Joan Cuxart

CASES-99: A Comprehensive Investigation of the Stable Nocturnal Boundary Layer

Abstract The Cooperative Atmosphere-Surface Exchange Study—1999 (CASES-99) refers to a field experiment carried out in southeast Kansas during October 1999 and the subsequent program of investigation. Comprehensive data, primarily taken during the nighttime but typically including the evening and morning transition, supports data analyses, theoretical studies, and state-of-the-art numerical modeling in a concerted effort by participants to investigate four areas of scientific interest. The choice of these scientific topics is motivated by both the need to delineate physical processes that characterize the stable boundary layer, which are as yet not clearly understood, and the specific scientific goals of the investigators. Each of the scientific goals should be largely achievable with the measurements taken, as is shown with preliminary analysis within the scope of three of the four scientific goals. Underlying this effort is the fundamental motivation to eliminate deficiencies in surface layer and turbul...

Stable Atmospheric Boundary Layers and Diurnal Cycles: Challenges for Weather and Climate Models

The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For thi...

Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.

Intercomparison of Single-Column Numerical Models for the Prediction of Radiation Fog

Abstract The short-term forecasting of fog is a difficult issue that can have a large societal impact. Radiation fog appears in the surface boundary layer, and its evolution is driven by the interactions between the surface and lower layers of the atmosphere. Current NWP models poorly forecast the life cycle of fog, and improved NWP models are needed before improving the prediction of fog. Six numerical model simulations are compared for two cases from the Paris-Charles de Gaulle (Paris-CdG) fog field experiment. This intercomparison includes both operational and research models, which have significantly different vertical resolutions and physical parameterizations. The main goal of this intercomparison is to identify the capabilities of the various models to forecast fog accurately. An attempt is made to identify the main reasons behind the differences among the various models. This intercomparison reveals that considerable differences among models exist in the surface boundary layer before the fog onset...

Study of a Sea-Breeze Case through Momentum, Temperature, and Turbulence Budgets*

A simulation with the Meso-NH model over the island of Mallorca, Spain, has been made in a case of synoptichighpressure(5June2010)thatallowedthedevelopment ofseabreezes(SB)inthethreemainbasins of the island. The results compare well to the available observations and are qualitatively very close to a previous idealized study with no synoptic forcing made by Ramis and Romero in 1995. The temporal and spatial structure of the SB in the southeastern basin is analyzed with the use of the momentum, temperature, and turbulence kinetic energy budgets provided by the numerical model. Five stages of evolution from before dawntoaftersunsetarediscussed,identifyingthemainphysicalmechanismsatplay.Themorninglandheating warms the land and the air over it until an air temperature gradient is created and a marine flow accelerates inland, dragged by turbulence in the low layers. The upper part of the inland current and the layers just above are dominated by compensatory motions, which oppose the corresponding pressure gradient at these levels. These mechanisms last while the SB is active, with significant effects from the local topography, and they decrease in intensity as sunset approaches. This relatively simple case has been used to check the goodness of two analytical models of the SB that perform relatively well because they use turbulence as a surrogate for the missing advection terms in the layers above 200m. These models are formulated here in a more consistent manner in the turbulence parameterization than were the original propositions.

Evaluation of the surface energy budget equation with experimental data and the ECMWF model in the Ebro Valley

In numerical models of the climate system and in other applications, the surface energy budget is usually considered closed, allowing for estimation of missing terms as the residual of the others. Real measurements of this budget show signi_cant uncertainties in the values of each ux and imbalances that range between 5% and more than 50%, as shown in recent literature. In this article, a derivation of the surface energy budget equation from the prognostic temperature equation is presented and the hypotheses are discussed. Minor terms, which are usually neglected, such as tendency or advection, are estimated. Then, the two-year statistics for a station in the Ebro Valley are analyzed, focusing on the imbalance, which is found to increase as the other terms in the equation increase, with values on the order of 30% of the net radiation. The same location seen by the model of the European Center for Medium-Range Weather Forecast (ECMWF) is analyzed. Large di_erences between observations and model simulation results occur at a daily scale although the average terms are comparable, with a systematic overestimation of the ground and sensible heat uxes by the model.

Landsat and Local Land Surface Temperatures in a Heterogeneous Terrain Compared to MODIS Values

Land Surface Temperature (LST) as provided by remote sensing onboard satellites is a key parameter for a number of applications in Earth System studies, such as numerical modelling or regional estimation of surface energy and water fluxes. In the case of Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Terra or Aqua, pixels have resolutions near 1 km 2 , LST values being an average of the real subpixel variability of LST, which can be significant for heterogeneous terrain. Here, we use Landsat 7 LST decametre-scale fields to evaluate the temporal and spatial variability at the kilometre scale and compare the resulting average values to those provided by MODIS for the same observation time, for the very heterogeneous Campus of the University of the Balearic Islands (Mallorca, Western Mediterranean), with an area of about 1 km 2 , for a period between 2014 and 2016. Variations of LST between 10 and 20 K are often found at the sub-kilometre scale. In addition, MODIS values are compared to the ground truth for one point in the Campus, as obtained from a four-component net radiometer, and a bias of 3.2 K was found in addition to a Root Mean Square Error (RMSE) of 4.2 K. An indication of a more elaborated local measurement strategy in the Campus is given, using an array of radiometers distributed in the area.

The Upslope-Downslope Flow Transition on a Basin Sidewall

AbstractThe late afternoon upslope–downslope flow transition on the west inner sidewall of Arizona’s Meteor Crater, visualized by photographs of smoke dispersion, is investigated for 20 October 2006 using surface radiative and energy budget data and mean and turbulent flow profiles from three towers, two at different distances up the slope and one on the basin floor. The bowl-shaped crater allows the development of the upslope–downslope flow transition with minimal influence from larger-scale motions from outside and avoiding the upvalley–downvalley flow interactions typical of valleys. The slow downslope propagation of the shadow from the west rim causes a change in the surface radiation budget and the consequent loss of heat from the shallow atmospheric layer above the western slope at a time when the sun still heats the crater floor and the inner east sidewall. The onset of the katabatic flow is visualized by the dispersion of the smoke, and the onset occurs at the same time at the two slope towers. Th...

Comparison of Three Methods for Estimating Land Surface Temperature from Landsat 8-TIRS Sensor Data

After Landsat 8 was launched in 2013, it was observed that for Thermal Infrared sensor (TIRS) bands, radiance from outside of an instrument’s field-of-view produced a non-uniform ghost signal across the focal plane that varied depending on the out-of-scene content (i.e., the stray light effect). A new stray light correction algorithm (SLCA) is currently operational and has been implemented into the United States Geological Survey (USGS) ground system since February 2017. The SLCA has also been applied to reprocess historical Landsat 8 scenes. After approximately two years of SLCA implementation, more land surface temperature (LST) validation studies are required to check the effect of correction in the estimation of LST from different retrieval algorithms. For this purpose, three different LST estimation method algorithms (i.e., the radiative transfer equation (RTE), single-channel algorithm (SCA), and split-window algorithm (SWA)) have been assessed. The study site is located on the campus of the University of Balearic Islands on the island of Mallorca (Spain) in the western Mediterranean Sea. The site is considered a heterogeneous area that is composed of different types of surfaces, such as buildings, asphalt roads, farming areas, sloped terrains, orange fields, almond trees, lawns, and some natural vegetation regions. Data from 21 scenes, which were acquired by the Landsat 8-TIRS sensor and extracted from a 100 × 100 m2 pixel, were used to retrieve the LST with different algorithms; then, they were compared with in situ LST measurements from a broadband thermal infrared radiometer located on the same Landsat 8 pixel. The results show good performances of the three methods, with the SWA showing the lowest observed RMSE (within 1.6–2 K), whereas the SCA applied to the TIRS band 10 (10 μm) was also appropriate, with a RMSE ranging within 2.0–2.3 K. The LST estimates using the RTE algorithm display the highest observed RMSE values (within 2.0–3.6 K) of all of the compared methods, but with an almost unbiased value of −0.1 K for the case of techniques applied to band 10 data. The SWAs are the preferred method to estimate the LST in our study area. However, further validation studies around the world are required.

“Transport in the Atmosphere-Vegetation-Soil Continuum” by A. Moene and J. C. van Dam

Multidisciplinary problems usually are very difficult challenges. This is the case when the exchange of matter and energy between the surface and the atmosphere is considered, since it involves an ensemble of processes all determining the evolution of the climate system. The involvement of a number of scientific disciplines, with different methodologies and backgrounds, makes communication between interested scientists difficult and the progress of understanding may be slower than needed. Atmospheric boundary-layer scientists are familiar with similar complex problems. Usually their background is in atmospheric physics and they are concerned with the study of a complex turbulent flow. Since this flow is in contact with the surface, it is very much determined by the characteristics of the soil, the vegetation, or man-made materials. Therefore, interacting with soil physicists, hydrologists and plant physiologists is essential for improving our understanding and developing applications such as numerical models of the climate system. The book (Transport in the Atmosphere-Vegetation-Soil Continuum, Cambridge University Press, 2014, 446 pp) by A. Moene and J. van Dam attempts to respond to the needs for dialogue between the different scientific communities working at the surface-atmosphere interface. A. Moene is a meteorologist and J. van Dam a soil physicist with a good knowledge of vegetation processes. Taking advantage of their teaching expertise at the University of Wageningen in the Netherlands, they have combined their talents to produce this book for the benefit of the public. The subject matter is restricted to the atmospheric column up to 100 m above the surface, a height taken as the upper limit of the atmospheric surface layer, and the soil 3 m below the surface, at which depth the thermal annual cycle is usually absent. The analysis is mostly restricted to a single vertical column, to correlate with typical physical parametrization schemes in numerical models of the Earth system. Three-dimensional effects, such as those produced by topography or surface horizontal heterogeneity, are not treated in depth.