The eastern Ebro basin is composed of an extensive irrigated plain, surrounded by rainfed slopes and wooden mountain ranges and open to the west to the agricultural western Ebro basin. The sea breeze ...generated at the coast is able to surmount the Catalan prelitoral range through its lowest heights, reaching the basin by its easternmost part. It is a well‐known feature in the region, called Marinada. A network of Automatic Weather Stations is used here to analyse a period of 19 years (2003–2021). A filtering procedure is developed which selects the events when the Marinada is present, based on detecting clear sky, weak wind conditions and the wind direction from the coast in the afternoon. The analysis of these days show that the Marinada propagates along the basin in the afternoon meanwhile observations of the specific humidity show a sudden increase as the temperature cools down, resulting on a cold and humid advection. It is also found that the timing of the arrival of the Marinada depends on the mesoscale/synoptical circulations already present in the region (westerlies or a thermal low).
During the afternoon of the warm months, the sea breeze generated in the Catalan coast can surmount the mountains and reach the Ebro river eastern sub‐basin (wind known as Marinada). A methodology is proposed based on surface observations to select days under the influence of the Marinada. The Marinada acts as a cold and humid advection that interacts with the presence of the locally generated winds.
Cold‐air pool evolution in a wide Pyrenean valley Conangla, Laura; Cuxart, Joan; Jiménez, Maria Antonia ...
International journal of climatology,
20/May , Letnik:
38, Številka:
6
Journal Article, Publication
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This study on cold‐air pool formation in the wide Cerdanya Valley in the Pyrenees mountain range was conducted using available observational information from September 2010 to August 2014. Cold‐air ...pools occur during almost 60% of the nights, mainly during winter. Cold pools develop even under significant synoptic pressure gradients. Additionally, drainage currents transporting air down‐valley occur most of the nights. In particular one representative cold‐air pool event has been analysed in detail by a high‐resolution mesoscale simulation, combined with an analysis of data from both ground‐based stations and satellites. Radiative processes dominate the evolution of cold‐air pools, together with turbulence in the lowest layers, while drainage flows down from the high mountains mainly through the tributary valleys and from the valley sidewall slopes play a key role in bringing air to the pool. Cold pool formation begins approximately 1 hr after sunset, and it extends across most of the valley bottom, with a very strong thermal inversion close to the surface that has a depth of up to 100 m in the lowest parts of the valley. Wind veers down‐valley along the main axis 2–3 hr after sunset and the wind direction is approximately maintained until after sunrise.
This study on cold‐air pool formation in the wide Cerdanya Valley in the Pyrenees mountain range was conducted using available observational information from September 2010 to August 2014. Cold‐air pools occur during almost 60% of the nights, mainly during winter. Cold pools develop even under significant synoptic pressure gradients. Additionally, drainage currents transporting air down‐valley occur most of the nights. In particular, one representative cold‐air pool event has been analysed in detail by a high‐resolution mesoscale simulation. (c) Difference between the local spatial deviation of the LST at the end of the night (October 1–2, 2011) and at the beginning. The blue polygon notes the area selected as the Cerdanya main valley, also imposing an altitude lower than 1,500 m asl. Crosses show the AWS locations.
The evolution of the nocturnal atmospheric boundary layer at the foothills of a major mountain range is studied, exploring the influence of thermally driven winds. Observations made in Lannemezan (in ...the foothills of the Pyrenees, about 10 km from the exit of the Aura Valley) during the Boundary‐Layer Late Afternoon and Sunset Turbulence (BLLAST) experimental field campaign are taken together with high‐resolution mesoscale simulations of some selected intensive observation periods. We find that the nocturnal boundary layer features in Lannemezan are different from those reported in flat terrain regions due to the influence of thermally driven winds. Close to sunset, a thermal gradient between the plain and the mountains favours the initiation of mountain–plain circulations, while at the mountain slopes locally generated downslope winds are formed. The organization of the flow in the Aura Valley generates a valley exit jet close to midnight, which propagates through the foothills while its speed and height decrease. As a result, a maximum wind speed of about 5–10 m/s from the southern sector is found in Lannemezan between 50 and 200 m above the ground, depending on the intensity and direction of the mesoscale winds. Turbulence (above and below the jet nose) and the jet features affect the relative importance of the terms of the surface energy balance and the temperature evolution at lower levels. The organization of the flow at lower levels in the Aura Valley and the thermal gradient between the valley and the foothills are the most important factors in the formation and maintenance of the valley exit jet, strongly modulated by the intensity and direction of the mesoscale winds.
50 m‐wind vectors at 0400 UTC in the region covering the Aura Valley and the Pyrenees foothills where Lannemezan is located.
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.
Abstract
Measurements in seven sites in the Campus of the University of the Balearic Islands (UIB; Mallorca, Spain) during an experimental campaign to study the contribution of local surface ...heterogeneities on the surface energy budget at one point have been used to characterize the differences in extreme daily temperatures between the sites during a summer month. Absolute temperature differences in this month reached up to 1.92 (with a median of 0.73) and 2.02 (median of 1.21)°C for daily maximum and minimum, respectively. Higher differences in the minimum temperature can be attributed to the stably stratified and weak turbulent conditions at night that enhance local differences in the surface energy fluxes, especially in an area with strong variability of the surface characteristics like the UIB Campus. Instead, during daytime, maximum temperature differences are smoothed due to the convection and the horizontal advection due to the sea‐breeze. Two sites with longer records allowed to study the seasonal variations of these differences, which were substantially lower in the colder months. These results suggest that relocation of observatories, even at distances as short as 200 m, may introduce important inhomogeneities in the temperature series. Therefore, raw values of series from nearby stations should not be used to infill missing data in other series without adequate statistical adjustments.
A simple conceptual view of downslope flows allows a derivation of a diagnostic relationship between the maximum depth of the flow, its speed, the slope angle and the cooling flux. This relationship ...is obtained considering that the turbulence cooling causing the air to flow downslope is essentially compensated by the warming by compression as the flow reaches higher pressure levels. The obtained relationship is consistent with the bulk heat budget for the along‐slope flows, and its agreement with existing prognostic layer‐averaged models is checked. Finally, the depth of the flow obtained from the diagnostic relationship is compared against the observational estimations of katabatic flows from two experimental campaigns.
A simple formula for downslope flows is presented and compared to recently obtained experimental data. It relates the wind speed, the slope angle, the surface cooling flux, the depth of the flow, and the background thermal stratification. It is consistent with previous findings based on the bulk heat budget equation for the along‐slope wind. The figure shows the variation of the depth of the downslope flow (Dmax) allowing only one variable to change. The values for the variables when fixed are v = 2 m·s−1, H = −0.01 K·m·s−1, ∂
zθ = 0.0033 K·m−1, and α = 15°.
The effect of terrain heterogeneities in one-point measurements is a continuous subject of discussion. Here we focus on the order of magnitude of the advection term in the equation of the evolution ...of temperature as generated by documented terrain heterogeneities and we estimate its importance as a term in the surface energy budget (SEB), for which the turbulent fluxes are computed using the eddy-correlation method. The heterogeneities are estimated from satellite and model fields for scales near 1 km or broader, while the smaller scales are estimated through direct measurements with remotely piloted aircraft and thermal cameras and also by high-resolution modelling. The variability of the surface temperature fields is not found to decrease clearly with increasing resolution, and consequently the advection term becomes more important as the scales become finer. The advection term provides non-significant values to the SEB at scales larger than a few kilometres. In contrast, surface heterogeneities at the metre scale yield large values of the advection, which are probably only significant in the first centimetres above the ground. The motions that seem to contribute significantly to the advection term in the SEB equation in our case are roughly those around the hectometre scales.
Irrigation in semi-arid regions induces thermal heterogeneity across a range of spatial scales that impacts the partitioning of energy at the surface, the development of the atmospheric boundary ...layer, and the bidirectional interactions between the atmosphere and the surface. In this analysis, we use data from the Land Surface Interactions with the Atmosphere in the Iberian Semi-Arid Environment (LIAISE) experiment combined with a coupled land–atmosphere model to understand the role of the scales of irrigation-induced, thermal heterogeneity on the surface fluxes and consequently, the development of the diurnal convective boundary layer. The surface heterogeneity is characterized by Bowen ratios that range from ∼0.01 in the irrigated areas to ∼30 in the non-irrigated areas; however, the observed boundary-layers dynamics in both locations are similar. In this analysis, we address the questions of how the surface fluxes impact the development of the boundary-layer dynamics and how the boundary layer influences the diurnal cycle of surface fluxes. To interpret the observations, we introduce a heterogeneity scaling scheme where length scales range from local scale (∼100 m) to regional scale (∼10 km) to investigate the role of scale on surface representation in numerical models and to address the discrepancy between surface observations and their representation in weather and climate models.
We find that at the surface, both the available energy and its partitioning depend on spatial scale. The observed boundary-layer properties can be explained through the composite of surface fluxes at the regional scale. Surface fluxes at the local scales are unable to replicate the observed boundary layer — even when including large-scale contributions. We find that non-local boundary layer processes like advection are important for partitioning energy at the local scale. We explore the connection between surface fluxes and the development of the boundary layer and the potential non-local effects on boundary-layer development.
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•Surface energy partitioning depends on the scale of the heterogeneity.•The observed Bowen ratios range from 0.01–30, but the boundary layers were similar.•The observed boundary layer is formed by surface fluxes at the regional scale.•At local scales, non-local boundary-layer processes impact surface fluxes.
The turbulent structure parameters of temperature (
C
T
2
) and humidity (
C
Q
2
), and their cross-structure parameter (
C
Q
T
), are investigated using data collected with the airborne-measurement ...platform Helipod during the LITFASS-2003 campaign. The flights took place within the atmospheric surface layer over heterogeneous terrain including forests, a lake and farmland. We find variability in
C
T
2
along such flight legs, with values of
C
T
2
over forested surfaces one order of magnitude larger than over farmland, and two orders of magnitude larger than over the lake. However, a quantitative relationship between the magnitude of
C
Q
2
and the surface type is not found, most likely due to a similar surface latent heat flux between the land-use types. However, when the different flight legs are taken together and data grouped by land-use type, values of
C
Q
2
are significantly lower over the lake than over the other surfaces. A classification of
C
Q
T
is only possible between water and land surfaces, with lower values over water. We find the correlation coefficient
R
Q
T
in the range of 0.4–1.0, which is less than unity, and thus violates the assumption of unity in Monin–Obukhov similarity theory.
The energy, momentum and mass exchanges between the soil and the atmosphere take place in the surface layer. The difference between the screen temperature of the air at 2 m (T2) and the land surface ...temperature (LST) is a key parameter in their determination. Here an analysis of this difference is made with data from a site at the island of Mallorca, in the Western Mediterranean. While previous studies are often built for relatively short temporal intervals (from several days to few months), the current work analyzes 2-year-long series with a 30 min temporal resolution, in particular the diurnal and seasonal variability. The uncertainty of LST measured from near the ground is estimated to be at least 2
∘
C. The difference between T2 and LST (T2–LST) in the center of the day ranges between − 3 and − 10
∘
C, arriving at about − 18
∘
C for dry summer days. Statistically it is shown that these values are related to the net radiation heating of the surface transmitted upwards by thermal turbulence. The values in summer are much larger because the upper dry soil does not allow the heat to be transmitted efficiently into the ground. At night T2–LST is usually under 3
∘
C, with weak correlations of T2–LST with net radiation or ground flux, indicating that these processes are active and efficient transporting heat in most cases. The better correlated quantities at night with T2–LST are the upper soil temperature, the LST and the turbulent heat flux, reflecting the importance of the state of the surface. In summer nights T2–LST takes very small values, below 1
∘
C, even allowing unstable stratification. This is a result of a soil in which the first centimeters are very warm after sunset, not being able to send heat into the dry soil and releasing it into the atmosphere.
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