Microwave Remote Sensing of Water Vapor in the Atmosphere

the ground and from an airborne plat-form have been built at the Institute of Applied Phys¬ ics, University of Berne. The paper presents the method of microwave remote sensing and gives an overview of recently achieved results with regard to water vapor distribution as a function of altitude and Iatitude. First results of an imaging radiometer for the two dimensional distribution of liquid water is presented.


Introduction
For a long time greenhouse warming and the depletion of Ute OZOtlC layer have been studied as two independcnl probten areas. It has been thought that the Stratosphäre with only about 10-20% of the atmos¬ phere in terms of mass can play only a limited role in climate change. However, Ihere has been increas¬ ing evidence in recent years that the stratosphere is a sensitive component of the climate System, which can aflect the troposphere through various coupling mechanisnis: such as radiative transfer, changing the eharacteristics of tropospheric waves by affecting the upper boundary condition of the troposphere or through the downward propagation of zonal-mean anomalies (Shbpherd 2000). A detailed understanding of the processes by which atmospheric composition affects climate directly, through the distributions of waler vapor, other greenhouse gases and aerosols, and indirectly through chemistry and vice versa is thus vilal in order lo predict the future State of the atmos¬ phere. A summary of the latest findings in the field of global change is given in the reports of the Intergov-ERNMENTAL PaNBLON Cl.lMATh ClIANGE (IPCC) (2001) 011 climate change and in the WORLD Meieorologi-CAL Organization (WMO) (1998) assessments of the ozone layer. Both are updated at regulär intervals. Ozone and water vapor play key roles in the subjeet under discussion.
Walcr vapor plays a crucial role in atmospheric proc¬ esses through its radiative, chemical and dynamical properties. In the upper troposphere it is one of the inain greenhouse gases that absorbs longwave terrestrial radiation. Its distribution is strongly influenced by both large scale circulation and localized convection. Chemically, water vapor is a major source of the hydroxyl radical, the primary oxidant in the tropo¬ sphere that is able to react with most pollutants. Water vapor is also a valuable tracer of atmospheric motion due to its long chemical lifetime. Part of water vapor enters the stratosphere by vertical transport in the tropical tropopause with the air being freeze dried by the low tropopause temperature and spreads to middle latitudes with a certain phase lag. Air passing this way through the tropopause and carried upward through the large scale circulation is marked by the mixing ratio of entry in the same way as a magnetic tape recorder is marked by the recording head. This effect that is clearly visible has been called atmos¬ pheric tape recorder (Mote et al. 1996). In addition, ILO in the stratosphere is produced by oxidation of CH4. Both sources contribute approximately one half to the available water vapor in the middle atmos¬ phere. The sink of H20 in the middle atmosphere is photolysis. While the methane oxidation is well understood, understanding of tropical stratospheretroposphere exchange is weak. More insight in these processes might be obtained by observations of the isotopic content of water entering the tropical strat¬ osphere as the Saturation vapor pressure of isotopes heavily depends on temperature (Keith 2000). How¬ ever, only very few measurements of water isotopes exist. Another unsolved issue in atmospheric water vapor is the discovery of a narrow layer of H20 located at approx. 70 km altitude in the mesosphere where the H20 mixing ratio reaches the highest values observed in the middle atmosphere (Summers et al. 1997). This layer can only be explained by a local source of H20 Suggestions have been made that this layer could be produced by an influx of small comets releasing water in the atmosphere. However, Summers & Siskind (1999) proposc a recombination of O and H2 on meteoric dust as this source of mesospheric water vapor.
Measurements of water vapor in the upper tropo¬ sphere and in the stratosphere require an enormous technical effort due to large gradients around the tro¬ popause and the stratospheric mixing ratios of a few ppmv in contrast to the moist tropospheric air masses. In addition, in the stratosphere, the spatial and tem¬ poral variability of the H20 abundance is relatively small, i.e. changes of a few tenths of 1 ppmv need to be detected with a similar aecuraey of the measurement. No Single instrumenl today is capable of H20 measurements at all altitudes, with adequate global and temporal coverage. Therefore a combination of different techniques is necessary in order to investigate the spatial and temporal variability of water vapor. In situ measurements are mainly limited to altitudes below approx. 35 km and are performed on balloons or from aircraft, which are restricted by set cruising attitudes. Signatures of tropospheric and stratospheric exchange have e.g. been studied in this way by Ovarlez et al. (1999). A method that is particularly well suited to investigate the vmr-profile from the ground is microwave radiometry that retrieves the profile from pressure broadened transition lines. Measurements of water vapor in the middle atmos¬ phere have successfully been performed e.g. by Ned- for the subtropics where the tropopause is higher. Gemessene Spektren der Wasserdampflink bei 183.310 GHz für verschiedene geographische Breiten. Höhere Werte In den Fluge/n der Spektrallinie weisen auf eine höhere Wasserdampfmenge in einer Flughöhe von 11 km hin Dies trifft insbesondere für die Subtropen zu, wo die Tropopause oberhalb der Flughöhe liegt. Specire de la ligne de transition de la vapeur d'eau ä 183.310 GHz, mesure ä differentes latitudes. Des valeurs elevees sur les ftancs de la ligne indiquent de plus grandes quantites de vapeur d'eau presentes ä Taltitude de vol d'envlron 11 km. ("est le cos en particulier pour la region subtropicale, oü Taltitude de la tropopause est plus elevee. oi.tniA et al. (1997,1998,1999), who observed a clear annual cycle and an upward trend in middle atmos¬ pheric water vapor of 0.15 ppmv/yr in the altitude ränge of 40-60 km concluding that there has been a signilicant increase in the amount of water vapor entering the middle atmosphere. However, the exaet cause is unknown. Seele & Hartogh (1999) observed with microwave radiometry a pronounced annual cycle of water vapor in the polar middle atmosphere that is slronger than what has been reported for midlatitudes. A more global coverage of data has been obtained by different salellite sensors e.g. LIMS, AI MOS, HALOE and MLS. A climatology of water vapor mixing ratios for October 1991 -June 1997 was presented Crom the microwave limb sounder (MLS) on UARS (Upper Atmosphere Research Sateilite), by Sioni et al. (2000). An assessment of the present siate of knowledge can be found in the SPARC (Strat¬ ospheric Processes And their Role in Climate) report on upper tropospheric and stratospheric water vapor (SPARC 2000).
Research in tropospheric water (mainly water vapor, but also cloud liquid and frozen water) has increased in recent years due to its importance for processes relevant to radiation, meteorology, climate, hydrological cycles, biogeochemistry and human activities, such as telecommunications. One handicap in this research has not been removed so far, namely the lack of aecurate methods to measure atmospheric humidity.The large errors observed for water vapor soundings is certainly affected by the heterogeneity and temporal variability of water in the atmosphere. But this is not the only reason for the problem. Most measurement methods rely on comparisons with radiosonde data. That widely used humidity sensors on radiosondes can have more significant errors than was expected so far, was recently demonstrated by Westwater et al. (2000) for the Vaisala Humicap RS80 sensor. Although correction algorithms for aging can reduce the errors, they cannot be removed effectively. Route de vol de la campagne de mars 2000 qui couvrait des latitudes s'etendant des regions subtropicales jusqu'au pole Nord. Un vol au-dessus de l'lslande a ete specialement programme pour recueillir des donnees ä Tinterieur du vortex polaire qui se trouvait alors sur cette region.
observations are necessary. Even if there are more and more satellite observations available it has to be kept in mind that ground based measurements providing high quality ground truth data will be needed for Vali¬ dation purposes also in the future. Ground based sen¬ sors and satellite Systems will remain complementary.
The aim of the paper is to show that microwave radiometry is a powerful remote sensing technique for the detection not only of the column density of water vapor or liquid water but in addition also of the water vapor profile. Data from microwave radiometers allow the investigation of the atmosphere from the ground to the mesopause thus providing valuable Information for atmospheric research. Results from different instruments are given.

Microwave radiometry
Microwave radiometry is a passive remote sensing technique which detects emission lines of atmospheric constituents. For a review of the methodology refer to Janssen (1993) or Kämpfer (1995), for example. A spectral analysis of the pressure broadened lines allows the retrieval of the altitude profile of the species under investigation over the height ränge of typically 20 km to 70 km, which corresponds to altitudes from the lower stratosphere to the mesopause. As a direct detection of the spectral features in the micro¬ wave region is technically not possible, with the exception of frequencies below approx. 20 GHz, microwave radiometers operate in the so called heterodyne mode.
In this mode, the incoming high frequency signal from the atmosphere is superposed with a highly stable signal from a local oscillator in a non linear element, a so-called mixer, thus transforming the complete spec¬ tral Information to an intermediate frequency where sufficient amplification and spectral analysis is possible. Spectral analysis typically is performed in an acousto optical spectrometer where the microwave frequen¬ cies are converted to ultrasonic waves that disperse a monochromatic light beam onto an array of visible week and Covers almost all of the northern latitudes from the tropics to the arctic Typical spectra measure¬ ments are sfiown in Figure 1, whereas Figure 2 shows the flight route for the campaign in spring 2000 which led up to the north pole and allowed measurements of the water vapor distribution at the pole for the first time. Figure 3 shows the latitudinal distribution of water vapor with superimposed values of the potential vorticity. The edge of the polar vortex is clearly visible in the altitude profiles of stratospheric water vapor that we retrieved from our spectra. 3.3 A new instrument for water vapor in the middle atmosphere In order to measure middle atmospheric water vapor also during less favorable conditions and particularly from low altitudes, a different spectral line has to be used. The transition at 22.235 GHz has an opacity which is sufficiently weak to be used from an Observa¬ tion site such as Berne (550 m a.s.l.). An instrument has been built based on a concept which minimizes optical components in order to reduce any internal reflections leading to baseline effects. JTie radiometer has an outstanding System temperature of 160 K in Single sideband mode (Deuber 2001;Deuber et al. 2002). An impression of the instrument is given in Figure 4.
First spectra of the water vapor transition line with an extremely high spectral resolution of 15 kHz is given in Figure 5. This high spectral resolution will allow the retrieval of the altitude profile of water vapor up to the mesopause thus giving insight into processes which are poorely understood.

Tropospheric water vapor
In contrast to the stratosphere where water vapor is a trace gas, water vapor in the troposphere is found in abundance and is a highly variable factor contributing to weather and climate. In order to determine the columnar water vapor distribution in the troposphere along with a temperature profile, we used ASMU-WARA (All Sky Multi Wavelength Radiometer) which has the unique advantage of allowing the Observation of the parameters of interest over the whole sky by means of scanning in azimuth and elevation. Local inhomogeneities can thus be easily detected (Martin 2003). In addition to several Channels in the microwave region, the instrument has an infrared sensor which Valeur integree du contenu en eau liquide (CEL) de Tatmosphere au-dessus de Berne, le 16 avril 2002. L'angle du diagramme polaire indique l'azimut et le rayon indique Vangle zenithal d'Observation. La courbe blanche correspond ä un CEL de 0 mm, la courbe noire ä un CEL de 0.5 mm. Les points de mesure sont representes par les points noirs. is used for cloud detection. In addition to allowing the Observation of water vapor, this instrument also opens up the possibility to observe cloud liquid water, a parameter which can not be measured, for e.g. with balloon soundings and which thus gives insight into the total water content of a cloud. An example of the two dimensional distribution of the liquid water content of the sky is given in Figure 6. As the time resolution of these maps is a few minutes, they provide insight into the dynamical processes of the atmosphere. 4 Conclusions information on the column densities of the troposphere. Due to the Operation from an aircraft, latitudinal variations of water vapor in the middle atmosphere can be investigated. As the chemical lifetime of water vapor is high, it is possible to study dynamical processes, as was the case within the polar vortex. The distinction of differ¬ ent isotopes might give insight into the origin of water in the middle atmosphere. Phenomena in the troposphere can be accessed with a novel method providing maps of the sky for integrated water vapor and liquid content. Therefore this method is well suited for investigations in climate research as is done within project START-WAVE of NCCR-climate within Switzerland.
Microwave remote sensing is an excellent means of studying processes in the atmosphere related to water vapor and liquid water content. The analysis of spectral fea- Summary: Microwave Remote Sensing of Water Vapor in the Atmosphere Water vapor in the atmosphere plays a crucial role in climate and in atmospheric processes. Due to its long chemical lifetime it can be used as a tracer for investigations of dynamical processes in the middle atmos¬ phere. Microwave radiometry is one of the few remote sensing methods which is capable of inferring Informa¬ tion on the water vapor content of the troposphere to the mesosphere, however with a different altitude res¬ olution. Different microwave radiometers that can be operated from the ground and from an airborne platform have been built at the Institute of Applied Phys¬ ics, University of Berne. The paper presents the method of microwave remote sensing and gives an overview of recently achieved results with regard to water vapor distribution as a function of altitude and Iatitude. First results of an imaging radiometer for the two dimensional distribution of liquid water is presented. Resume: La sensibilite ä distance de la radiometrie micro-ondes ä la vapeur d'eau dans Patmosphere La vapeur d'eau joue un role crucial dans les proces¬ sus climatiques et atmospheriques. Gräce ä son temps de vie chimique tres long, cette molecule peut etre utilisee comme traceur pour l'etude de processus dynamiques dans la moyenne atmosphere. La radiometrie micro-ondes est une technique de teledetection passive permettant d'interferer l'information sur le contenu en vapeur d'eau de Patmosphere de la troposphere jusqu'ä la mesosphere, avec une resolution verticale dependant des regions considerees. Plusieurs radiometres micro-ondes ont ete construits a l'Institut de Physi¬ que Appliquee de l'Universite de Berne, et sont utilises au sol et ä bord d'un avion. L'article presente la techni¬ que de la radiometrie micro-ondes et donne un survol des resultats recents obtenus sur la distribution de la vapeur d'eau en fonction de l'altitude et de la Iatitude, depuis la region sub-tropicale jusqu'au Pole Nord. Sont egalement presentes les premiers resultats d'un nouveau radiometre permettant de mesurer depuis le sol la distribution de la vapeur d'eau dans deux dimensions.
Teaching in Geographypertinent questions -Why is water vapor interesting? -How is it possible to measure the altitude distribu¬ tion of water vapor without being in situ? -What are the characteristics of MIAWARA? -What were the results of the different methods? -