Detailed Observations of Five African Easterly Waves During NAMMAF. J. Schmidlin (NASA) , B. J. Morrison (SSAI), E. T. Northam (SSAI), J. Gerlach (NASA)

NASA Wallops Flight Facility, Wallops Island, Virginia

22 Aug 2006 / 00 UTCTD at T-6 hrs

TS Debby at T+24 hrs• Associated with active TD to south • Active deep convection• Followed by strong surge in low- level easterlies• Strong buoyancy maintained prior to, during, and after wave passage

26 Aug 2006 / 02 UTCNO Development

• Low level cyclonic circulation passed directly over Praia• Convective activity suppressed• Dominated by Saharan Air Layer• Low buoyancy and CAPE

03 Sep 2006 / 14 UTCNO Immediate DevelopmentBecame Gordon on 11 Sep?

• Associated with Monsoon Trough over southern Cape Verde Islands • Active deep convection• Buoyancy drops at wave passage and only slowly recovers

09 Sep 2006 / 14 UTCNO Development

• Associated with eddy in Monsoon Trough approx 200 km south of Praia • Marginal convective activity• Relatively shallow wave• Southerly flow aloft due to upper- level trough to northwest• Slow decrease in buoyancy prior to wave passage, then large drop at wave passage with recovery afterwards

13 Sep 2006 / 03 UTCTD at T-15 hrs

TS Helene at T+21 hrs• Associated with active TD south- southwest of Praia • Active deep convection• Preceded by strong surge in low-level easterlies• High buoyancy maintained prior to, during, and after wave passage

SUMMARYPersonnel from the NASA Wallops Flight Facility Upper Air Instrumentation Research Project (UAIRP) were deployed to Praia on the island of Santiago in Cape Verde during the NAMMA field program. They conducted round-the-clock GPS radiosonde soundings from 18 August through 14 September. A total of 150 soundings were done at a frequency of 6 per day (every 4 hours) until 09 September and then 4 per day (every 6 hours) afterwards due to site power problems. 91% of the ascents exceeded altitudes equivalent to 50 hPa.

Five African Easterly Waves (AEWs) were clearly present during the observational period and are shown in the vertical time series of wind north-south (v-) component, above, and in the geopotential height time series at selected levels in the figure beneath. The axes of these waves in the low-level tropical easterly flow passed over Praia on August 22, 26, and September 3, 9, and 13. We identify them as W22, W26, W03, W09, and W13, respectively. An additional wave, may have passed through sometime around August 31, but was very weak and not well resolved. A total of 6 AEWs over an observational period of 28 days give an approximate periodicity of 4.7 days, which is within the average periodicity of 3-5 days reported by Burpee (1972) and 2-5 days reported by Reed et. al. (1977).

The first and last AEWs, W22 and W13, observed at Praia were already associated with Tropical Depressions (TDs) centered to the south. Both of these disturbances continued to develop into Tropical Storms as they moved west of Cape Verde. The three intervening AEWs did not develop, although it might be speculated that W03 may have persisted in the tropical easterlies and eventually developed into Tropical Storm Gordon 8 days later in the Eastern Caribbean. If we assume that 2 out of 6 waves developed during the 28 day observational period, this represents a 33% development rate - much higher than is observed on average for the development of tropical disturbances (e.g. 10% as reported by Frank, 1971).

Selected analyses for each observed AEW are presented at the right. Of specific interest to NAMMA objectives included examination of the effects of the Saharan Air Layer (SAL) and Mid-Level Easterly Jet (MLEJ) on AEW development. The SAL was clearly observed as an elevated mixed layer of warm dry air bounded by inversions in a number of soundings from August 23 - 28 and September 6 - 9. The presence of the SAL during the passage of W26 may have suppressed development of this wave due to entrainment of dry air to inhibit convection resulting in low parcel buoyancies.

Significant acceleration of the mid-level easterly flow, indicative of the MLEJ, was observed to coincide with W22 and W13, both of which continued to develop, but was mostly absent during the other wave passages. Karyampudi, et. al. (1999) have noted transverse vertical circulations with rising motion on the southern boundary of MLEJ and their potential impact on wave development.

References

Burpee, R.W., 1972: The origin and structure of easterly waves in the lower troposphere of north Africa. J. Atmos. Sci, 29, 77-90.

Frank, N., 1971: Atlantic tropical systems of 1970. Mon Wea. Rev., 99, 281-285.

Karyampudi, V. M., S. Palm, J. Reagen, H. Fang, W. Grant, R. Hoff, C. Moulin, H. Pierce, O. Torres, E. Browell, and S. Melfiet,1999: Validation of the Saharan Dust Plume Conceptual Model Using Lidar, Meteosat, and ECMWF Data. Bull. Amer. Meteor. Soc., 80, 1045-1074.

Reed, R.J., D.C. Dorqist, and E.E. Recker, 1977: The structure and properties of African wave disturbances as observed during phase III of GATE. Mon. Wea. Rev., 105, 317-333.