Main Scientific Goals
The main scientific goals of this programme are:
- High quality observations and study of precipitable water vapour (PWV) total column content and vertical profile.
- Analysis of intra-hourly variability as well as daily and annual mean cycles of PWV for different locations and altitudes of Tenerife and La Palma islands.
- Study of radiative forcing due to water vapour and clouds.
- Study of monthly and annual mean series, analyzing their homogeneity and evaluating their anomalies and evolution over time to detect posible trends.
Several measurement techniques are used in this programme.
RS-92 and RS-40 Vaisala radiosondes
From the vertical profiles of relative humidity obtained with RS-92 radiosondes, precipitable water content in the atmospheric column is calculated by integrating numerically (using the trapezoidal rule) the density function of atmospheric water vapour for the base and top of each atmospheric stratum. The integration is performed from ground level to 12 km altitude. By default, the PWV profile is supplied for the following layers: 1) from ground up to 1.5 km; 2) from 1.5 km to 3 km altitude in layers of 0.5 km thickness; 3) from 3 km altitude up to 12 km in layers of 1 km thickness.
From 13 December 2017, the RS-92 radiosondes have been replaced by RS-40 Vaisala radiosondes. The RS-40 radiosonde has a higher temporal resolution (1s) compared to the RS-92 radiosonde (2s), this results in a greater number of levels in the vertical profiles for pressure, temperature and humidity. However, at high altitudes (~14 km in the stratosphere), we have frequently detected a weak decrease in altitude in the RS-40 Vaisala Tenerife radiosonde data. It could be due to the combination between an excessively high temporal resolution and longer response times and errors of the different meteorological sensors and GNSS. For these reasons, those records are filtered from the files before they are evaluated.
At the moment, no individual corrections are being applied to the RS-92 radiosonde data in order to correct for possible inhomogeneities with respect to the RS-40 radiosonde data. Instead, we will analyze the homogeneity of the monthly mean series for the total PWV column and, if neccesary, homogenize them by adjusting the medians on the possible breakpoints.
Precipitable total water content in the atmospheric column is estimated from the absorption of water vapour in a narrow band around 941 nm from a MFRSR (Yankee Environmental Systems, Model MFR-7). From the PWV value deduced from RS92, we can characterize, on the one hand, the filter parameters of the water vapour channel using the Campanelli technique (Campanelli et al, 2010; Romero- Campos et al., 2011a), and on the other hand, through the Langley-modified technique, we can obtain the extraterrestrial irradiances for 941 nm, from which we extract the corresponding calibration constant. Finally, 1- minute PWV is obtained.
From 1996 to 2004, the MFRSR has been the radiometer used to evaluate the PWV at Izaña Observatory. Since 2004, more precise mea surements of PWV have been carried out by AERONET-Cimel Network sunphotometers but with a lower temporal resolution. On 30 January 2019, the stepper motor of MFRSR was damaged and we are currently waiting for the replacement.
Global Navigation Satellite System technique
The Global Navigation Satellite System (GNSS) technique consists in determination of PWV in the atmospheric column from the observed delay in radio signals at two different frequencies emitted by a network of Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS) satellites received in our GNSS receiver .
Global Navigation Satellite System receiver at Izaña Atmospheric Observatory.
Currently, we work with nine GNSS receiver stations at different heights, eight of them in Tenerife and one in La Palma island. The atmospheric pressure in places where the GNSS antennas are located is a key parameter for obtaining the PWV from the zenith total delay (ZTD) and zenith hydrostatic delay (ZHD).
Locations of Global Navigation Satellite System stations.
The four reference meteorological stations used to obtain accurate surface pressure records with GNSS stations are:
Reina Sofía Airport-Tenerife South, IZO, SCO and La Palma Airport. The GNSS network and data acquisition are managed by the Spanish National Geographic Institute (IGN).
The PWV is calculated from ZTD and pressure values at the stations. An important task we do is estimate the pressure in the GNSS sites where measurements of surface pressure are not available. To do this, we calculate, based on the hydrostatic equation, a mean density (weighted by gravity) of the air in the air column between the nearest reference weather stations and our GNSS station located at different altitude on the field.
The final evaluation of the PWV obtained by the three techniques described above is performed by comparing the results with each other. At IZO these techniques have been evaluated using the FTIR as reference instrument. A detailed analysis is provided in Schneider et al. (2010).
Microwave Radiometer: a new acquisition
During 2019, the reception of a high-precision microwave radiometer for continuous atmospheric profiling is planned. The acquired model is the RPG-LHATPRO-G5 series of Radiometer Physics Rohde & Schwarz Company which will allow us to obtain tropospheric temperature and humidity vertical profiles with a vertical spatial resolution of 200 m to 400 m, depending on the altitude level, and a temporal resolution of 1 second. The radiometer will be installed at IZO and it is especially designed to measure at low humidity. It works with two channels: 60 GHz oxygen absorption line for temperature profiling and 183 GHz water vapour line to obtain humidity and water vapour profiles from the brightness temperature measurement using an Artificial Neural Network (ANN) algorithm.
Botey, R., J. A. Guijarro y A. Jim nez. “Valores normales de precipitación mensual 1981 – 2010”. Agencia Estatal de Meteorología. NIPO: 281-13-007-X. 2013.
Campanelli, M., A. Lupi, T. Nakajima, V. Malvestuto, C. Tomasi and V. Estellés (2010). Summertime columnar content of atmospheric water vapor from ground-based Sun-sky radiometer measurements through a new in situ procedure. J. Geophys. Res., 115, D19304, doi: 10.1029/2009JD013211.
Hagemann, Stefan & Bengtsson, Lennart & Gendt, G. (2003). On the determination of atmospheric water vapor from GPS measurements. Journal of Geophysical Research-Atmospheres, v.108 (2003). 108. 10.1029/2002JD003235.
Lanzante, J., Resistant, Robust and Non-Parametric Techniques for the Analysis of Climate Data: Theory and Examples, including Applications to Historical Radiosonde Station Data. International Journal of Climatology, Vol. 16, 1197-1226, CCC 0899-8418/96111197-30, by the Royal Meteorological Society, 1996.
Romero Campos, P.M., Emilio Cuevas Agulló, Omaira García Rodríguez, Alberto J. Berjón Arroyo y Victoria E. Cachorro Revilla.: Aplicación de la Técnica de Campanelli para la calibración de los canales de vapor de agua de fotómetros CIMEL en el Observatorio Atmosférico de Izaña. NTD no 2. NIPO: 784-11-010-6. Centro de Investigación Atmosférica de Izaña. Agencia Estatal de Meteorología (España), 2011.
Romero-Campos P.M., Marrero C., Alonso S., Cuevas E., Afonso S., and Ortiz de Galisteo J.P.: Una Climatología del Agua Precipitable en la Región Subtropical sobre la Isla de Tenerife basada en Datos de Radiosondeos. NTD no 6 de AEMET. NIPO: 281-12-007-5. Centro de Investigación Atmosférica de Izaña. Agencia Estatal de Meteorología (España), 2011.
Schneider, M., Romero, P. M., Hase, F., Blumenstock, T., Cuevas, E., and Ramos, R.: Continuous quality assessment of atmospheric water vapour measurement techniques: FTIR, Cimel, MFRSR, GPS, and Vaisala RS92, Atmos. Meas. Tech., 3, 323-338, doi:10.5194/amt-3-323-2010, 2010.
Pedro Miguel Romero Campos (AEMET; Head of programme)
Ramón Ramos (AEMET; Head of Infrastructure)
Sergio Afonso (AEMET; Ozone and meteorological soundings expert technician)
Dr Yballa Hernández (AEMET; lidar and ceilometer Fellowship) left IARC in November 2018