By Carlos Román Cascón, Carlos Yagüe and J. Fidel González-Rouco, GuMNet, Complutense University of Madrid, Spain.
She was born during the summer of 2016 at La Herrería Forest (El Escorial), 40 km Northwest from the busy city of Madrid, and a few hundred meters from the quiet Royal Monastery of San Lorenzo de El Escorial – where many Spanish Kings from the last centuries rest. But the most important thing was that she was situated just at the foothills of the majestic Mount Abantos (1753 m), a prominent peak of the Guadarrama Mountain Range.
With this beautiful environment, it is not strange that she started talking just a few days later after her birth, telling us details about how her day was. That was the 7th of June of 2016, but she has been doing the same during more than six years now. Indeed, her everyday life is sometimes quite cyclical, which is good for her, and for us. But, as everyone, she also has some days in which everything is different, which is also good for her, and for us.
Although she does not have an official name yet, many people know her as:
The Surface-Energy-Balance (SEB) tower of La Herrería Forest
Or just La Herrería (HER) or la torre, in Spanish. These are the most common names in which this robust tower, belonging to Guadarrama Monitoring Network (GuMNet) [1,2], is cited in the scientific literature. Her perseverance, her peculiarity, and her excellent service deserved a post in this blog.
The routine life of la torre was indeed scheduled by the Mount Abantos closeness, which causes recurrent and cyclical winds during fair-weather days, conditions that are relatively typical in the area. These winds – known as mountain breezes – are characterised by wind blowing down the slope during the nighttime (katabatic) and upslope during the daytime (anabatic) [3,4]. But she has also experienced periods characterised by extreme conditions: in summer after long periods without rain leading to dry soils  and in winter days under cold temperatures and snowy ground, as happened during the post-period of the famous Filomena storm , in January 2021.
Apart from the special and challenging environment where the tower is, her singularity also falls in her capability for measuring all the components that participate in the surface energy balance (SEB). Firstly, the sensors located at the upper part of the 4-component radiometer measure the incoming shortwave (solar, SW) and longwave (atmospheric, LW) radiation that reaches the surface, while the sensors at the bottom part are oriented towards the ground to capture the outgoing amount of solar energy reflected by the surface and the terrestrial emission (LW). The sum of the incoming components minus the outgoing ones provides us with the radiative energy available at the surface, commonly known as the Net Radiation (Rnet):
Rnet = SW↓ + LW↓ – SW↑ – LW↑
From the measurements of HER, we can expect a positive Rnet during daytime, negative during nighttime, and positive during the whole diurnal cycle. This means that some energy is available, free, there…
This net radiation, which resulted from the play between the solar radiation, the surface, and the atmospheric and land surface radiative properties, is now employed by the surface turbulent fluxes to redistribute the heat in the lower atmosphere. Here, the term “heat” redistribution includes both the sensible and the latent heat fluxes (SH and Le, respectively). While the former is related to the vertical differences in temperature, the latter is due to vertical differences in water vapour concentration (which contains “latent” energy awaiting changes in phase).
And that’s when the high-frequency sensors of la torre become the key players. The IRGASON instrument can measure the three components of the flow – both the horizontal (u, v), and the vertical (w) ones -, the temperature (T), and the concentration of water vapour (q). But what makes this instrument special is that it does it at a very high frequency, namely 10 or 20 times per second. This is needed to apply the so-called Eddy-Covariance (EC) technique, traditionally considered as the most reliable technique to calculate surface turbulent fluxes. These fluxes are characterized by irregular, turbulent, and chaotic swirls of different sizes (known as eddies) that are almost always invisible and that can transport this energy. Although this seems something difficult to measure, the reality is that the use of the EC technique makes possible the estimation of the vertical energy transport from the IRGASON measurements (using covariances, wT for SH and wq for Le). This is done thanks to the fast and precise measurements, used to know when the former part of a small eddy is arriving to la torre and when it is leaving her, always transported (or “advected”) by the mean wind. And we can do this for different eddy sizes, just statistically analysing the evolution in time of the covariance of two variables and adding up the contribution from the different eddy sizes. With these atmospheric fluxes, we already have the vertical transport in the atmosphere, but this is not enough yet…
What is happening in the soil? Within the soil, there exists another flux that also appears due to the available energy that was heating the surface during the daytime (or to the loss of energy that was cooling it during nighttime). As in the lower atmosphere, this effect at the surface causes temperature gradients that stimulate the fluxes. And la torre can also measure it within the soil, now thanks to the heat flux plate that is buried at around 4 cm depth, providing us with the soil heat flux (G) that closes the SEB:
Rnet = SH + Le + G
This equation points out the closure of the SEB, something that physically should happen in the nature to avoid a continuous heating of the surface. However, the SEB closure is still one of the main challenges of the current studies in observational micrometeorology. But the long-time series that la torre have already provided us with have served to investigate this topic at this challenging site, with very recent interesting results! .
And if that was not enough, she continues telling us histories about her life, also taking high-quality measurements of atmospheric variables at different heights: temperature, humidity, wind speed, wind direction, or atmospheric pressure. Besides, the IRGASON measures the CO2 concentration at a high frequency [4, 8], which is also used to calculate the CO2 flux by means of the EC technique. Last but not least, all these data are also complemented by continuous soil temperature and soil moisture measurements at different depths, the latter also telling us histories about the past rainfall, a variable that is also measured at the site.
The uniqueness of the commented measurements provided by La Herrería tower have fed numerous scientific studies aimed at investigating different topics. We hope she can continue telling us histories about her life in this awesome environment for several years, something needed to feed the scientific studies done by researchers interested in improving the knowledge of the land-atmosphere interactions in mountainous areas.
Lastly, we cannot forget that she (and us), we are all very grateful to the unconditional help from Patrimonio Nacional del Estado (Delegación S. L. de El Escorial), to the support of the Spanish Meteorological Agency (AEMET) and the funding from the Spanish Government through the research projects ATMOUNT-II (Ref. CGL2015-65627-C3-3-R and LATMOS-I (Ref. PID2020-115321RB-I00).
 Vegas-Cañas, C.; González-Rouco, J.F.; Navarro-Montesinos, J.; García-Bustamante, E.; Lucio-Eceiza, E.E.; García-Pereira, F.; Rodríguez-Camino, E.; Chazarra-Bernabé, A.; Álvarez-Arévalo, I. An Assessment of Observed and Simulated Temperature Variability in Sierra de Guadarrama. Atmosphere 2020, 11, 985. https://doi.org/10.3390/atmos1109098
 Arrillaga, J.A., and the GuMNet Consortium Team. GuMNet – A High Altitude Monitoring Network in The Guadarrama Mountain Range (Spain). 22nd Symp. Boundary Layers and Turbulence. American Meteorological Society (AMS). Salt Lake City, 2016.
 Arrillaga, J.A., Yagüe, C., Román-Cascón, C., Sastre, M., Maqueda, G. & Vilà-Guerau de Arellano. J. From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO2 variability. Atmospheric Chemistry and Physics, 19, 4615-4635, 2019. https://doi.org/10.5194/acp-19-4615-2019
 Román-Cascón, C., Yagüe, C., Arrillaga, J. A., Lothon, M., Pardyjak, E.R., Lohou, F., Inclán, R.M., Sastre, M., Maqueda, G., Derrien, S., Meyerfeld, Y., Hang, C., Campargue-Rodríguez, P. & Turki, I. Comparing Mountain breezes and their impacts on CO2 mixing ratios at three contrasting areas. Atmospheric Research, 221, 111-126, 2019. https://doi.org/10.1016/j.atmosres.2019.01.019
 Valverde, J. How does the soil moisture impact the surface energy fluxes? An observational study at “La Herrería” Forest. Master thesis (Physics Faculty, UCM). 2020.
 Fernández-Castillo, P. Storm Filomena. Analysis of an extraordinary snowfall event in Spain. Degree thesis (Physics Faculty, UCM). 2022.
 Pocino, J. A. Surface Energy Balance (SEB) closure analysis under different environmental conditions. Master thesis (Physics Faculty, UCM). 2022.
 Rodicio, A. Study of the annual cycle of greenhouse gases in the area of “La Herrería” Forest (Guadarrama Mountains). Master thesis (Physics Faculty, UCM). 2018.