Detailed Observations of Five African Easterly Waves During NAMMA

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Detailed Observations of Five African Easterly
Waves During NAMMA F. 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 UTC
• TD at T-6 hrs
• TS Debby at T24 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 UTC
• NO 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 UTC
• NO Immediate Development
• Became 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 UTC
• NO 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 UTC
• TD at T-15 hrs
• TS Helene at T21 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

SUMMARY Personnel 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.