Vertical heat transports in the ocean and their effect on time-dependent climate change

In response to increasing atmospheric concentrations of greenhouse gases, the rate of time-dependent climate change is determined jointly by the strength of climate feedbacks and the efficiency of processes which remove heat from the surface into the deep ocean. This work examines the vertical heat transport processes in the ocean of the HADCM2 atmosphere–ocean general circulation model (AOGCM) in experiments with CO₂ held constant (control) and increasing at 1 per year (anomaly). The control experiment shows that global average heat exchanges between the upper and lower ocean are dominated by the Southern Ocean, where heat is pumped downwards by the wind-driven circulation and diffuses upwards along sloping isopycnals. This is the reverse of the low-latitude balance used in upwelling–diffusion ocean models, the global average upward diffusive transport being against the temperature gradient. In the anomaly experiment, weakened convection at high latitudes leads to reduced diffusive and convective heat loss from the deep ocean, and hence to net heat uptake, since the advective heat input is less affected. Reduction of deep water production at high latitudes results in reduced upwelling of cold water at low latitudes, giving a further contribution to net heat uptake. On the global average, high-latitude processes thus have a controlling influence. The important role of diffusion highlights the need to ensure that the schemes employed in AOGCMs give an accurate representation of the relevant sub-grid-scale processes. Received: 8 July 1999 / Accepted: 17 November 1999     

Surface energy balance measurements at a tropical site in West Africa during the transition from dry to wet season

Summary In one of the first micrometeorological experiments at a tropical site in West Africa, direct measurements of all surface energy balance components were carried out. The experiment NIMEX-1 in Ile-Ife, Nigeria (7°33′ N, 4°33′ E), was conducted from February 19, 2004 to March 9, 2004, during the transition from the dry to the wet season. Three typical weather situations could be observed: firstly, monsoonal winds from the southwest blew over desiccated soils. Almost 100% of the available energy at the surface was transformed into sensible heat flux. Secondly, after several thundershowers, monsoonal winds swept over soils of increased water content, which led to a partitioning of the available energy corresponding to Bowen ratios between 0.3 and 0.5. Thirdly, harmattan winds advected dry dusty air from northern directions, which reduced the incoming shortwave radiation. Again, Bowen ratios range from 0.3 to 0.5 during daytime, whereas latent heat fluxes are still high during the night due to the advection of very dry air. No systematic non-closure of the surface energy balance could be found for the NIMEX-1 dataset. Unlike other experiments in Europe, most of the ogives for the sensible and latent heat flux were found to be convergent during NIMEX-1 in Ile-Ife. This can be attributed to the homogeneity of the surrounding bush, which lacks the defined borders found in agriculturally cultivated landscapes.     

Impact of increased vertical resolution on simulation of tropical climate

Summary The aim of this study is to describe the behaviour of tropical dynamics in the ECHAM4 model when increased vertical resolution around the tropopause and in the planetary boundary layer is used. In this work we perform experiments with the ECHAM4 model using T30 horizontal resolution and 19 and 42 vertical levels. The impact of the increased vertical resolution on the simulation of tropical clouds and precipitation has been investigated. Therefore, the dynamic fields related to tropical convection have been analyzed. The results suggest a beneficial effect of the increased number of vertical levels on the convective scheme performance and on the related dynamic fields over the Tropics. The improvement of the rainfall climatologies in the 42-level model has been explained via the impact of vertical resolution on the cloud structure. In the cloud spectrum of the L42 simulation, a third peak appears around 600 hPa, revealing that when using higher vertical resolution the convective parametrization starts to represent cumulus congestus clouds.     

Estimation of Surface-Layer Scaling Parameters in the Unstable Boundary Layer: Implications for Orographic Flow Speed-Up

A method is proposed for estimating the surface-layer depth \((z_s)\) and the friction velocity \((u_*)\) as a function of stability (here quantified by the Obukhov length, L) over the complete range of unstable flow regimes. This method extends that developed previously for stable conditions by Argaín et al. (Boundary-Layer Meteorol 130:15–28, 2009), but uses a qualitatively different approach. The method is specifically used to calculate the fractional speed-up \((\varDelta S)\) in flow over a ridge, although it is suitable for more general boundary-layer applications. The behaviour of \(z_s \left( L\right) \) and \(u_*\left( L\right) \) as a function of L is indirectly assessed via calculation of \(\varDelta S\left( L\right) \) using the linear model of Hunt et al. (Q J R Meteorol Soc 29:16–26, 1988) and its comparison with the field measurements reported in Coppin et al. (Boundary-Layer Meteorol 69:173–199, 1994) and with numerical simulations carried out using a non-linear numerical model, FLEX. The behaviour of \(\varDelta S\) estimated from the linear model is clearly improved when \(u_*\) is calculated using the method proposed here, confirming the importance of accounting for the dependences of \(z_s\left( L \right) \) and \(u_*\left( L \right) \) on L to better represent processes in the unstable boundary layer.     

The meteorological observations of Bento Sanches Dorta, Rio de Janeiro, Brazil: 1781–1788

Bento Sanches Dorta was an astronomer and geographer in the Portuguese colony of Rio de Janeiro in Brazil from 1781 to 1788. He recorded daily readings of meteorological and geomagnetic variables during that period. This dataset provides, to the best of our knowledge, the earliest known continuous 8-year-long instrumental meteorological observations for any South American site. His data show that the winters in this period were relatively cool, and that 1785 was the rainiest and hottest year, and 1787 the driest and coolest. The records display a distinct seasonal cycle and a variability that are comparable with the modern data.     

Application of remote sensing and multivariate analysis to the agroclimatic characterization of a Mediterranean region

Summary This paper presents a climatological application of the combination of remote sensing data and multivariate analysis. It proposes a methodology to perform an agroclimatic characterization of a region with the aim of determining homogeneous areas. This methodology is applied to Catalonia, in the northwest of the Mediterranean Sea, in the Iberian Peninsula. To this end various multivariate analysis techniques have been applied to data from 107 meteorological stations and to digitally processed AVHRR images from a NOAA satellite (ground surface temperature and thermal inertia). Values of emissivity and albedo for different land uses and phenomenological states, in addition to the most suitable algorithms for the soil surface temperature, have been obtained. Combining this information with orographic characteristics and the Papadakis classification, a final map is obtained with 68 homogeneous zones for the period between April and October, the most important for the agriculture of the area studied.With 5 Figures     

Tracer experiments with turbulently dispersed air ions

Unipolar air ions released into the wind constitute a tracer which can be measured with high resolution. An ion source produces a cloud with homogeneous charge density, insensitive to source strength, dependent on time since formation only. It is well suited for tracing concentration changes due to turbulence, less suited for cloud size tracing. A tight array of 8 sensors has been used to examine turbulently dispersed ions. High-resolution records are presented and discussed. The highest concentrations measured could be undiluted source material. The frequency distribution within a plume did not differ from that in a multitude of puffs. The distribution seems to be log-normal with a geometric standard deviation of about 2.45. The time resolution used corresponds to volume resolutions of 40, 225, and 650 cm³. Sample size had no apparent effect.     

SL-AV atmospheric model version using σ-<Emphasis Type="Italic">p</Emphasis> hybrid vertical coordinates

The SL-AV atmospheric model version using hybrid vertical coordinates combies the advantages of sigma and isobaric coordinates. The formulation and discretization of model equations maintain the equivalency of the new model version to the basic sigma version in the special case, when hybrid coordinates coincide with sigma coordinates. The SL-AV model version with hybrid vertical coordinate is verified with medium-range weather forecasts. The decrease in the errors of predicted geopotential height and wind as compared to the sigma model version is demonstrated. The use of hybrid coordinates also leads to a certain increase in forecast skill scores for some meteorological parameters characterizing aviation significant weather.     

Projected impact of deep ocean carbon dioxide discharge on atmospheric CO2 concentrations

An evaluation of oceanic containment strategies for anthropogenic carbon dioxide is presented. Energy conservation is also addressed through an input hydrocarbon-fuel consumption function. The effectiveness of the proposed countermeasures is determined from atmospheric CO₂ concentration predictions. A previous box model with a diffusive deep ocean is adapted and applied to the concept of fractional CO₂ injection in 500 m deep waters. Next, the contributions of oceanic calcium carbonate sediment dissolution, and of deep seawater renewal, are included. Numerical results show that for CO₂ direct removal measures to be effective, large fractions of anthropogenic carbon dioxide have to be processed. This point favors fuel pre-processing concepts. The global model also indicates that energy conservation, i.e. a hydrocarbon-fuel consumption slowdown, remains the most effective way to mitigate the greenhouse effect, because it offers mankind a substantial time delay to implement new energy production alternatives.     

Climate change and human activities: a case study in Xinjiang, China

We examined both long-term climate variability and anthropogenic contributions to current climate change for Xinjiang province of northwest China. Xinjiang encompasses several mountain ranges and inter-mountain basins and is comprised of a northern semiarid region and a more arid southern region. Climate over the last three centuries was reconstructed from tree rings and temperature series were calculated for the past 50 years using weather station data. Three major conclusions from these analyses are: (1) Although temperature varied considerably in Xinjiang over the last 200 years, it was non-directional until the last 50 years when a substantial warming trend occurred; (2) The semiarid North Xinjiang was representative of the northern hemisphere climate, while the more arid South Xinjiang resembled the southern hemisphere climate, meanwhile, (3) The entire Xinjiang province captured the global-scale climate signal. We also compared human contributions to global change between North and South Xinjiang, including land cover/land use, population, and greenhouse gas production. For both regions, urban areas acted as heat islands; and large areas of grassland and forest were converted to barren land, especially in North Xinjiang. Additionally, North Xinjiang also showed larger increase in population and greenhouse gas emissions mainly associated with animal production than those in South Xinjiang. Although Xinjiang province is a geographically coupled mountain–basin system, the two regions have distinct climate patterns and anthropogenic activities related to land cover conversion and greenhouse gas production.