Preliminary report from
EuroSOLVE Workpackage 2:

Mountain Waves and PSCs

Compiled by: Harald Flentje, DLR
31 March 2000

Introduction

While the last two stratospheric winters were comparatively warm, the stratosphere in late 1999 cooled down quickly, such that large-area PSCs occurred from mid-December onward in the presence of cold temperatures below 195K. Temporarily, the stratification and wind-direction were favourable for stimulation of gravity waves over either Greenland , Spitsbergen and the Scandinavian mountain ridge. The meso-scale adiabatic cooling of the vertically displaced air masses, as fore- and hindcasted by the MM5 model (Doernbrack, DLR), ranged up to more than 10K. Therefore, dense lee wave PSCs were frequently observed, especially over northern Scandinavia.

The Eurosolve PSC-Workpackage combines experimental and theoretical investigations by several groups:
NILU (Groundbased lidar at ALOMAR - O3 and aerosol)
DLR Oberpfaffenhofen (MM5 meso-scale model and airborne lidar – O3 and aerosol)
CNRS-SA (Airborne lidar; aerosol)
CNRS-LOA (Balloon-borne radiance measurements)
Univ. Bonn (Ground based lidar at ESRANGE - aerosol)
ETH Zürich (Meso-scale modelling)

Mesoscale models

The meso-scale MM5 forecasts of DLR run successfully from 1st December to mid-March and provided a very useful daily basis for mission planning. Simultaneously the HRM model has been run in full physics mode by ETHZ. Initial technical problems with the HRM model are now overcome and a couple of hindcast runs are presently performed (see Figure 1a,b).

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a

b

Figure 1. Dry (a) and moist (b) run of the HRM model for 2100 UTC on January 14, 2000, where in one of the waves the temperature was 2 K lower in the moist simulation compared to the dry run (ETHZ).

Aircraft measurements

On the basis of the MM5 forecasts 8 science flights ( incl. 2 transfers) of the FALCON lidar (see Figure 2, 3) and 4 science flights (one of these transfer to France) of the ARAT lidar (see Figure 4) have been carried out during the late January/early February SOLVE deployment. The flight legs were chosen to be quasi-Lagrangian as referred to the PSC level (about 30 - 50hPa). During the missions various extended PSCs, either synoptic as well as wave induced, have been measured. Strong wave amplification with PSCs in 20-27 km altitude occurred on Jan 25 to 27 and again after Feb 4, 2000, while in between synoptic PSCs below 19 km, sometimes down to the tropopause, have been observed. The PSCs exhibited considerable internal, partly filamental, structure. Inside the mountain wave PSCs, frequently, narrow layers of ice particles were embedded in liquid particles. Also pure ice clouds were observed as in the lee wave on January 26, 2000 (see below) which exhibited backscatter ratios up to 800 at 1064 nm. On Jan. 25 the ARAT performed two successful, coordinated flights with a PSC in situ analysis gondola. PSCs with a layered structure were observed in the balloon trajectory between 20 and 22 km altitude. Two more ARAT flights took place on Jan. 26 and 27. On Jan. 27 the FALCON tracked two balloons (HALOZ and TRIPLE). A diminishing mountain wave PSC was observed, which in its lower, mainly liquid part was caught by the balloons.

The low synoptic PSCs layers (Figure 3), observed after Jan. 31 were investigated in coordination with the NASA?s ER-2 aircraft in the region between northern Scandinavia and Spitzbergen. The PSCs exhibited very low backscatter ratios of only up to 3 in the infrared. Especially on Feb 3 a coordinated mission with the ER-2 has been performed during which the particles were simultaneously measured by the in situ probes onboard the ER-2 and the FALCON lidar. A focus of our investigations will be put on these layers, since e.g. the MASP in situ measurements indicate the existence of large particles while the lidar signal was dominated by relatively small particles (colour ratio larger than 1). Perhaps there were relatively few quite large particles (the term "rocks" rose during the campaign discussions).

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Figure 2. MM5 forecast (left) and OLEX section of backscatter ratio (right) perpendicularly to the mountain wave on Jan 26, 2000, with underlaid orography. The MM5 run shows temperature contours in a vertical 2-D section. The temperature anomalies are induced by gravity waves above the Scandinavian mountains. The response of the wave to the ridges is obvious in the lidar data.
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Figure 3. 2-D section of backscatter ratio @ 1064 nm, measured on February 3, 2000, on the way from the North Cape to Spitsbergen. The flight was closely co-ordinated with the NASA's ER-2, from which the contrail can presumably bee seen around 15.5 km altitude at 72.7°N. Below 13 km the overlap of the laser beam with the telescope was no yet complete. Not corrected for extinction.
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Figure 4. (a) ARAT flight path on Jan. 25, 2000. (b) Corrected parallel signal at 532 nm (arbitrary units) versus time from balloon co-ordinated ARAT mission on Jan 25 at 1838 UTC. The tracking of the balloon was very efficient. PSCs with a layered structure were observed between Kiruna and the mid-distance to the Finnish border around 21 to 24 km altitude.

Ground-based measurements

Two ground based lidars were positioned up- and downwind of the Scandinavian mountains at Andøya and the ESRANGE, respectively. Unfortunately Tromsø (and north Norway as a whole) experienced a very overcast, snowy winter, so that PSC observation there was possible only temporarily, e.g. on Dec. 21, Jan. 21, 22, 26, 27, 28, 29 and Feb. 6. On most of these days, except Jan. 10, 27 and 28 PSC signals were detected. Fortunately the relatively good data coverage coincides with the second SOLVE deployment, where intensive Eurosolve activities took place. At the moment no measurement was possible since March 3.

The lidar at ESRANGE was operating from the end of November until the beginning of February. On 19 days of measurement about 106 hours of data were collected. On 8 days PSCs were detected, mostly around 20 km altitude with relatively low backscatter ratios of 1-2 in the parallel and 5-15 in the perpendicularly polarised channel. The highest PSC reached up to 25.5 km, the lowest was observed at 15 km. On all PSC days the temperatures above ESRANGE were between TNAT and TICE on all other days temperatures were higher than TNAT (see Figure 5). On three days PSC signatures are compatible with the detection of water-ice PSCs even though synoptic temperatures were too high for PSC II existence. Lee-waves on these days lowered the stratospheric temperatures sufficiently for water-ice PSC formation.

Figure 5. A first comparison of lidar PSC detection and the evolution of the temperature in the 50 hPa pressure level above the ESRANGE during the period December 1, 1999 to February 29, 2000. The solid line is the ECMWF T106 analysis temperature above the ESRANGE. Lidar measurement dates are marked by plus-signs (at 176 K), PSCs at the 50 hPa level are marked by times-signs for Type I (at 194 K) and by triangle-signs for Type II (at 187 K). Days with PSCs at the other levels 20 and 50 hPa are marked by squares and asterisks (near 180 K). The dashed horizontal lines represent the PSC formation temperatures in the 50 hPa level. In this winter PSCs were always detected whenever the temperature was below the PSC I formation temperature on the measurement date. Conversely, no PSCs were detected on measurement days, when temperatures were above the PSC I formation temperature.
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Balloon-borne measurements

The balloon-borne MicroRADIBAL instrument measured the radiance and polarisation degree of the sunlight scattered by the atmosphere at 730, 865, 1000, 1620 nm on January 26, 2000. It was launched at 0900 UT from ESRANGE-Kiruna and performed measurements between 13 and 23 km. The measurements are obtained at several altitudes during the ascent of the balloon. At many altitudes the measurements show that the layer was very inhomogeneous, so the preliminary analysis has been conducted on radiance scattering diagrams only (see Figure 6). The temperature profiles from the two PTU soundings (at 0458 UT and 0929 UT) indicate values lower than 195 K around 19-25 km, so that our measurements probably correspond to PSC observations, with large particles. The polarisation measurements are currently analysed and non-spherical particles will be considered in a further analysis.

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Figure 6. (a) the normalised radiance versus scattering angle at the four wavelengths at 22 km. We see that there is no good symmetry with respect to the sun incident plane (inhomogeneity of the layer), especially in the forward direction, the signal is much larger than the molecular component, the aerosols are thus well detected, the scattering diagrams are very sharp in the forward direction, that is characteristic of rather large aerosols, this statement is confirmed by the weak spectral variations observed in the forward direction. (b) Comparison of the measured scattering diagrams at two wavelengths, to diagrams obtained with a submicron size distribution (rm= 0.25 mm,
s = 0.3) and with a micron size distribution (rm= 1 mm, s = 0.3) for spherical particles. The bad agreement obtained with the first model shows that we are not in presence of mean ("classic") aerosols. The agreement is better with the second model, and the aerosol slant optical thickness is rather large: About 1 at 865 nm.