Low stratospheric water vapor measured by an airborne DIAL

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D24, PAGES 31,351-31,359, DECEMBER 27, 1999

Low stratospheric water vapor measured by an airborne DIAL

Gerhard Ehret, • Klaus P. Hoinka, 2 Joal Stein, 3 Andreas Fix, • Christoph Kiemle, • and Gorazd Poberaj 1

Abstract. Water vapor measurements are presented which were performed with a newly developed airborne water vapor differential absorption lidar (DIAL). The transmitter of this DIAL system consists of an injection-seeded optical parametric oscillator at a signal wavelength of •940 nm capable of measuring very low values of water vapor (10 ppmv). The instrument was tested in an experiment where the aircraft crossed a potential vorticity streamer at about 12 km altitude. The downward looking DIAL can scan vertical cross sections in the atmosphere up to a range of 6 km, with a resolution of about 30 km in the horizontal and 250 m in the vertical. The measured data show significant water vapor structures of 5-100 ppmv within the stratospheric air of the streamer. The measured cross section is compared with fields simulated by a mesoscale model and analyzed from operational data. The comparison of the three data fields shows good agreement in water vapor structures and magnitudes. In addition, the streamer exhibits a clear structure in the aerosol backscatter lidar scan.

1. Introduction

Humidity is one of the key meteorological parameters for weather and climate. Low tropospheric humidity is frequently measured accurately in situ by aircraft and radiosondes. In addition, airborne differential absorption lidar (DIAL) applied in the near-infrared spectral region has been demonstrated to remotely sense the two-dimensional distribution of water vapor in the lower troposphere with high spatial and temporal resolution [Kiemle et al., 1997]. In the upper troposphere and lower stratosphere the situation is quite different because in this region, the water vapor mixing ratio does not exceed 100 ppmv. In that region the usability of humidity data measured by radiosondes is limited by restricted spatial coverage and reduced accuracy. Humidity data derived from satellite have only recently become more trustworthy [e.g., Jackson et al., 1998]. Major limitations of these measurements are their poor vertical resolution. In situ data taken by aircraft are reliable since measurement systems have been successfully developed to measure values of very low water vapor mixing ratios [Busen and Buck, 1995; Ovarlez and Ovarlez, 1994].

The airborne DIAL technique offers a unique opportunity to measure water vapor mixing ratios along extended cross sections. The instrument basically measures the backscatter of laser pulses emitted into the atmosphere at two adjacent wavelengths with one wavelength centered on an appropriate water vapor absorption line (hereinafter referred to as "on line") and another nonabsorbing wavelength selected for the reference measurement ("off line"). The methodology of DIAL and its applicability for tropospheric water vapor have been the sub-

•Lidar Gruppe, Deutsches Zentrum ftir Luft- und Rauhmfahrt (DLR), Oberpfaffenhofen, Germany.

2Institut ffir Physik der Atmosphiire, DLR, Oberpfaffenhofen, Ger- many.

3Centre National de Recherches Meteorologiques/GAME, Meteo- France, Toulouse.

Copyright 1999 by the American Geophysical Union.

Paper number 1999JD900959. 0148-0227/99/1999JD900959 $ 09.00

ject of extensive studies [Ismail and Browell, 1989; BOsenberg, 1998]. Browell e! al. [1998] demonstrated that an airborne DIAL using novel transmitter and receiver technology in the 820 nm spectral region can measure two-dimensional water vapor distributions covering tropospheric layers of up to more than 10 km.

This instrument is very successful in atmospheric regions with large ppmv water vapor content. However, in the upper troposphere and lower stratosphere with absolute values of water vapor usually at less than 100 ppmv the selected spectral region is not appropriate. The reason for this is that the detection sensitivity achievable with DIAL generally scales with the line strength of the selected water vapor absorption line. For measurements in the lower and midtroposphere where the values are significantly larger than 100 ppmv, appropriate lines can be found in the 4 v overtone vibrational band of H20 at 730 or 820 nm. However, when little water vapor is present, the use of much stronger lines is necessary. In this case, appropriate lines can be found, for example, in the 3 v overtone vibrational band of H20 at 940 nm. Its strongest lines are more than an order of magnitude stronger than those in the 4 v band. To the authors' knowledge, airborne DIAL measurements of water vapor using such strong lines have not yet been reported.

In the present paper, a H20-DIAL, operating at a wavelength of 940 nm, was used to measure low values of water vapor mixing ratio as are usually encountered in the upper troposphere and the lower stratosphere. To demonstrate the measurement capabilities of this instrument, it was deployed aboard the DLR aircraft Falcon in order to perform measurements in November 1998. Therefore a distinct atmospheric flow pattern was chosen, so-called potential vorticity streamer, which contained strong horizontal gradients of water vapor. The mixing ratio measured by the DIAL along a cross section will be compared with data obtained from the European Cen- tre for Medium-Range Weather Forecasts (ECMWF) operational analysis, radiosonde data, and output of a mesoscale numerical model. The operational analysis uses ECMWF data at a horizontal truncation and a vertical resolution of T213/

L31. The model used is a nonhydrostatic mesoscale model,

31,351

31,352 EHRET ET AL.: WATER VAPOR DIAL

MESO-NH [Lafore et al., 1998] which contains a two-way interactive grid-nesting technique [Stein et al., 1999].

The paper is structured as follows: Section 2 describes the H20-DIAL system; section 3 discusses the synoptic situation with an imbedded potential vorticity streamer; section 4 compares low stratospheric water vapor mixing ratios; and section 5 gives a summary and discussion.

2. DIAL System and Data Processing In the following the abbreviation DIAL is used to denote the

H:O-DIAL. The signal wavelength of an optical parametric oscillator (OPO) serves as a radiation source for the DIAL measurements in the 940 nm spectral region. The wavelength selected here is 942.825 nm. The OPO uses potassium titanyl phosphate (KTP) as nonlinear crystal and is pumped by the second harmonic of a Q-switched, injection-seeded Nd:YAG laser at a repetition rate of 10 Hz. Narrowband radiation is achieved by injection seeding, with an external cavity diode (ECL) laser system. This system is tunable from 940 to 960 nm with continuous tuning ranging up to 60 GHz. Its spectral width was measured to be less than 10 MHz and enables

precise control of the pulsed OPO radiation by tuning the wavelength of the seed beam to the center of the selected water vapor absorption line. Wavelength setting is established by means of water vapor absorption measurements using a 30 m multipass cell. The water vapor pressure in the multipass cell is chosen to be only 1 hPa in order to avoid line saturation effects even for the strongest lines. Under this condition, the line shape is dominated by Doppler broadening with a line width of • 1 GHz. This narrow linewidth ensures precise wavelength setting for the on-line measurements. In order to main- tain proper injection seeding and to keep the frequency stable under flight conditions, a computer-controlled feedback loop is used which stabilized the ECL frequency to the edge of the selected water vapor line. Recently, Ehret et el. [1998] demonstrated that such an injection-seeded OPO system fulfils the spectral requirements of a precise water vapor DIAL measurement in the upper troposphere and lower stratosphere. The residual systematic error caused by the spectral properties of the OPO is 2%.

For the water vapor measurements reported here, the wavelength was set to be close to the center of the selected line. The residual frequency shift of 250 MHz has been accounted for in the absorption cross-section calculation. The long-term in- flight stability was measured to be of the order of 30 MHz. This ensures that systematic errors caused by wavelength drift are negligible. For DIAL, two wavelengths are needed. This has been achieved by using the different spectral widths of seeded and unseeded OPO. Since the bandwidth of the unseeded

OPO radiation (100 GHz) is much broader than that of the seeded system (•0.2 GHz), the output of the unseeded OPO can serve for the nonabsorbing off-line measurements [Fbc et el., 1998]. This is achieved by switching between an unseeded and a seeded operation at 10 Hz.

The backscattered photons are collected with a nadir- viewing 35 cm Cassegrain telescope installed on the aircraft. An avalanche photodiode is used for signal detection. The individual signals are digitized with 12 bit resolution at a sam- pling rate of 10 MHz. The vertical resolution of a single shot is 15 m. The horizontal resolution depends on the shot-pair frequency of 5 Hz. The original two-dimensional field consists of data with a spacing of 40 m horizontally and 15 m vertically.

The data evaluation procedure is as follows: In order to suppress instrument noise, which typically dominates the raw signal, the data must be smoothed. This is done by applying a running mean over 33 range bins vertically and over 1200 shot pairs in the horizontal. The smoothed on-line and off-line backscatter profiles are converted into the water vapor field by applying the DIAL equation [Schotland, 1974]. For each height, the molecular absorption cross section is calculated from line parameters measured in the laboratory (V. Weiss, personal communication, 1999). For this, ambient temperature and pressure as a function of altitude are necessary to com- pensate for the height-dependent line broadening and line shift. The vertical profile data are obtained from aircraft ascent and descent soundings during takeoff and landing combined with along-flight track radiosonde data. In the data reduction the residual water vapor absorption on the broadband off-line return signals has been considered, as well as the Rayleigh- Doppler broadening on the narrowband on-line returns [Ans- mann and B6senberg, 1987]. The Rayleigh-Doppler correction applied in this study took into account the aerosol concentrations (see Plate 2).

The smoothing procedure strongly influences the horizontal and vertical resolution of the resulting water vapor field. The scale of the running mean in the horizontal depends on the aircraft velocity; with a flight velocity of 200 m/s it is about 50 km. However, the low-pass filter characteristic implemented by the running mean results in a horizontal resolution of 30 km using a 90% attenuation threshold. On the other hand, the vertical resolution is calculated by an autocovariance method [Kiemle et al., 1997]. Thus the final data field allows structures of the order of 30 km (250 m) in the horizontal (vertical) considered to be real atmospheric ones.

The signal-to-noise ratios of the averaged on-line and off- line returns decrease from 2500 (near range) to 63 (far range) and from 2500 to 440, respectively. Nevertheless, the statistical error remains at a fairly constant level of 5% throughout the whole measurement range. This is because at lower altitudes, there is higher humidity, leading to more water vapor absorption, which improves the sensitivity of the retrieval method, so compensating for the poor signal-to-noise ratio in the far range. For further details on the determination of the statistical error, see Kiemle et al. [1997].

In addition, the DIAL off-line wavelength is used to deter- mine the aerosol distribution. This wavelength is sensitive to enhance the contrast between aerosol scattering and scattering from air molecules. The backscatter ratio, defined here as the ratio of total (aerosol plus molecular) to pure molecular back- scattering, is a measure of this contrast. Molecular backscat- tering was calculated using air density values derived from the above mentioned aircraft soundings. As with the water vapor data retrieval, a smoothing procedure is applied. The resulting vertical and horizontal resolutions of the backscatter ratio

measurement are 150 m and 8.5 km, respectively. In principle, the DIAL aboard the aircraft could be posi-

tioned to sense upward, downward, or sideways. Because of decreasing atmospheric backscatter, the range in the upward looking mode is limited to about 3 km. The side view range is of the order of 5 km. The downward looking mode has the advantage that the density of the backscatter medium increases, allowing more atmosphere to be scanned between the aircraft and the Earth's surface. To scan the entire troposphere (•10 km) requires two different water vapor lines. By using just one line the maximum range is limited to about 6 km. For

EHRET ET AL.: WATER VAPOR DIAL 31,353

reasons of simplicity in the present experimental test, a one- line setup was used.

3. Stratospheric Intrusion on November 12, 1998 To demonstrate the capability of the DIAL, in-flight mea-

surements were performed in November 1998. The flight plan was to operate the aircraft at the uppermost possible flight altitude of about 12 km with the instrument looking downward to get water vapor mixing ratio structures within the upper troposphere topped by the relatively dry stratosphere. A situation with a strongly descending tropopause was selected which is characterized by considerable horizontal gradients in most meteorological parameters, particularly in the water vapor mixing ratio. On November 12, 1998, a northwest- southeast-elongated trough stretched from the British Isles to the Isle of Sardinia. These troughs have been termed "potential vorticity streamers" because they are characterized by dis- tinctive structures in the upper tropospheric potential vorticity (PV) field [Appenzeller and Davies, 1992].

Figure 1 shows the height field of the tropopause above Europe. The tropopause height is derived from ECMWF operational analysis data by applying a potential vorticity threshold value of 2.5 pvu; 1 pvu (potential vorticity unit) is equal to 1 x 10 -6 km 2 kg -• s -•. Here we have applied a slightly stronger threshold value than the WMO-defined value of 1.6 pvu [WMO, 1986]; for more details on determining the tropopause surface, see Hoinka [1998]. Within the streamer above central France the tropopause descends down to less than 7.5 km, whereas at its eastern and western flanks, the tropopause is found at a height ranging between 10 and 11 km. This tropopause descent is associated with a corresponding transport of relatively dry stratospheric air toward lower altitudes, usually called stratospheric intrusion. The presence of a tongue of dry air extending into the troposphere appears as a dark band in the Meteosat water vapor image (Figure 2).

Figure 3 depicts PV, potential temperature, and meridional wind (all ECMWF) across this streamer above the baseline given in Figure 1. The flow around the streamer shows the typical flow from the northwest (hatched, out of the plane) at the western flank of the streamer and a much weaker south-

easterly flow at its eastern flank. The PV field clearly depicts the stratosphere with high values in PV (hatched) and the troposphere with lower values. The tropopause descent can also be seen clearly. Depending on the threshold value, the tropopause descends down to 4 km (1 pvu), 5 km (2 pvu), 7 km (2.5 pvu), and 7.5 km (3 pvu).

Because the intention is to compare high-resolution DIAL measurements with low-resolution ECMWF analyses it is necessary to show that the low-resolution data represent the meteorological situation to a certain extent. For this purpose, Figure 4 exhibits in situ aircraft data taken on November 12, 1998 (1500-1700 UTC), and ECMWF operational analysis data (1800 UTC). This is reasonable because on this day the synoptic system has not changed very much. The ECMWF analyses of 1200 and 1800 UTC indicate that the streamer propagated about 100 km eastward (not shown); during this period the maximum of tropopause descent moved by about 200 km toward the southeast. Obviously, these two data sets have different horizontal resolutions: the ECMWF data have a

resolution of about 20-30 km compared to the in-flight data, taken by the Falcon, which have a resolution of about 200 m.

The potential temperature (Figure 4a), zonal wind (Figure

Figure 1. Tropopause height on November 12, 1998, 1500 UTC, derived from ECMWF operational data defined by a threshold value of 2.5 pvu. The height increment is 0.5 km. The hatched area indicates where the tropopause descends below 9 km. The dashed line indicates the baseline of the cross section

shown in Figure 3. The solid line indicates the baseline of a selected part of the flight track shown in Figure 4.

4b), and meridional wind data (Figure 4c) of both series show similar structures. Figure 4a also depicts the ECMWF potential vorticity. It shows from the left to the center the PV at flight level between Zurich and the Pyrenees ("track A") and from the center to the right the return flight from the Pyrenees toward Zurich ("track B"). The potential temperatures encountered on track A are lower than those measured during the return flight because the flight level has changed from 220 hPa (track A) to 180 hPa (track B). At the westernmost return point close to the Pyrenees (in the center of the figure) the tropopause is located at a relatively high altitude of about 11 km (Figure 1). Consequently, the PV at flight level drops down to 3 pvu. In the same region, there was a strong jet of up to 50 m/s (Figures 4b, 4c). Both tracks exhibit high PV values at flight level when crossing the streamer at high altitudes. In the center of the streamer at about 1545 UTC (track A) and 1645 UTC (track B) a slight decrease is apparent which is also obvious in the PV structure given in Figure 3.

This brief comparison emphasizes that the relatively coarse ECMWF analysis data represent to a considerable extent the generally smaller scale structure of the atmosphere. In general, it suggests that because both data series agree well on temperature and wind values, they will also agree well with the water vapor mixing ratio data measured by the DIAL.

4. Low Stratospheric Water Vapor ' In the following the DIAL data are compared with both the

ECMWF operational analysis data and the numerical simulation performed by the mesoscale model MESO-NH [Lafore et al., 1998]. All three data sets have different spatial resolutions. The ECMWF analysis data set, which is used here, provides data on 31 model levels; this version contains about twice as


Figure 2. Meteosat water vapor image of November 12, 1998, 1500 UTC. The figure shows the same section as that given in Figure 1.

many levels in the tropopause region as compared to the data set with standard pressure level data. For the model level data the vertical resolution in the layer between 350 hPa and 100 hPa ranges from 20 hPa at the top to 35 hPa at the bottom of this layer. The horizontal grid resolution used is 0.5 ø in latitude and longitude. For the region under consideration this represents a horizontal grid resolution of about 35 km.

The initial conditions for the model are provided by the ECMWF operational analysis (T319). The model initially runs during the first period from 0000 to 0600 UTC for November 12, 1998, with a grid distance of 40 km. From 0600 to 1800 UTC a nested grid with a resolution of 10 km is implemented within the coarse model. The vertical grid is nonlinear, spaced every 50 m at the lower boundary, and reaching 1000 m at a height of 10 km. At greater heights the grid is regularly spaced. The size of the coarse domain is 4000 km by 4000 km, centered in southern France. The size of the fine domain is in the

east-west direction 1450 km and north-south 1200 km. The

time steps are 100 s (coarse model) and 33 s (fine). The synoptic forcing is provided by the boundary conditions, which are updated every 6 hours using the corresponding ECMWF operational analyses. In the two-way interactive grid-nesting procedure [Stein et al., 1999], the two models interact at every coarse time step in the following way: The coarse model gives the lateral boundary conditions to the fine mesh model; in this model the data are averaged at the resolution of the coarse model. A Rayleigh damping toward these values is applied to the coarse mesh model in the region common to both models. This procedure is applied from 0600 to 1800 UTC, after running only the coarse grid model from 0000 to 0600 UTC.

Plate 1 shows cross sections of water vapor mixing ratio on November 12, 1998: measurements of the DIAL (top), simulations of the model MESO-NH (middle), and analyses of ECMWF data (bottom). The baseline of these cross sections is given in Figure 1 (solid line). The aircraft crossed the PV streamer between 1622 and 1713 UTC on its flight from the Pyrenees to Zurich (track B). The model cross section was constructed by combining three subsections at three different model times in order to account for the temporal evolution during the flight from west to east: 1630 UTC western part, 1650 UTC central part, and 1705 UTC eastern part. During this time the temporal evolution was weak, which suggested this approach should be taken. The three parts were fitted to each other by a weak smoothing.

The deep intrusion of dry air measured by the DIAL between 1645 and 1705 UTC is reasonably represented by the model simulation as well as by the gross synoptic situation as given by the ECMWF operational analysis data. However, the evaluated width of the streamer is narrowest in the DIAL data.

The water vapor satellite image (Figure 2) shows that the streamer consists of two branches above the British Isles form-

ing one streamer over central France. This spatial feature of the streamer is not apparent in the ECMWF tropopause analysis (Figure 1), whereas the MESO-NH is able to simulate this feature (not shown). All three data sets indicate a dry intrusion splitting into two descending tongues between 6 and 9 km altitude. The two-branched pattern is most prominent in the DIAL data and appears less strong in the MESO-NH data. The ECMWF analysis exhibits two descending branches with the western one formed only weakly. All three data sets show


that within the stratosphere the strongest descent occurs at the •' 370 eastern flank of the streamer between 8 and t0 km. In the upper troposphere, in the western part of the streamer, the • 360 descent is obvious between 6 and 8 km.

The structure in the water vapor mixing ratio shown in the E 350 three data sets indicates that the intrusion bends toward the. • 340 west between the upper troposphere and the midtroposphere. The midtropospheric PV structure shows a similar bending • 330 toward the west (Figure 3), where, for example, the 2 pvu 60 isoline forms a tropopause fold. Plate 1 shows the important • 50 fact that the intrusions defined by the PV and water vapor are •E 40 found at different locations. The PV (Plate 1, solid lines) '- 30

descent is less pronounced in the western part of the streamer, 2O

whereas this region is characterized by very low water vapor o

mixing ratios. At the eastern edge of the streamer, larger PV N 10 0

values occur in the stratospheric intrusion, but the mixing ra- 0 tios are still relatively high. Both features suggest that there are • -10 differences in mixing PV and water vapor. The descent of -o -20

stratospheric air indicated by the dashed line in Plate 1 shows '• -30 that for a threshold value of 2.5 pvu the ECMWF data give a ._o -40 more pronounced tropopause descent than that simulated by

the model. However, considering the WMO [1986] threshold '• -50 value of 1.6 pvu, both data sets exhibit a broad descent stretch- E -60 ing horizontally over more than tOO km. This underlines the difficulty of defining a "realistic" tropopause, the transition zone between the troposphere and the stratosphere.

The DIAL data indicate that the magnitude of the water vapor mixing ratios ranges between 60 and 80 ppmv within the streamer at an altitude of 6 km. In low descending tropopause folds, water vapor mixing ratios between t0 and 50 ppmv were

b

ß c

--- ECMWF

....... I 1530 1600 1630 1700

time (UTC)

12•

10 •

8._0 o

6 > ._m

4• o

2 •'

Figure 4. Wind and temperature data on November 12, 1998: (a) potential temperature and potential vorticity (dashed line), (b) zonal wind, (c) meridional wind. The Falcon data were taken between 1500 and 1700 UTC (thin lines); the ECMWF data are from 1800 UTC (solid lines).

,,..

4 -,,

2

0 250 500 750 1000

distance (km)

Figure 3. Cross section above the baseline given in Figure t (dashed line) between the Pyrenees (to the left) and Munich (to the right) for November 12, 1998, 1800 UTC (ECMWF): potential temperature and meridional wind (top), potential vorticity (bottom). The hatched area indicates northerly wind which comes out of the plane (top). In the bottom figure the hatched area indicates the stratosphere limited by 2.5 pvu. The increments are 5 K and t0 m/s (top), and t pvu (bottom). The area bounded by the dashed line is given enlarged in Plate t.

measured in situ by aircraft [Vaughan et al., 1994]. In general, the magnitude of the measured water vapor mixing ratios compares sufficiently well with those simulated and analyzed. All three data sets show the strongest gradients in water vapor mixing ratio within the tropopause region close to the 100 ppmv isoline. The vertical gradients west of the streamer appear particularly strong in the measured data, moderate in the simulated data, and weakest in the analysis data; the reason for this is the different vertical resolution of the three data sets.

Additionally, the layers with 50-100 ppmv are located at different heights for the three data sets. Beneath the tropopause region, for example, between 8 and 9 km at the western flank of the intrusion, the DIAL measurement fails above 300 ppmv. This is due to the strong increase in optical density caused by the strength of the absorption line specially selected to detect the weak stratospheric concentrations. The ECMWF (MESa- NH) data indicate an increase up to 800 (tOO0) ppmv at an altitude of 6 km at the western flank. Above the tropopause region in the stratosphere and at lower altitudes within the streamer the DIAL data show distinct horizontal structures

that appear neither in the simulation nor in the analysis. The following section compares the humidity structure more

quantitatively. Two distinct vertical profiles were selected for this purpose: one east of the streamer (profile a) and another within the streamer (profile b). The profiles are located at 46.5øN/7øE (profile a) at 580 km in Plate 1 and 46øN/6øE (profile b) at 470 km. Figure 5 compares vertical profiles of water vapor mixing ratios measured by the DIAL with MESa- NH, ECMWF, and radiosonde data of Payerne (46.82øN/ 7.95øE). In order to allow a comparison between single-profile


11

lO

time (UTC) 1630 1640 1650 1700 1710

11

10

•______ no data ] DIAL 6-•

no data

MESO-NH

11

10

6 •_ECMWF 0 lOO

0 10 2o 3o 4o

200 300 400 500 600

d•stance (km)

50 60 70 80 90 100 200 300 400 500

water vapor mixing ratio (ppmv)

Plate 1. Cross sections of water vapor mixing ratio (ppmv) on November 12, 1998, above the baseline given in Figure 1 (solid line) between 43.8øN/1.4øE (left-hand side) and 47.0øN/7.7øE (right): data taken by the DIAL between 1622 and 1713 UTC (top), data simulated by the mesoscale model MESO-NH (center), ECMWF operational analysis, 1800 UTC (bottom). Note that there is a jump in scaling at the 100 ppmv isoline: the increment is 10 ppmv below it, and 100 ppmv above it. The solid lines indicate the potential vorticity (increment 1 pvu). The dashed line indicates the tropopause (threshold value of 2.5 pvu).


data provided by the radiosonde, ECMWF analyses, and the model with the DIAL data, it is necessary to average the latter horizontally over 25 km. For the DIAL data this is equivalent with a time period of---2 min. Thus the DIAL profile within the streamer represents the period 1658-1700 UTC, and the profile above the eastern flank is averaged over the period 1706- 1708 UTC. These profiles are compared to the corresponding MESO-NH and ECMWF grid point data.

The profiles of water vapor mixing ratio of all three data sets compare very well within the streamer (profile b). The dry stratospheric air descends down to less than 6 km and reaches about 150 ppmv. To the east of the streamer (profile a) where the tropopause is located at much higher altitude, similar low values are already exceeded at an altitude of more than 9 km. The strong gradient tropopause region is reached between 8 and 10 km depending on the data type. The vertical gradient across the tropopause appears strongest in the radiosond• data, whereas the ECMWF analysis data show the weakest vertical gradient. Obviously, the vertical resolution strongly determines the sharpness of the gradient. This is in accordance with Danielsen et al. [1987], who pointed out that objective analyses tend to produce smaller PV gradients and to under- estimate the downward extension of a tropopause fold. Be- neath the tropopause layer at 8 km the values of water :,apor mixing ratio reach and exceed 200 ppmv. The profile derived from the model data compares sufficiently with the ECMWF profile, which might be due to the fact that the model is synoptically forced by ECMWF analyses. The observed radiosonde profile comes closest to the measured DIAL profile. In

( o % 46'5øN / 7øE 11 - • o 10 _ e•/ o

9- ß •

'

', - --

0 2bo 300 water vapor mixing ratio (ppmv)

MESO-NH ECMWF Radiosonde

Figure 5. Vertical profiles of water vapor mixing ratio (ppmv) on November 12, 1998: (a) east of the streamer (46.5øN/7øE, 1707 UTC); (b) above the center of the streamer (46øN/6øE, 1659 UTC). Measurement data (DIAL, solid lines); simulation data (MESO-NH, dashed lines); analysis data (ECMWF, dots); radiosonde data of Payerne, 1200 UTC (open circles).

Table 1. Total Precipitable Water Content in a Vertical Column for Vertical Profiles Given in Figure 5

Layer East of Within Limits, Streamer a, Streamer b,

km Data mm mm

1.3-30.0

6.0-11.7

8.7-11.7

ECMWF 8.108 7.295 ECMWF 0.217 0.078 MESO-NH 0.311 0.073 DIAL '" 0.060

ECMWF 0.007 0.009 MESO-NH 0.013 0.012 DIAL 0.019 0.014

,

aEast of the streamer (46.5øN/7øE). bWithin the streamer (46øN/6øE).

general, the measured profiles show larger values than the analyzed and simulated profiles in the layer between 8 and 9 km. In the upper troposphere the magnitude of the profile values differs due to the fact that the tropopause zone is located at different altitudes. In the lower stratosphere, however, the DIAL, ECMWF, and model data compare very well, which is in agreement with Hoinka [1999], who showed that data from ECMWF, aircraft, and satellite compare very well in the lower stratosphere and in the tropopause region. Only the radiosonde data indicate larger magnitudes in the water vapor mixing ratio. However, at high altitudes, humidity measured by radiosondes is of limited accuracy.

Furthermore, the total precipitable water content in a vertical column is evaluated above the same locations as in Figure 5. The water vapor signal associated with a streamer is typically an order of magnitude less compared to that of the ambient upper tropospheric air. The ECMWF data show that for this event the total precipitable water content of the entire column is about 10% smaller in the streamer region than outside of the streamer (Table 1). Above 6 km (480 hPa) within the streamer the water content amounts to less than one third of that cal-

culated for the profile east of the streamer. For the layer between 8.7 and 11.7 km the values are slightly larger outside the streamer, except for the ECMWF analysis; the DIAL data give rise to the maximum value. Figure 5 shows that for the layer under consideration the magnitude of the precipitable water derived from ECMWF data is lowest, whereas the precipitable water values are at a maximum when derived from DIAL.

Browell et al. [1987, 1994] determined ozone and aerosol distribution within a tropopause fold by an airborne DIAL system, showing that regions of high aerosol scattering and enhanced ozone mixing ratios were correlated with descending air from the lower stratosphere. In a similar way, lidar measurements of aerosols across the streamer are presented in Plate 2, which, like Plate 1, depicts a false-color display of the backscatter ratio distribution along the atmospheric cross section beneath the aircraft. On both flanks of the streamer there

are middle to upper tropospheric clouds. These clouds are optically thick to the laser beam, which results in a shadow beneath them. The middle to upper troposphere typically has a smaller aerosol loading than the stratosphere. In the present case, strong loading of aerosol is apparent slightly above the tropopause and within the streamer, as shown by the red to brown coloring. As the air descends within the streamer toward the west, its backscatter intensity decreases. This coincides with an enhancement of water vapor mixing ratio, as seen in Plate


11'

10

1630 1640 time (UTC)

1650

.

.

1700

.

, .

1710

.

.

o. 100 2oo

1.00 1.06

3oo 400 500

distance (km) , • . .. . •.• , ,•-•..•-.•,-..,.• ,..• • •, ..... , ,, ,

1.12 1.18 1.24

backscatter ratio

600

1.30

Plate 2. Cross section of backscatter ratio on November 12, 1998, above the baseline given in Figure 1 (solid line) between 43.8øN/1.4øE (left-hand side) and 47.0øN/7.7øE (right). The color bar increment is 0.02.

1. In the center of the streamer there is even a small region with higher backscatter ratios of up to 1.28. However, the local maximum at 6 km altitude and 1700 UTC is assumed to orig- inate from the lower troposphere.

In the present case the backscatter ratios increase from 1.10, measured just beneath the tropopause to 1.28 at about 1000 m above the tropopause, for example, at a distance of 200 km from the west. A rough estimate of the effect of stratospheric background aerosol shows that the corresponding backscatter ratio within the lowest 1000 m of the stratosphere is about 1.13 [Jursa, 1985], which is much less than the measured value of 1.28. Following J•ger and Hornburg [1998] the three years be- fore the measurement can be considered as "background period" when stratospheric aerosol was free from major volcani- cally induced increases.

Single vertical profile measurements of aerosol mixing ratios and aerosol concentrations show shallow layers of strong values close to the tropopause, both above and below it [e.g., Hofmann et al., 1975]. They pointed out that strong values just above the tropopause, at about 12 km, could be a transient feature, although such aerosol layers near the tropopause are often observed during winter and spring. It is not clear why the low stratosphere should have been observed to be strongly loaded with aerosols in the present case. In the region where the measurement was performed, heavy upper tropospheric and lower stratospheric air traffic produces a significant quan- tity of aerosol. This might contribute significantly to the aerosol concentration in the lower stratosphere above southern and central France between the Alps and the Pyrenees.

5. Discussion

In the present paper a newly developed H20-DIAL system was employed for the first time for airborne remote sensing of

very low water vapor mixing ratios. This instrument is particularly useful for measuring water vapor in the lower stratosphere. The capability of the DIAL has been shown by a comparison with data obtained from operational analyses (ECMWF) and from a numerical simulation (MESO-NH). All three data sets show good agreement in structure and magnitude of the low stratospheric water vapor within the potential vorticity streamer selected for a first test of the instrument.

However, there are problems in verifying the measured data. The reason for this is that the simulated and analyzed humidity data do not give sufficient details of the real atmosphe/-ic situation to verify the measured water vapor data. For the ECMWF operational analyses this might be because the resolution is too coarse. The usual problem with numerical simulations is to correctly select the time in the model and the data to compare with the atmospheric measurements. There is a large degree of uncertainty in the low values of humidity measured by radiosondes. These low magnitudes of water vapor typically occur at high altitudes and low temperatures. At present, only volume-integrated humidity data can be derived from satellite data.

On the other hand, there is a strong demand for remotely sensing three-dimensional, small-scale water vapor structures at low concentrations. The new DIAL system that has been presented is a powerful tool to do this, by obtaining two- dimensional sections, and thus can be used for a broad variety of applications. A further improvement will be the increase of the spatial resolution. This can be achieved by pumping the narrowband optical parametric oscillator not with the 10 Hz flashlamp-pumped laser but by a novel 100 Hz diode-pumped Nd:YAG laser. This technical improvement will increase the horizontal resolution by a factor of 10.

Nevertheless, it is clear that there needs to be more verifi-


cation of the water vapor measured by the DIAL. One good approach would be to collect humidity data in a second aircraft flying beneath the one with the downward looking DIAL aboard. However, this would give just one data line through the data curtain provided by the DIAL. During the Mesoscale Alpine Programme (MAP 1999) [Bougeault et al., 1998] it is planned to carry out verification experiments.

Acknowledgments. The operational analysis data were kindly provided by the ECMWF within a "Special Project" under the title "The climatology of the global tropopause." We thank Martin Wirth (DLR) for helping with the OPO stabilization and the data acquisition during flight operation; Arnold Tafferner and Martin Leutbecher (both DLR) for providing us with forecast data; and DLR flight facility for processing the aircraft data. We thank Nicole Asencio (Meteo-France) for helping to prepare the model simulations and Joel Van Baelen (CNRM) for helping to plan the flight.

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(Received April 15, 1999; revised August 20, 1999; accepted August 31, 1999.)

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Low stratospheric water vapor measured by an airborne DIAL