Characteristics of Convection in an African Easterly Wave Observed During NAMMARobert Cifelli, Timothy Lang, Steven A. Rutledge

Colorado State University

Acknowledgements

This work is supported by NASA NAMMA grant NNX06AC11G under the supervision of Dr. Ramesh Kakar. Paul Kucera provided the NPOL data.

Background and Motivation

Previous studies have shown that precipitation characteristics vary with African Easterly Wave (AEW) phase, depending on geographic region. Gu et al (2004) showed that precipitation tends to occur ahead (behind) the trough

for waves south (north) of 15N. Kiladis et al. (2006) found that convection shifts from ahead of the wave trough to

behind the wave trough as the AEW propagates across west Africa and into the east Atlantic.

Because these studies relied on indirect methods to observe precipitation (i.e., sounding, reanalysis, and satellite IR

data), detailed understanding of differences in convective properties could not be discerned. Further, the differences in characteristics of convection within AEWs that ultimately develop into tropical cyclones and those that do not remains unclear.

The NAMMA (NASA African Monsoon Multidisciplinary Analysis) - AMMA data set provides a unique opportunity to study the characteristics of convection in AEWs with ground-based radar in both continental and maritime locations (see panel 3). In particular, this study seeks to understand how modulations of AEWs at the synoptic scale are

manifested in radar observed precipitation characteristics in different geographic locations. Ultimately, we seek to identify distinctive precipitation characteristics (if any) of AEWs that develop into tropical cyclones.

To begin to address this issue, we examine data from one AEW (NAMMA wave 5) that was sampled by the TOGA and NPOL radars on 2-3 September 2006 . This wave was well sampled by both ground-based radars as well as the NASA DC-8 as it propagated from west Africa into the eastern Atlantic. The wave was also the likely precursor to hurricane Gordon, the most intense tropical storm of the 2006 season in the Atlantic basin.

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NASA African Monsoon Multidisciplinary Analyses (NAMMA) Objectives

• AMMA:Characterize evolution and structure of African Easterly

Waves (AEWs) and Mesoscale Convective Systems (MCSs) over western Africa, and their associated impacts on regional water and energy budgets

• NAMMA:Examine formation and evolution of tropical hurricanes in the

eastern and central Atlantic and their impact on the U.S., and the composition and structure of the Saharan Air Layer (SAL), and whether aerosols affect cloud precipitation and influence cyclone development.

• NAMMA is the “downstream” component of AMMA

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Approximate region of TOGA coverage

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• The Cape Verde Islands are in a unique transition region between intense land-based convection over Africa and weaker maritime convection over the open ocean

1998-2004 JJA TRMM Climatology of AMMA-NAMMA Region

The progression of NAMMA wave 5 between 2 and 3 September is shown in these four panels from METEOSAT IR data. Brightness temperature and Saharan Air Layer (SAL) intensity is indicated by the gray and color shading, respectively. The approximate position of the wave trough is indicated by the dashed yellow line. Convection is located in the vicinity of NPOL on 2 September (panel a) and progressively translates toward TOGA on 3 September (panels c and d).

(a)

(b)

(c)

(d)

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12 hour operational forecasts from the GFS model on 2 September (top) and 3 September (bottom). Note the change in position of the African Easterly Jet (left panels - dashed purple lines) and the change in vorticity structure ahead of the trough (right panels - trough indicated by solid black lines) between 2 and 3 September. As shown in panel 9, the change in the AEJ has a large impact on the organization of convection observed by the radars.

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Analysis Procedure

• AEW trough passage at both the TOGA (Praia, CV) and NPOL (Dakar, Senegal) sites determined through change in 700 mb meridional wind (N-->S)

• TOGA and NPOL radar data QC’d and interpolated to Cartesian grid extending 120 km from each radar with a grid spacing of 2 km in x,y, and z dimensions

spurious echos (clutter, side lobes, anomalous propagation) removed

4 (2)dB bias removed from TOGA (NPOL) data, based on comparisons with TRMM-PR

within each radar volume, contiguous echo regions identified (features) and horizontal and vertical characteristics analyzed

• Examine satellite data over 5º x 5º region centered on radar location to see if ground-radar analyses are consistent with larger-scale observations

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Time series of wind and relative humidity for 1-3 September at Dakar (left) and 2-4 September at Praia (right). The approximate time of the trough passage is indicated by the vertical dashed line in each panel. Note the existence of relatively strong shear (middle panels) ahead of the trough passage, especially at Dakar. The trough passage is also marked by a significant increase in mid-level (700 mb) relative humidity (bottom panels), especially at Praia.

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Radar CAPPI’s and cross-section examples of convective organization at NPOL (left) and TOGA (right). The NPOL example is from a time period when the AEJ is in the vicinity of the radar (panels 6 and 8). Based on studies from COPT81, the AEJ shear is important in organizing the convection into a classic squall line structure. In contrast, the TOGA example is during a period when the AEJ has moved north of the Cape Verdes and the shear is greatly reduced. The convection observed by TOGA at this time is much less organized.

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Time series of horizontal precipitation feature characteristics observed by the TOGA (red) and NPOL (blue) radars. The absicca is time relative to the trough passage at each location. Note that the size of features (especially MCSs) in panel (a) is generally larger at NPOL, consistent with the TRM climatology shown in panel 4. Also, the feature areas (panels c and d) show different trends: broad increase ahead of the trough at TOGA compared to a bimodal structure at NPOL. The bimodal structure is due to the more pronounced influence of the diurnal cycle at Dakar compared to Praia.

(a)

(b)

(c)

(d)

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Same as panel 10 except for echo top height and rainfall characteristics. The most intense activity at TOGA in terms of rain volume and rain rate (panels b and c) occurs ahead of the trough. The same trend occurs at NPOL; however, the pattern is more complicated due to the influence of the diurnal cycle.

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Radar reflectivity contoured frequency by altitude (CFAD) for TOGA (left) and NPOL (right) ahead (top) and behind (bottom) the trough. Contours indicate relative frequency of occurrence (%) of radar reflectivity at a given height. Note the steeper vertical gradient of radar reflectivity above the melting level for the distributions behind (panels b and d) the trough compared to ahead of the trough (panels a and c). This pattern suggests stronger vertical drafts in the “pre” trough convection, consistent with trends in lightning flash density (not shown).

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Histogram of IR brightness temperature as a function of AEW phase for NPOL (blue) and TOGA (red). The IR data are based on a 5 x 5 box surrounding each radar. The colder Tb temperatures at NPOL ahead and behind the trough are somewhat at odds with the ground radar zero dBZ echo tops (see panel 11) but are in agreement with the 20 dBZ echo tops. The difference is probably due to the larger region sampled by the satellite compared to the radar.

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Time series of TRMM 3B42 rain rates based on a 5 x 5 region surrounding the TOGA (top) and NPOL (bottom) radars.

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Summary• The AMMA-NAMMA radar data sets provide an unprecedented opportunity to study the

detailed evolution of convection within AEWs, traversing continental to oceanic environments.

• One event has been examined in detail using two of the ground radars in a coastal (Dakar) and maritime (Praia) environment, as well as upper air sounding and satellite data.

• Convection in both the coastal and maritime locations is maximized (by almost all feature characteristics in 10-11) ahead of the AEW trough.

• The evolution of feature characteristics in the maritime location showed a relatively smooth transition in intensity during the AEW passage. The structure was more complicated at the coastal site due to the combined effect of the AEW passage and diurnal convection associated with land-sea breeze convergence. The result was to produce a bimodal pattern in the evolution of most of the precipitation feature characteristics at the NPOL radar site.

• Next step is to tie in large scale dynamics (vorticity) as it relates to precipitation features and to fold in MIT radar data from a continental location (Niamey).

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