3.6 Radiosonde profile Comparison
3.6.5 Examples: 13th and 15th Feb 2014
Comparing mean atmospheric profiles can hide deficiencies in the performance of reanalysis products. Here two examples are briefly discussed to illustrate profile biases at one instance in time and in differing synoptic situations.
962
Figure 3.15: Mean sea level pressure and temperature over the Amundsen Sea Embayment at:
Left (a) 1200UTC on 13th February; and right (b) 1200UTC on 15th February (both from ERA-I). The wind vector shows the wind direction recorded on the research vessel, the observed near-surface wind speed was 12 m s−1on the 13th and 10 m s−1on the 15th. The background colour contour shows the 2-m temperature (Celsius) with labelled isobars showing the mean sea level pressure (hPa).
On both the 13th and 15th February a radiosonde was launched from the JCR when it was located close to the ice shelf of PIG. On 13th February the implied wind direction from the isobars is slightly south of east, surface friction is the likely cause of south-easterly near-surface winds recorded on the JCR. A large, deep cyclone with centre to the north east of PIG is driving the synoptic weather pattern (see Fig.3.15a), the isobars are close together, suggesting strong winds. The mean sea level pressure on board the JCR dropped through the day suggesting a westward movement of the cyclone, which is supported by ERA-I (not shown).
72
ANEVALUATION OF FOUR GLOBAL REANALYSIS PRODUCTS USING IN SITU OBSERVATIONS IN THEAMUNDSENSEAEMBAYMENT
The synoptic conditions on 15th February show a weaker low pressure system centred north of the JCR with central pressure of 974 hPa (see Fig. 3.15b). Field notes reveal that light snow had fallen overnight but had stopped by 1120 UTC, the launch time. Low cloud associated with the cyclone was reported and the surface temperature recorded on the JCR was −7.6 ◦C . Despite ERA-I pressure contours suggesting easterly near-surface flow, the wind direction recorded on the JCR was from the south east, blowing off PIG ice shelf.
256 259 262 265 268 271 800
Figure 3.16: Radiosonde and reanalysis profiles from 1620UTC on 13th February 2014: Top left (a) Temperature; top right (b) Specific Humidity; bottom left (c) Wind speed; and bottom right (d) Relative Humidity.
The observed temperature profile from 13th Feb (Fig. 3.16a) shows a 5 ◦C temperature inversion at 880 hPa. JRA-55, ERA-I and MERRA all produce a temperature inversion that is both vertically broader and typically somewhat lower than observed; they all fail to reproduce the sharp temperature gradient over a few tens of metres - unsurprisingly given their coarse vertical resolution. All of the
3.6.RADIOSONDE PROFILECOMPARISON 73
reanalysis products struggle to reproduce a humidity inversion at the same altitude and are therefore too dry, particularly above the inversion (Fig. 3.16b). The wind speed profile from the radiosonde shows a strong LLJ at the same altitude as the base of the temperature inversion. The reanalysis products that contain a temperature inversion all produce a LLJ with peak wind speed at approximately the same altitude as observed. CFSR, which doesn’t reproduce the inversion, does produce a LLJ but it is weaker and higher than in the other reanalysis products and the observations.
Despite the shortcomings discussed here, it is clear that the reanalysis products are doing a relatively good job of reproducing the atmospheric profiles. In this case the temperature inversion and LLJ are likely linked to the synoptic-scale cyclone, perhaps a frontal system, and due to the larger scale of the forcing mechanism it appears the reanalysis products successfully identify features such as the inversion and LLJ.
256 259 262 265 268 271 800
Figure 3.17: Radiosonde and reanalysis profiles from 1200UTC on 15th February 2014: Top left (a) Temperature; top right (b) Specific Humidity; bottom left (c) Wind speed; and bottom right (d) Relative Humidity. The coloured lines represent the same reanalysis products as in Fig.3.11.
74
ANEVALUATION OF FOUR GLOBAL REANALYSIS PRODUCTS USING IN SITU OBSERVATIONS IN THEAMUNDSENSEAEMBAYMENT
The radiosonde temperature profile from 15th Feb (Fig.3.17) shows a low level temperature inversion with its base at 960 hPa, its top at 935 hPa and a temperature increase of approximately 4 ◦C . Of the reanalysis products only MERRA produces an inversion of similar strength and depth, although in its case the inversion extends down to the surface. From the surface to 935 hPa MERRA is 4◦C colder than observed.
ERA-I and CFSR both accurately reproduce temperatures above 900 hPa but fail to produce a near-surface inversion and hence over-estimate the surface temperature by 4◦C .
Coincident with the base of the temperature inversion a LLJ is recorded in the observed wind speed profile (Fig.3.17c). Only ERA-I produces a feature that could be described as a LLJ and while this does match the maximum wind speed, it is more than 50 hPa (approx. 500 metres) above the observed jet and it is too broad. Above the jet the observed wind speed drops and all of the products over-estimate the wind speed between the 940 hPa and 800 hPa pressure levels. In this example the reanalysis products struggle to reproduce the temperature and wind speed profiles; with both the observed near-surface temperature inversion and low-level jet absent in three of the four reanalysis products. With the JCR located 2 km from PIG ice shelf and the wind blowing off the glacier it is possible that katabatic or orographic enhancement of the near-surface wind may have caused an acceleration of the jet and this may explain the inaccurate profiles from the reanalysis products.
• Mean atmospheric profiles show that ERA-I and CFSR are most accurate at reproducing temperature and humidity profiles (see Fig. 3.11). All products accurately reproduce the surface wind speed but underestimate the wind speed aloft (Fig.3.11c).
• All products contain a cold temperature bias in the group of radiosondes which were launched close to the Antarctic continent (Fig. 3.12). The largest temperature bias is seen in MERRA with the product having a tendency to produce a strong near-surface inversion (Fig.3.12).
• The wind speed bias between 950 and 850 hPa is caused by underestimations of the wind speed in profiles containing a LLJ, this indicates that the magnitude and frequency of LLJs are being under-represented in the reanalyses (Fig.3.13).
3.6.RADIOSONDE PROFILECOMPARISON 75
• Evaluation of individual examples indicates that reanalyses have more skill at reproducing features such as temperature inversions and LLJs when there occurrence is linked to large-scale forcing mechanisms such as synoptic cyclones (compare Figs.3.16and3.17).
76
ANEVALUATION OF FOUR GLOBAL REANALYSIS PRODUCTS USING IN SITU OBSERVATIONS IN THEAMUNDSENSEAEMBAYMENT