Estimation of Regional Evapotranspiration by Combining Aircraftand Ground-Based Measurements

Two methods are examined for combining measurements from instrumented aircraftand towers to estimate regional scale evapotranspiration. Aircraft data provided spatially averaged values of properties of the surface, the evaporative fraction and maximum stomatal conductance. These quantities are less sensitive to meteorological conditions than the turbulent fluxes of heat and water vapour themselves. The methods allowed aircraft data collected over several days to be averaged and thus to reduce the random error associated with the temporal under-sampling inherent in aircraft measurements. Evaporative fraction is estimated directly from the aircraft data, while maximum stomatal conductance is estimated by coupling the Penman–Monteith equation to a simple model relating surface conductance to the incoming shortwave radiation and specific humidity saturation deficit. The spatial averages of evaporative fraction and maximum stomatal conductance can then be used with routine tower data to estimate the regional scale evapotranspiration. Data from aircraft flights and six ground based sites during the OASIS field campaign in south–east New South Wales in 1995 have been used to check the methods. Both the evaporative fraction and the maximum stomatal conductance derived from the aircraft data give information on the spatial variability of the surface energy budget at scales from 10 to 100 km. Daily averaged latent heat fluxes estimated using these methods for the OASIS study region agree with the available observations in quasi-stationary conditions or in weakly non-stationary conditions when the data from several aircraft flights are averaged to reduce the impact of short term imbalances in the surface energy budget.     

Estimating Buoyancy Heat Flux Using the Surface Renewal Technique over Four Amazonian Forest Sites in Brazil

The surface renewal (SR) method was applied for the first time to measurements of air temperature over four Amazonian forest sites and different seasons in order to obtain estimates of buoyancy heat flux. The required calibration of this method against eddy covariance resulted in a value for a specific parameter that is close to the range reported in other studies, contributing to the generalization of the SR method to different kinds of canopies. The comparison with fluxes obtained using the eddy covariance technique revealed a good match between the two methods for different sites, heights and seasons. Sites with high levels of non-stationarity in the signals of temperature and wind speed presented higher scatter in the regression with fluxes from eddy covariance. For a particular site with previously reported influence of low-frequency motions, the regression was only satisfactory, i.e., slope parameter close to unity and small offset, when oscillations with periods longer than $\approx $ ≈ 13 min were filtered out. The SR method has a great potential due to the simplicity of the instrumentation required. However, care should be taken when measuring under the influence of mesoscale motions, which can lead to high levels of non-stationarity, compromising the fundamental concepts of the SR theory.     

Vertical behaviour and meteorological properties of air masses in the southwest of the Iberian Peninsula (1997–2007)

Air masses are characterized by physical (temperature, humidity) and chemical (transported gases and aerosols) properties, being associated their arrival to different meteorological scenarios. The knowledge of the air masses over a region is fundamental as complementary information in several atmospheric studies, being the calculation of back-trajectory the most widely used tool whenever air masses are analyzed. A study of air masses has been carried out in southwestern Iberian Peninsula using 5-day kinematic back trajectories computed by the HYSPLIT model at three heights (500, 1,500 and 3,000 m) from 1997 to 2007. The main aims have been to characterize their vertical behaviour and their thermal and humidity properties. Thirteen trajectory clusters have been defined, showing the northerly and westerly clusters a high coupling degree. Seasonal daily variation of potential temperature and specific humidity has been analyzed, obtaining higher differences among clusters in the cold season.     

Diurnal variation of deep cloud systems over the Indian region using INSAT-1B pixel data

Summary Among the various time scales of deep clouds, diurnal variation is found to be prominent over the tropics. The present study examines the diurnal variation in the number and area cover associated with the cloud systems over the tropical Indian region using INSAT-1B pixel data. Three periods, namely, April–May 1988, July–August 1988, and January–February 1989 are considered. The dependence of diurnal characteristics on the temperature threshold, life duration and size of cloud systems, and land-sea contrast has been explored. The diurnal characteristics of cloud systems living for more than a day have been studied for the first time. It is shown that cloud systems exhibit strong diurnal dependence at the coldest temperature threshold used (201 K). Also, the diurnal variation is more for larger cloud systems and for longer living systems. In general, more deep cloud activity is found from the satellite data during the pre-dawn and early morning hours. Precipitation is enhanced during morning to early noon hours. Further, using data from a recent field experiment, clear evidence of diurnal variation in precipitation over the Bay of Bengal is also presented. Received March 20, 2000/Revised October 3, 2000     

MODELLING CLIMATE CHANGE IMPACTS ON ECOSYSTEMS USING LINKED MODELS AND A GIS

Future climate change scenarios have been applied to a linked model at five UK sites. Results indicate a decrease in grassland productivity under a changed climate, resulting from lower soil water content, with possible large consequences to future UK water resource planning. The effect of running the grassland sub-model in isolation, compared with running it as part of a linked model showed that linked models should be used for climate change impacts studies. The location specific response to climate change was also highlighted. The model is mechanistic, consists of four sub-models (grassland, water balance, nitrate and evapotranspiration) and is spatially explicit.     

A verification framework for interannual-to-decadal predictions experiments

Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model’s ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic metric applied to the initialized hindcasts and comparing different ways to ascribe forecast uncertainty. Verification is advocated at smoothed regional scales that can illuminate broad areas of predictability, as well as at the grid scale, since many users of the decadal prediction experiments who feed the climate data into applications or decision models will use the data at grid scale, or downscale it to even higher resolution. An overall statement on skill of CMIP5 decadal hindcasts is not the aim of this paper. The results presented are only illustrative of the framework, which would enable such studies. However, broad conclusions that are beginning to emerge from the CMIP5 results include (1) Most predictability at the interannual-to-decadal scale, relative to climatological averages, comes from external forcing, particularly for temperature; (2) though moderate, additional skill is added by the initial conditions over what is imparted by external forcing alone; however, the impact of initialization may result in overall worse predictions in some regions than provided by uninitialized climate change projections; (3) limited hindcast records and the dearth of climate-quality observational data impede our ability to quantify expected skill as well as model biases; and (4) as is common to seasonal-to-interannual model predictions, the spread of the ensemble members is not necessarily a good representation of forecast uncertainty. The authors recommend that this framework be adopted to serve as a starting point to compare prediction quality across prediction systems. The framework can provide a baseline against which future improvements can be quantified. The framework also provides guidance on the use of these model predictions, which differ in fundamental ways from the climate change projections that much of the community has become familiar with, including adjustment of mean and conditional biases, and consideration of how to best approach forecast uncertainty.     

Multi-Source Emission Determination Using an Inverse-Dispersion Technique

Inverse-dispersion calculations can be used to infer atmospheric emission rates through a combination of downwind gas concentrations and dispersion model predictions. With multiple concentration sensors downwind of a compound source (whose component positions are known) it is possible to calculate the component emissions. With this in mind, a field experiment was conducted to examine the feasibility of such multi-source inferences, using four synthetic area sources and eight concentration sensors arranged in different configurations. Multi-source problems tend to be mathematically ill-conditioned, as expressed by the condition number κ. In our most successful configuration (average κ = 4.2) the total emissions from all sources were deduced to within 10% on average, while component emissions were deduced to within 50%. In our least successful configuration (average κ = 91) the total emissions were calculated to within only 50%, and component calculations were highly inaccurate. Our study indicates that the most accurate multi-source inferences will occur if each sensor is influenced by only a single source. A “progressive” layout is the next best: one sensor is positioned to “see” only one source, the next sensor is placed to see the first source and another, a third sensor is placed to see the previous two plus a third, and so on. When it is not possible to isolate any sources κ is large and the accuracy of a multi-source inference is doubtful.     

The effect of sea-ice extent in the North Atlantic on the stability of the thermohaline circulation in global warming experiments

Different climate models simulate different behavior of the Atlantic meridional overturning circulation (MOC) under the same global warming scenario. We propose a plausible explanation for this and argue that a proper simulation of the present-day climate in the subpolar North Atlantic is important. This is illustrated using results from idealized global warming experiments, in which both the radiative forcing scenario and the model employed are the same, with the only major difference being the initial subpolar North Atlantic climate. The initial conditions are made progressively colder, with more extensive sea-ice cover in the northern North Atlantic.The key result is that starting from conditions which are too cold in the North Atlantic and with sea-ice that is too extensive leads to an MOC that is more stable to the radiative forcing. Furthermore, under considerably underestimated sea surface temperatures in subpolar regions, the MOC can even intensify. A reduction of freshwater flux associated with the reduction of sea-ice melt is shown to be important for such unusual behavior of the MOC. Other mechanisms are also considered, but not deemed as important in explaining published inter-model differences.     

The role of the subtropical jet stream during heat wave events over north-central Greece

Summary The role of the subtropical jet stream (SJ) in the occurrence of heat waves in South Balkans and Greece is sought here. For this purpose ECMWF grid-point data is examined, concerning the Balkan heat wave of 5–9 July 1988, that cost human lifes, at least in Greece. For the city of Thessaloniki, Greece, a temperature budget is presented, as a function of time. It turns out that the most important heating mechanism is the adiabatic heating. Horizontal mass convergence at the maximum wind level (200 hPa) causes descent and adiabatic heating. The convergence occurs in association with the Hadley Cell, as well as with the right exit quadrant of an anticyclonically curved subtropical jet streak. As air parcels that exit the above jet streak slow down and turn anticyclonically, a strong ageostrophic wind current is established towards and to the right of the flow direction. This ageostrophic current converges above the northeastern Balkans. Downward ageostrophic motion emerges from the above area of horizontal convergence and heads towards the SSW, affecting the Balkans. From the above case study, it is concluded that intense heat waves are favoured in the South Balkans and Greece when the SJ is anticyclonically curved to the north of the Balkans and a jet streak is situated to the north west of the Balkans.     

The Hadley circulation: assessing NCEP/NCAR reanalysis and sparse in-situ estimates

 We present a comparison of the zonal mean meridional circulations derived from monthly in situ data (i.e. radiosondes and ship reports) and from the NCEP/NCAR reanalysis product. To facilitate the interpretation of the results, a third estimate of the mean meridional circulation is produced by subsampling the reanalysis at the locations where radiosonde and surface ship data are available for the in situ calculation. This third estimate, known as the subsampled estimate, is compared to the complete reanalysis estimate to assess biases in conventional, in situ estimates of the Hadley circulation associated with the sparseness of the data sources (i.e., radiosonde network). The subsampled estimate is also compared to the in situ estimate to assess the biases introduced into the reanalysis product by the numerical model, initialization process and/or indirect data sources such as satellite retrievals. The comparisons suggest that a number of qualitative differences between the in situ and reanalysis estimates are mainly associated with the sparse sampling and simplified interpolation schemes associated with in situ estimates. These differences include: (1) a southern Hadley cell that consistently extends up to 200 hPa in the reanalysis, whereas the bulk of the circulation for the in situ and subsampled estimates tends to be confined to the lower half of the troposphere, (2) more well-defined and consistent poleward limits of the Hadley cells in the reanalysis compared to the in-situ and subsampled estimates, and (3) considerably less variability in magnitude and latitudinal extent of the Ferrel cells and southern polar cell exhibited in the reanalysis estimate compared to the in situ and subsampled estimates. Quantitative comparison shows that the subsampled estimate, relative to the reanalysis estimate, produces a stronger northern Hadley cell (∼20%), a weaker southern Hadley cell (∼20–60%), and weaker Ferrel cells in both hemispheres. These differences stem from poorly measured oceanic regions which necessitate significant interpolation over broad regions. Moreover, they help to pinpoint specific shortcomings in the present and previous in situ estimates of the Hadley circulation. Comparisons between the subsampled and in situ estimates suggest that the subsampled estimate produces a slightly stronger Hadley circulation in both hemispheres, with the relative differences in some seasons as large as 20–30%. 6These differences suggest that the mean meridional circulation associated with the NCEP/NCAR reanalysis is more energetic than observations suggest. Examination of ENSO-related changes to the Hadley circulation suggest that the in situ and subsampled estimates significantly overestimate the effects of ENSO on the Hadley circulation due to the reliance on sparsely distributed data. While all three estimates capture the large-scale region of low-level equatorial convergence near the dateline that occurs during El Nino, the in situ and subsampled estimates fail to effectively reproduce the large-scale areas of equatorial mass divergence to the west and east of this convergence area, leading to an overestimate of the effects of ENSO on the zonal mean circulation. Received: 16 September 1998 / Accepted: 22 April 1999