean effective onset of monsoon varies from 18th June at Sabour to 28th June at Patna. The trends in the date of effective onset of monsoon indicate that the date tends to be early in all the sites except Madhepura. But a significant delayed trend in the onset at a rate of 2.8% of the mean/30 years has been observed for Madhepura. The early trend of the effective onset of monsoon and increasing trends of length of monsoon season have been observed for Samastipur, Sabour and Patna.     

A cloud-resolving regional simulation of tropical cyclone formation

The development of Tropical Cyclone Diana (1984) is simulated with a mesoscale model using 1.2 km grid spacing over a regional-scale (>1000 km) domain in the first known experiment of this kind. With only a synoptic-scale disturbance in the initial conditions, the model first develops a mesoscale convective system along a remnant frontal zone, which yields a mesoscale vortex. After a period of quiescence, banded convection organizes about the vortex from isolated, grid-resolved cells, with the system becoming warm-core and intensifying into Tropical Storm Diana.     

Rapid distortion theory for mixing efficiency of a flow stratified by one or two scalars

The mixing efficiency of unsheared homogeneous turbulence in flows stratified by one or two active scalars was calculated with rapid distortion theory (RDT). For the case with one scalar the mixing efficiency η depends on the Schmidt number Sc = ν/D and the Grashof number Gr = NL²/ν, where ν is the kinematic viscosity, D is the molecular diffusivity, N is the buoyancy frequency, and L is a length scale representative of the large eddies. For the case with two scalars the efficiency also depends on the density ratio R_ρ, which compares the density difference caused by temperature and the density difference caused by salt. In the one scalar case when Gr is large, η decreases as Sc increases. The mixing efficiency increases with Gr up to a maximum value, as in numerical simulations and experiments. The maximum mixing efficiency of approximately 30% for low Sc is consistent with simulations, while the maximum efficiency of 6% for heated water is consistent with laboratory measurements. However, RDT underpredicts the maximum efficiency for saltwater and also the value of Gr at which the efficiency becomes constant. The predicted behavior of the mixing efficiency for two active scalars is similar to that for one scalar, and the efficiency decreases as R_ρ decreases, as in experiments and semi-empirical models. These calculations show that results from simulations with low Sc likely overestimate the efficiency of turbulence in strongly stratified flows in lakes and oceans.