# Observations of surface radiation and stratospheric processes at Thule Air Base, Greenland, during the IPY

Giovanni Muscari, Claudia Di Biagio, Alcide Di Sarra, Marco Cacciani, Svend Erik Ascanius, Pietro Paolo Bertagnolio, Claudio Cesaroni, Robert L. de Zafra, Paul Eriksen, Giorgio Fiocco, et al.

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Giovanni Muscari, Claudia Di Biagio, Alcide Di Sarra, Marco Cacciani, Svend Erik Ascanius, et al. Observations of surface radiation and stratospheric processes at Thule Air Base, Greenland, during the IPY. Annals of Geophysics, 2014, 57 (3), 10.4401/ag-6382. insu-03448811

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Special section: Geophysical monitorings at the Earth's polar regions

Observations of surface radiation and stratospheric processes
at Thule Air Base, Greenland, during the IPY

Giovanni Muscari<fnref n="1" />,*, Claudia Di Biagio<fnref n="2" />,<fnref n="3" />,<fnref n="7" />, Alcide di Sarra<fnref n="2" />, Marco Cacciani<fnref n="4" />,
Svend Erik Ascanius<fnref n="5" />, Pietro Paolo Bertagnolio<fnref n="1" />,<fnref n="3" />, Claudio Cesaroni<fnref n="1" />,<fnref n="4" />, Robert L. de Zafra<fnref n="6" />,
Paul Eriksen<fnref n="5" />, Giorgio Fiocco<fnref n="4" />, Irene Fiorucci<fnref n="1" />, Daniele Fuà<fnref n="4" />

Article history

Received June 30, 2013; accepted October 25, 2013.

**Subject classification:**

Polar atmosphere, NDACC, Radiative budget, Stratospheric ozone.

ABSTRACT

Ground-based measurements of atmospheric parameters have been car-
ried out for more than 20 years at the Network for the Detection of At-
mospheric Composition Change (NDACC) station at Thule Air Base
(76.5 °N, 68.8 °W), on the north-western coast of Greenland. Various in-
struments dedicated to the study of the lower and middle polar atmos-
phere are installed at Thule in the framework of a long standing
collaboration among Danish, Italian, and US research institutes and uni-
versities. This effort aims at monitoring the composition, structure and
dynamics of the polar stratosphere, and at studying the Arctic energy
budget and the role played by different factors, such as aerosols, water
vapour, and surface albedo. During the International Polar Year (IPY),
in winter 2008-2009, an intensive measurement campaign was conducted
at Thule within the framework of the IPY project "Ozone layer and UV
radiation in a changing climate evaluated during IPY" (ORACLE-O3)
which sought to improve our understanding of the complex mechanisms
that lead to the Arctic stratospheric O3 depletion. The campaign involved
a lidar system, measuring aerosol backscatter and depolarization ratios
up to 35 km and atmospheric temperature profiles from 25 to 70 km alti-
tude, a ground-based millimeter-wave spectrometer (GBMS) used to de-
rive stratospheric mixing ratio profiles of different chemical species
involved in the stratospheric ozone depletion cycle, and then ground-based
radiometers and a Cimel sunphotometer to study the Arctic radiative
budget at the surface. The observations show that the surface radiation
budget is mainly regulated by the longwave component throughout most
of the year. Clouds have a significant impact contributing to enhance the
role of longwave radiation. Besides clouds, water vapour seasonal changes

produce the largest modification in the shortwave component at the sur-
face, followed by changes in surface albedo and in aerosol amounts. For
what concerns the middle atmosphere, during the first part of winter
2008-2009 the cold polar vortex allowed for the formation of polar stratos-
pheric clouds (PSCs) which were observed above Thule by means of the
lidar. This period was also characterized by GBMS measurements of low
values of O3 due to the catalytic reactions prompted by the PSCs. In mid-
January, as the most intense Sudden Stratospheric Warming event ever
observed in the Arctic occurred, GBMS and lidar measurements of O3,
N2O, CO and temperature described its evolution as it propagated from
the upper atmosphere to the lower stratosphere.

**1. Introduction**

In 1988, the atmospheric physics group of the
"Sapienza" University of Rome (UR) and the Danish
Meteorological Institute (DMI) started planning the in-
stallation of a lidar for the investigation of the Arctic
stratosphere at Thule Air Base (TAB; 76.5°N, 68.8°W),
Greenland, where the DMI had started setting up a sci-
entific station dedicated to the Arctic climate. Thule Air
Base is a military base of the United States Air Force
(USAF), founded in 1951 and located on the north-west-
ern coast of Greenland, bordering Baffin Bay. The main
base is located in a valley facing westward to the sea
and enclosed between South Mountain and North
Mountain, which each have an elevation of about 250
m above sea level. Thule is equipped with a dock, op-

<footnote n="1">¹ Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy</footnote>

<footnote n="2">² ENEA, Laboratory for Earth Observations and Analyses, Santa Maria di Galeria, Italy</footnote>

<footnote n="3">³ Università di Siena, Dipartimento Scienze fisiche, della Terra e dell'ambiente, Siena, Italy</footnote>

<footnote n="4">⁴ La Sapienza - Università di Roma, Dipartimento di Fisica, Rome, Italy</footnote>

<footnote n="5">⁵ Danish Meteorological Institute, Copenhagen, Denmark</footnote>

<footnote n="6">⁶ State University of New York, Department of Physics and Astronomy, Stony Brook, NY, USA</footnote>

<footnote n="7">⁷ Now at LISA, UMR CNRS 7583, Universités Paris Est Créteil et Paris Diderot, Institut P.S. Laplace, Créteil, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Left: geographical map of Greenland, with the position of Thule Air Base indicated with a red arrow. Right: aerial view of Thule.</figcaption></figure>

erational only in summertime, and an airport which is operational all year round. An aerial view of the base is shown in Figure 1.

The Rayleigh lidar was installed at Thule during fall 1990 (see Figure 2). The main scientific objective was to improve knowledge of the stratospheric ozone depletion phenomenon that was observed to be very intense over Antarctica and anticipated to become important also over the Arctic. In particular, the collaboration set out to detect stratospheric aerosol/cloud particles by means of the UR ground-based lidar system which added to the DMI capability to measure stratospheric ozone profiles inside the polar vortex by ozonesondes and total ozone with a SAOZ spectrometer. The installation of the lidar took approximately 3 weeks of intense field work and involved 4 researchers from UR and DMI as well as several technicians and carpenters which set up electrical switches for the laser and built the roof hatch for the lidar's telescope. Initially, the lidar had only two receiving channels, used to observed the backscattered parallel- and cross-polarized signals from the stratosphere, which were digitalized and then saved by means of a 286 Olivetti PC.

During the following ~20 years, also in the context of large European measurement programs (i.e., the European Arctic Stratospheric Ozone Experiment, EASOE; the Second European Stratospheric Arctic Ozone Experiment, SESAME, and others), the DMI expanded its activities at Thule, additional instruments were installed, and the collaboration with the University of Rome was extended to other Italian (Istituto Nazionale di Geofisica e Vulcanologia - INGV, and ENEA) and US (National Center for Atmospheric Research - NCAR, and Stony Brook University) research institutes. In particular, in January 2009, the participation of DMI, UR, ENEA, and INGV to International Polar Year (IPY) activities were partly supported by the US National Sci-

ence Foundation, the Italian Antarctic Programme (PNRA) and Ministry for University and Research, and allowed the installation of additional instruments. The ground-based millimeter-wave spectrometer (GBMS) (Figure 3) was brought back to Thule (after having conducted 2 successful campaigns measuring the stratospheric trace gases N₂O, O₃ and ClO in 1992 and 1993, and again in 2001-2002 and 2002-2003) [e.g., Muscari et al. 2007] to contribute to the knowledge of stratospheric chemistry with measurements of stratospheric O₃ and HNO₃. The measurements during the IPY were conducted within the IPY project "Ozone layer and UV radiation in a changing climate evaluated during IPY" (ORACLE-O3) aimed at improving our understanding

<figure><img src="image_3.png" /><figcaption>**Figure 2.** The lidar installed at Thule.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** The GBMS installed at Thule, with the receiver facing the mm-wave window.</figcaption></figure>

of the complex mechanisms that lead to the Arctic stratospheric O$_3$ depletion, therefore including the physico-chemical processes that form the polar stratospheric clouds and the heterogeneous reactions that take place on the surface of PSC particles.

At the South Mountain laboratory (building #1985, at a 20-minute drive from main base), the NCAR and the DMI installed a Fourier Transform InfraRed spectrometer (FTIR) and a UV spectrometer in the late 90s. These instruments, together with the lidar and the GBMS, actively contribute to the Network for Detection of Atmospheric Composition Change (NDACC; http://www.ndsc.ncep.noaa.gov/), which is a global network of measurements stations for the monitoring of the atmospheric composition and climate.

In addition to observing processes that affect the stratospheric ozone, the NDACC station at Thule is dedicated to study the parameters that play a key role

in regulating the Arctic climate and its influence on the global scale. In fact, the Arctic region is among the most sensitive to climate change, due to peculiar conditions and some specific phenomena linked to the evolution of sea ice, its influence on the radiation budget and the involved feedback mechanisms [McGuire et al. 2006, Serreze and Francis 2006]. A growing attention has been dedicated in recent years to the study of the role of atmospheric aerosols, and in particular to the phenomenon of the Arctic haze [Quinn et al. 2007], which may affect the Arctic radiative balance. In order to investigate these phenomena, in 2002 the DMI installed a radiometer on the roof of building #1985 for measuring the ground-level shortwave irradiance (see Figure 4). In 2007 a Cimel sunphotometer (part of the AErosol RObotic NETwork, or AERONET; http://aeronet.gsfc.nasa.gov/) for the measurement of aerosol physical properties and column water vapour was added

<figure><img src="image_5.png" /><figcaption>**Figure 4.** The TSP-700 and the PIR 33499 installed on the roof of buiding #1985.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** The Cimel installed on the roof of building #1985.</figcaption></figure>

to it (see Figure 5), and in 2009 the lidar was upgraded through the addition of two new receiving telescopes for the investigation of the troposphere and surface boundary layer (see Figure 6).

In this study the surface radiative budget and the evolution of the stratospheric structure and composition at Thule during the IPY will be described using data collected with ground-based radiometers, the Cimel sunphotometer, the GBMS, and the lidar.

## 2. Instruments description

As previously discussed, different instruments are installed at Thule for measuring atmospheric composition and structure, the radiation budget at the surface, and the aerosol physical properties. The following sections contain a description of the instruments whose observations are used for these studies.

### 2.1. *The Rayleigh LIDAR*

The LIDAR (Light Detection And Ranging) is an active remote sensing instrument based on the same principle of operation of a radar but with a radiation source, a laser, emitting in the UV, visible, or near IR spectral range. Molecules and particles present in the atmosphere interact with the laser emitted light pulses through processes of scattering and absorption, and return an echo signal which is measured by the lidar receiving system.

A picture and the scheme of the lidar installed at TAB are shown in Figures 2 and 6. The emitting unit is a two-stage Nd:YAG laser with a second harmonic generator producing linearly polarized pulses at 532 nm, with a repetition rate of 10 Hz. A prism is used to direct the laser beam vertically into the atmosphere. The receiver

includes a 800 mm diameter Cassegrain telescope for the upper troposphere and middle atmosphere, and two 64 mm diameter telescopes for the troposphere. The signal collected by each of the three telescopes is focused and filtered to select only elastic backscattered signals. Each channel is equipped with a photomultiplier (PMT). A chopper is used in combination with the 800 mm telescope to cut off the signal from the lowest atmospheric levels to prevent the saturation of the photomultiplier tubes.

The acquisition system is composed by electronic units that include both analog and photon counting acquisition channels. The analog signal detection is used for the low altitude channels, while the photon counting is preferred for the high altitude ones. The total signal and the parallel polarized component (with respect to the polarization of the emitted radiation) are separately acquired for the low channels, while the parallel and the cross polarized components of the backscattered signal are acquired for the high channels.

The lidar has been operational during several years since its installation in 1990, particularly during winter seasons [i.e., di Sarra et al. 1992, Di Girolamo et al. 1994, Larsen et al. 1994, di Sarra et al. 1995, Marenco et al. 1997, di Sarra et al. 2002, Muscari et al. 2007, Di Biagio et al. 2010]. In 1993 the capability to perform daytime measurements of clouds and particles in the upper troposphere and lower stratosphere was added [di Sarra et al. 1998]. This improvement allowed to follow the evolution of stratospheric particles in the lower stratosphere after the 1991 eruption of volcano Pinatubo. At the same time, nighttime measurements of the middle atmosphere temperature profile were started [Marenco et al. 1997, Muscari et al. 2007].

During the IPY the lidar operational at TAB derived measurements of:

(1) aerosol backscatter and depolarization ratio profiles from 0.2 to 25-30 km altitude with a vertical resolution of 30 m and a temporal resolution of few minutes. Aerosol measurements were made both in daytime and nighttime;

(2) density and temperature profiles from 25-35 to 70 km with a vertical resolution of about 4.5 km. To increase the signal-to-noise ratio the signal was integrated for 1-5 hours (the integration time mainly depends on the weather conditions and on the possibility to make measurements). Density and temperature measurements were possible only during nighttime.

### 2.2. *Ground-based millimeter-wave spectrometer (GBMS)*

The ground-based millimeter-wave spectrometer (GBMS) [de Zafra 1995] measures rotational emission spectra of atmospheric chemical species such as O<sub>3</sub>,
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Scheme of the lidar installed at Thule; the receiving system for the upper atmosphere (high altitude channels) is on the left, while the two receivers for the low altitudes are on the right.</figcaption></figure>

$N_2O$, CO and $HNO_3$, as well as the $H_2O$ continuum, with a spectral window of 600 MHz tunable between approximately 230 and 280 GHz (or 7.7 and 9.3 cm⁻¹) [e.g., Cheng et al. 1996, de Zafra et al. 1997, Muscari et al. 2007, Fiorucci et al. 2008]. Figure 3 shows the GBMS in its present setup at Thule. It was designed and built at the Physics and Astronomy Department of the State University of New York at Stony Brook and comprises a front end receiver employing a cryogenically cooled SIS (superconductor-insulator-superconductor) double sideband mixer with an intermediate frequency (IF) of 1.4 GHz. The back end is composed of an acousto-optical spectrometer (AOS) with a spectral band pass of 600 MHz and a maximum resolution of 65 kHz. Details of the apparatus and observing method are covered in Parrish et al. [1988] and de Zafra [1995]. A condensed description of the observing method is outlined below. The GBMS observes the emission line of one chemical species at a time, usually for 1 to 5 hours (depending on S/N). Its 600 MHz window is then tuned to a different frequency interval (in the 230-280 GHz range) in order to observe the emission line of another chemical species.

Given the physical parameters of a specific rotational transition, its spectral line shape depends strongly on the vertical concentration profile of the observed species (typically unknown), strongly on the atmospheric pressure profile and less strongly on the temperature profile (both typically known). Therefore, by means of the observed line shape together with pressure and temperature vertical profiles, a mathematical

deconvolution process allows finding the emitting molecule's concentration as a function of altitude through various mathematical 'deconvolution' procedures [e.g., Rodgers 2000]. The overall spectral band pass and resolution of the GBMS are key elements to determine the 17-75 km altitude range where trace gases concentration can be measured. For water vapor, we do not observe an $H_2O$ emission line (e.g., at 183 or 325 GHz) but rather the emission from the $H_2O$ continuum existing between emission lines. Therefore, only the integrated column contents can be obtained from GBMS observations of water vapor [Fiorucci et al. 2008].

During normal data-taking operations the GBMS observes radiation from two different directions 75° to 80° apart, switched by a rotating reflective semicircular chopper wheel at ~1 Hz frequency. One observing direction is near the zenith (reference beam, or R) while the other points between 10° and 15° above the horizon (signal beam, or S). A dielectric sheet (made of Plexiglas) mounted in the R beam acts as a local partially transparent (and weakly emitting) 'grey body' source of broadband radiation to compensate for the lower total power received from atmospheric emission near the zenith (with a shorter geometrical path length with respect to the S beam), and allows a power balance to be achieved between the S and R beam directions. Different dielectric sheets are used, depending on atmospheric conditions, with their opacity ranging from ~0.2 to 0.8 Nepers at 275 GHz. This power balance is sensed by a phase-sensitive detector synchronized to the rotation of the beam-
switching chopper wheel, and maintained by a servo sys-
tem which adjusts the elevation angle of the S beam if
the (S-R) power level deviates from zero. As atmospheric
opacity increases or decreases (causing thermal emission
from the atmosphere to increase or decrease), the servo
mechanism, seeking to maintain power balance in the
two beams, will drive the S-beam angle upwards or
downwards. The opacity of the dielectric sheet, depend-
ent on its composition and thickness, will determine the
equilibrium angles for a given range of atmospheric opac-
ity. Atmospheric radiation from both beam directions en-
ters through a window made of type PP-2 Eccofoam
characterized by a very small opacity at millimeter wave-
lengths (~0.007 Nepers). This sensitivity of observing-
beam elevation angle to total water vapor opacity allows
a continuous measure of integrated water column
overhead, while the spectral data (line shape and inten-
sity), deconvolved against measured temperature and
pressure profiles, allows retrieval of the mixing ratio of
a particular molecular trace gas with altitude. Overall
accuracy estimates for various molecular mixing ratio
profile retrievals are altitude and species dependent,
and are given in Parrish et. al. [1988], de Zafra [1995],
and Fiorucci et al. [2011].

## 2.3. Broadband shortwave and infrared radiometers

Different pyranometers and pyrgeometers for the
measurement of the solar and infrared irradiance at
the surface have been installed at Thule in different pe-
riods starting from 2002. All the radiometers were
placed on the roof of building #1985 at South Moun-
tain (Figure 4).

A Yankee Environmental System ventilated Total
solar pyranometer (YES TSP-700) was installed by
DMI in 2002. The TSP is a WMO (World Meteorolog-
ical Organization) second class pyranometer and meas-
ures shortwave global irradiance in the spectral range
0.3-3.0 µm, ISW. The TSP was calibrated in 1998, and
checked once a year after its installation with a refer-
ence lamp: the integrated lamp irradiance measured by
the TSP in the different years is always within 2% of the
lamp nominal emission. The total uncertainty on the
shortwave global irradiance measurements is less than
2.5% for a solar zenith angle θ≤75° and less than 3.4%
for 75°<θ≤ 85° [Di Biagio et al. 2012]. In the period
February-August 2009 also an Eppley precision spectral
pyranometer (PSP) was installed at TAB by ENEA to
be compared with TSP. A further comparison with an-
other recently calibrated PSP was carried out at Thule
during the field campaign 2010-2011. Data indicate that
the TSP and PSP radiometers agree within ±3% for
θ<70%, thus confirming the TSP accuracy of ±2.5%.

Because of the annual evolution of the solar il-

lumination at TAB, the TSP is installed from mid-Feb-
ruary to mid-October each year. The minimum of the
solar zenith angle is ~52° at the summer solstice.

A ventilated Eppley precision infrared radiometer
(PIR) was installed at Thule in February 2009. The PIR
is a WMO first class pyrgeometer that measures the
downward longwave irradiance in the spectral range
3.5-50 µm, ISW. The downward irradiance is derived
from the measured signal and instrument temperatures
using the Albrecht and Cox [1977] formula. The PIR
calibration was verified by comparison with a recently
calibrated PIR installed at the ENEA station at Lampe-
dusa (http://www.lampedusa.enea.it/) in 2008. Addi-
tional calibrations were made at Thule by comparison
with two CGR4 pyrgeometers during 2010 and 2011.
The calibration error is estimated to be less than 1%.
The total uncertainty associated with the PIR meas-
urements is less than 1.7%.

The radiometers installed at #1985 have the hori-
zon free from obstacles; the domes of the instruments
are cleaned weekly and the data are acquired every
minute. An automatic weather station is also opera-
tional on the roof of building #1985.

## 2.4. The Cimel sunphotometer

A Cimel sunphotomer (Figure 5) [Holben et al.
1998] has been operational at Thule since 2007. The
Cimel sunphotometer measures direct sun and sky ra-
diances at eight narrowband channels (340, 380, 440,
500, 670, 870, 940, and 1020 nm). Cimel measurements
are used to derive the aerosol optical depth and the
water vapour column content, as well as other aerosol
physical properties. The Cimel is operated at Thule in
the period March-October, when the Sun is above the
horizon at least during part of the day.

The Cimel (see Figure 5) is composed by a sensor
head fixed to a robot base, which in turn is connected
to two motors that allow its rotation over the horizon-
tal and the vertical planes; the sensor head is connected
to a 33 cm long collimator able to point at the sun or at
the sky with a field of view of ±1.2°. The Cimel is pro-
vided with eight interference filters centered at the
eight used wavelength. The radiation transmitted by
each filter is then received by a silicon photodiode
which produces a voltage as output. The sequence of
measurements in the eight spectral bands requires
about 10 seconds; a series of three measurements, for a
total of 30 seconds, is taken for each observation.

The instrument measures in direct sun and sky
modes. In direct sun mode the instrument points at the
sun; these measurements are performed about every 15
minutes and are used to derive, by applying the Beer-
Bouguert-Lambert law, the aerosol optical depth and
the water vapour atmospheric content. In sky mode the instrument measures radiation coming from the sky in four spectral bands (440, 670, 870, and 1020 nm). Measurements are taken over the solar principal plane (at constant azimuth angle and varying the scattering angle) up to nine times a day, and along the solar almuncantar (constant elevation angle and varying the azimuthal angle) up to six times a day, both in the morning and in the afternoon. Sky mode measurements are used to derive aerosol optical and microphysical properties, such as size distribution, phase function, single scattering albedo, and asymmetry factor among others, through the application of inversion procedures [Dubovik and King 2000].

### 3. Field activities and meteorological constraints

During the 2009 IPY field campaign, most of the Italian instrumentation was set up in the main base, at building #216, which is a 10-minute walk from the NSF living quarters and even closer to the dining hall. Regularly, about once every 3 or 4 days, a trip to South Mountain was necessary in order to control the correct operation of the radiometers installed on the roof of building #1985. This required a 20-minute drive (each way) to South Mountain by means of a car provided by the DMI.

Lidar and GBMS data taking campaigns occur mainly in winter, when the stratospheric processes that generate ozone depletion take place and when the observing conditions are favourable. The Arctic middle stratospheric vortex forms in winter and vanishes in early spring, and it is inside this large vortex that GBMS and lidar observing efforts of stratospheric parameters have been concentrated. At the same time, the GBMS needs a low content of water vapor in the troposphere in order to measure the emission of stratospheric chemical constituents with sufficient accuracy, and this condition is realized only during winter months. Furthermore, lidar observations of the middle atmosphere are best carried out during darkness, as sunlight is a relevant source of noise. All these elements force the field team to operate at Thule during winter, when meteorological conditions are severe but long stretches of nighttime are available. Two or more intense snow storms are expected during each winter field campaign, with winds gusts that may reach 200 km/h and the windchill plunging to -60°C. Storms usually imply 3-4 days in which no travel outside or even inside the base is permitted and no instrumentation can be operated. During the field campaign of 2008-2009, a long stretch of bad weather with short-lived storms hit from February 5 to 11.

Figure 7 shows the 1951 to 2006 monthly mean of surface temperatures, winds, and precipitation over

<figure><img src="image_8.png" /><figcaption>Figure 7. 1951 to 2006 monthly mean of: (a) surface temperature, T, and its maximum and minimum extremes; (b) surface wind speed (ws) and maximum gust speed; (c) fractional amount (in percent) of precipitations as rain and snow; (d) mean and maximum rainfall and snowfall.</figcaption></figure>

Thule. In winter ground temperatures regularly reach -25°C; extreme minima of -45°C were measured during very cold years. The maximum probability of snow precipitations, ~20-25%, is in winter, with a mean of 15 cm of snow and a maximum of 60-90 cm between November and January.

### 4. Studies on the lower and upper atmosphere

#### 4.1. *Troposphere observations*

Figure 8 shows the daily mean meteorological parameters measured at Thule in the period 2007-2009. Daily surface temperatures vary between a winter minimum of about -33°C to a summer maximum of about +7°C. The variability is the strongest in winter. The daily mean T is above 0°C in June, July, and August, and below -20°C from November to April. The surface pressure also shows an annual cycle, with larger values in summer, up to about 1000 hPa, and lower values, down to about 960 hPa, in winter. The daily mean wind speed (WS) reaches peak values of 25 m/s, while it is < 5 m/s
in the majority of cases. The observed T and WS during 2007-2009 are in agreement with the long-term mean values reported in Figure 7, thus indicating no extreme cold or warm conditions in the three considered years. The wind direction (WD) shows a clear annual evolution, with WD<150° during winter, with frequent katabatic winds from the ice cap, and WD>150°, from the sea surface, throughout the rest of the year. The daily mean measured relative humidity over water (RH<sub>W</sub>) and the derived RH over ice (RH<sub>I</sub>) are also shown in Figure 8. RH<sub>I</sub> is between 60 and 95% in the period November to April, when T<−20°C, and lower in the remaining part of the year; T~−15°C was indicated by Curry et al. [1990] as the threshold temperature for the onset of the ice nucleation in the Arctic. RH<sub>W</sub> varies between 35 and 75% and shows a small annual cycle, with larger values in summer and lower in winter.

The all-sky daily mean shortwave (I<sub>SW</sub>) and long-wave (I<sub>LW</sub>) irradiances are shown in Figure 9. I<sub>SW</sub> shows a strong annual cycle, due to the annual evolution of the solar zenith angle, with values close to zero in February and October and peaks in the summer months. The maximum of about 360 Wm<sup>-2</sup> is observed in June, when the solar zenith angle reaches its minimum of ~52° and 24 hours of daylight are present. The daily I<sub>SW</sub> strong variability is primarily due to clouds, which may produce a reduction of the daily I<sub>SW</sub> as large as 85%.

The clear-sky shortwave radiation at the surface is mainly affected by water vapour absorption, which produces a reduction of I<sub>SW</sub> as large as 65 Wm<sup>-2</sup> (12%) during summer. The seasonal change of the albedo produces an increase of I<sub>SW</sub> by up to 25 Wm<sup>-2</sup> (4.5%) in spring. The annual mean radiative effect is estimated to be -(10÷11) Wm<sup>-2</sup> for water vapour and +(2÷3) Wm<sup>-2</sup> for the albedo. Aerosols also contribute, in smaller amounts, to the surface radiation budget [Di Biagio et al. 2012].

Surface conditions vary largely during the year, and snow and sea-ice coverage show a defined seasonal cycle. Measurements performed by the spectroradiometer MODIS [Xiong and Barnes 2005] on board Terra and Aqua satellites are used to derive the 8-day surface shortwave albedo over Thule (Figure 10, center panel). The surface albedo varies between 0.05 and 0.66, showing a strong annual cycle. The shortwave albedo is larger than 0.5 in spring, when the surface is covered by snow/ice, with a maximum between March and April. In summer, with the melting of surface snow and sea-ice, the albedo drops to values of about 0.05-0.1. The transition from high to low albedo occurs at Thule typically between mid-May and mid-June, in correspondence with the increase of the temperatures above freezing, and lasts for about three weeks.

Figure 10 (upper and lower panels) shows the

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Daily means of: (a) air temperature, T; (b) surface pressure, p; (c) wind speed, ws; (d) wind direction, wd; (e, f) relative humidity with respect to water, RH<sub>W</sub>, and ice, RH<sub>I</sub>, measured at Thule in the period 2007-2009. 10-day running means (solid lines) are also shown.</figcaption></figure>

aerosol optical depth (AOD) at 500 nm and the column water vapour derived at Thule by means of Cimel observations. The column integrated water vapour (wv) varies between 0.1 and 1.4 cm showing a well defined annual cycle with values lower than 0.6 cm in spring and early-autumn, and up to 1.4 cm, in summer. The water vapour peak occurs in July and the absolute maximum, 1.62 cm, was measured on July 17, 2009. The uncertainty on the retrieved wv is <12%.

The aerosol optical depth is generally lower than 0.2 and shows a defined annual cycle, with larger values during spring, in correspondence with the haze season [Quinn et al. 2007]. The lowest values of AOD are observed in summer, when pristine marine background conditions typically occur and the advection of air masses carrying aerosol originated at low latitudes is strongly suppressed. Isolated events of long range transport,
<figure><img src="image_10.png" /><figcaption>**Figure 9.** All-sky daily mean shortwave and longwave irradiance measured at Thule between 2007 and 2009 with the TSP-700 and the PIR 33499. The 10-day running mean (solid lines) is also shown.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Top panel: aerosol optical depth at 500 nm. Bottom panel: column water vapour (wv) derived from Cimel observations. Center panel: MODIS derived 8-day surface shortwave albedo during the period March-October of the years 2007-2009.</figcaption></figure>

mainly from Eurasia, may determine local large increases in optical depth. For instance, the absolute maximum of optical depth, ~0.25, observed at Thule on May 8, 2008, was related to an event of biomass burning occurred in central Russia, which largely affected the whole Arctic area in the period April-May of that year [e.g., Warneke et al. 2009]. Another example is the relative large values of optical depth observed in summer 2008 and 2009 (AOD > 0.1), which can be associated to episodes of transport of polluted particles from central and western Europe. AOD observations at Thule are in good agreement with data reported at several other Arctic sites in recent years [e.g., Tomasi et al. 2007 and 2012, Stone et al. 2010, Rodríguez et al. 2012].

The PIR was installed at Thule in 2009, and only one year of measurements of $I_{LW}$ is available. The daily $I_{LW}$ varies from ~130 Wm$^{-2}$ in winter to ~350 Wm$^{-2}$ in summer, with the maximum observed between July

and August; the annual cycle of $I_{LW}$ primarily follows that of surface temperature and, to a minor extent, that of water vapour. Clouds play a large role in affecting the magnitude of the longwave radiation at the surface; while atmospheric aerosols contribute in minor part. The effect of clouds is largest in winter, with an increase in $I_{LW}$ by up to 100 Wm$^{-2}$ during cloudy days, and somewhat smaller in summer.

As it is evident from Figure 9 and also from Figure 11 where the ratio between $I_{SW}$ and $I_{LW}$ is shown for 2009, the longwave radiation plays a large role throughout the year, and is the dominant component in all seasons except summer. Even in summer, the daily average LW component exceeds the SW component when thick clouds are present. The occurrence of clouds, generally leading to a reduction of $I_{SW}$ and an enhancement of $I_{LW}$, tends to increase the role of longwave radiation on the radiation budget.
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Ratio of the shortwave to longwave irradiance measured at Thule in 2009.</figcaption></figure>

## 4.2. Stratospheric observations during the IPY

The first part of winter 2008-2009 has been characterized by a stable and cold polar vortex which allowed the persistent formation of PSC particles. In mid-January of 2009, however, the most intense sudden stratospheric warming (SSW) ever observed occurred [Manney et al. 2009, Di Biagio et al. 2010]. SSWs strongly affect the dynamics and thermal structure of the Arctic stratosphere causing the breakdown of the eastward winter circulation, the build up of a westward circulation, and the reversal of the latitudinal temperature gradient. As the 2009 SSW developed, the stratopause lowered, the mean zonal circulation reversed, and ultimately the polar vortex in the lower stratosphere split in two (see Figure 12).

Figure 13 shows the lidar backscatter ratio (both parallel and cross polarized components) and the de-

polarization ratio on January 17 and 18, when the lidar detected a PSC layer. The layer extended between 16 and 23 km altitude on January 17 and then it moved to slightly lower altitudes (16-20 km) on January 18. The observed PSCs can be classified as type Ia due to their low backscatter ratio and moderate depolarization values [e.g., Browell et al. 1990, Toon et al. 1990]. This indicates that their particles are liquid and composed of nitric acid trihydrate (NAT), as also suggested by the temperature values at the altitudes of the observed PSC layer (not shown) which are below the NAT threshold.

Lidar and GBMS measurements at Thule observed the occurrence of the major SSW, sampling air inside the polar vortex at first and following the propagation of the SSW down to the lower stratosphere afterwards. The contour plots in Figures 14, 15, and 16 show the changes of the atmospheric chemical composition over Thule and temperature associated with the SSW. Figure 14 shows a sudden increase in N₂O mixing ratio (mr) which occurred on January 24 at around 35 km altitude and over the whole stratosphere between days 26 and 28. At higher levels, the vortex splitting and the vortex edge transit over Thule was marked by a rapid decrease in CO mr. CO data (not shown) indicate that in the upper stratosphere (45-50 km) the vortex broke up over Thule on January 19-20.

Concurrently, as warm, O₃-rich air from outside the vortex moved over Thule during the SSW, the GBMS measured an increase in O₃ mr in the upper stratosphere which reached a peak of ~8 ppmv at 35 km (Figure 15). The O₃ concentration in the lower stratosphere shows a clear sign of the passage of the vortex edge

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Potential vorticity (PV) contour maps at 550 K (22-23 km altitude) from ECMWF reanalyses showing the splitting of the Arctic polar vortex occurring (at this altitude) between January 23 (map on the left) and 27 (map on the right). The red arrows point to the north-western coast of Greenland where Thule is located.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 13.** Lidar derived backscatter (left panel) and depolaritazion (right panel) ratios between 12 and 25 km for the parallel and cross polarized components of the signal acquired on January 17 at 16:36 UT and 21:08 UT, and on January 18 at 03:57 UT.</figcaption></figure>

over Thule on January 26, in agreement with the N₂O
concentration displayed in Figure 14, but there are signs
of out-vortex air intrusions also a few days earlier. In
Figure 16 lidar temperatures show a sudden increase on

<figure><img src="image_15.png" /><figcaption>Figure 14. Contour map of GBMS N₂O vmr (in color, units of ppmv). Solid black contour lines depict temperature measurements obtained with the satellite-based Aura MLS instrument.</figcaption></figure>

late January 22. However, the warming in the upper
stratosphere started a few days earlier, as shown by the
superimposed Aura MLS temperatures, and lidar meas-
urements missed the onset of the SSW due to instru-
mental upgrades between January 16 and 22. The
maximum physical temperature of 289 K was recorded
by lidar near 40 km on January 22. In the following
days, the warming progressed downward reaching
about 15 km altitude on January 29, when the temper-
ature profile became nearly isothermal, particularly in
the altitude layer between 15 and 45 km.

5. What happened after the IPY and future developments

Starting in October 2010, all the atmospheric re-
mote sensing equipment located on base at building
#216 was moved to South Mountain, in a renovated
large building #1971 (see Figure 17). The 2010-2011
field campaign at the new location started in late Janu-
ary and ended in early March 2011 and was very suc-

<figure><img src="image_16.png" /><figcaption>**Figure 15** (left panel). Contour map of GBMS O<sub>3</sub> vmr (in color, units of ppmv). Solid black contour lines depict temperature measurements obtained with the satellite-based Aura MLS instrument. **Figure 16** (right panel). Contour map of temperature values (in units of K) obtained from lidar measurements (in color). Solid black contour lines depict temperature measurements obtained with the satellite-based Aura MLS instrument. Solid black triangles at the base of the plot show when lidar measurements were carried out.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure 17.** Building #1971 (left) and the DMI balloon house at its new location on South Mountain, next to building #1971 (right).</figcaption></figure>

<figure><img src="image_18.png" /></figure>

cessful. The move also involved the DMI equipment, including the balloon house used for preparing and launching the ozonesondes, which was previously located near the airport. Together with the relocation of additional remote sensing equipments already present on South Mountain at building #1985, such as the NCAR FTIR, the DMI UV spectrometer, and the ENEA radiometers, which occurred in summer 2011, this major move created a large, international, and multi-instrumental atmospheric remote sensing laboratory at Thule Air Base. The renovated building was modified to host both the GBMS and the lidar, with their specific requirements of electric power, windows and a roof hatch. The altitude of the site (225 m above sea level) and its distance to the main base has both positive and negative points. The GBMS can count on a decrease in atmospheric opacity, albeit small, whereas the lidar loses the possibility of observing the bottom layers of the boundary layer. On the logistics front, the new setup involves the daily use of a car to go back and forth from the main base to building #1971, which is a draw-back, but offers the advantages of having all the equipments in one laboratory, supervised in principle by a single unit of personnel.

During the winter campaign of 2011-2012, the lidar system will be upgraded with a more powerful laser source that will allow the lidar temperature measurements to reach higher altitudes and improve on their time resolution. The new laser will have the capabilities of being remotely controlled, which will be useful for operating it from main base or even from Italy, when necessary. This improvement will include mounting small motors that can adjust the direction of the laser beam and of the small telescopes to make it possible to align all the lidar receiving channels by remote control. In the future, lidar operations could be performed at Thule by local personnel that simply turn on and off the electronics and by specialized personnel at the University of Rome that can act on data taking runs and adjust the

alignment of the system when necessary. As a part of this collaboration, in the near future we also plan on installing at Thule a new 22 GHz spectrometer for the observation of water vapor stratospheric vertical profiles, at the moment under development at the INGV. This effort is aimed at long term measuring of one of the most important parameters of the middle atmosphere, a key element for detecting the first effects of climate change on the middle atmosphere and on the ozone depletion.

**Acknowledgements.** This material is based on work also supported by the National Science Foundation under grant 0936365 and by the Programma Nazionale di Ricerca in Antartide (PNRA) under grant 2009/A3.04. We would like to thank David Seibert of the Thule Air Base Weather Forecast Office for providing the 1951-2006 meteorological data, the ECMWF for its temperature and pressure reanalyses, and the MLS team at JPL for the Aura MLS temperature data. MODIS data are distributed by the Land Processes Distributed Active Archive Center (LP DAAC), located at the US Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (lpdaac.usgs.gov). Very sadly, Prof. Giorgio Fiocco passed away in July 2012. The Italian atmospheric remote sensing activities at Thule, which started back in 1990, the international collaborations and all the science that originated from these 20+ years at Thule have his signature, his crucial and enlightening contribution.

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<footnote>*Corresponding author: Giovanni Muscari, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Rome, Italy; email: giovanni.muscari@ingv.it.</footnote>

<footnote>© 2014 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.</footnote>