Precipitation Over Continental Africa and the East Atlantic: Connections with Synoptic Disturbances Matthew A. Janiga November 8, 2011

Motivation AEWs grow through both the mixed barotropic-baroclinic instability of the basic state and latent heat release from moist convection. Mesoscale convective systems can result in heavy rainfall and flooding.

The MIT Radar Data Set July 5 - September 27, 2006 and June 28 - September 30, Volume scans every 10 min. – 15 vertical tilts – Horizontal range of 150 km. – Nyquist velocity of 12.7 m/s. Operated from the Niamey, Niger airport. High resolution soundings also available. dBz

Automatic Unfolding and VAD Analysis With over volume scans in 2006 automation is key! Data is placed in hourly cylindrical coordinates bins. Step 1 – Relative unfolding: minimize variance in each bin. Step 2 – Absolute unfolding: the unfolding guess with the closest fit to a sine curve and greatest azimuthal continuity is used. 1) 2)

Estimation of Divergence 7/ UTC a stratiform case. Divergence and its standard error is calculated at each pressure and range

How Does Continental Africa Compare to the Rest of the Tropics? Comparison to EPIC NIAMEY

Regression between Rainrate and Divergence c/s = 0.98 JAS Mid-level convergence and downdrafts are stronger in the oceanic tropics. Values need to be taken with caution given attenuation problems with C- Band radars. Mapes and Lin (2005) Divergence s -1 per mm/hr

Convective and Stratiform Regressions JAS (2006) Convective Stratiform Convective Convective profile is similar. 2x3 times more low-level divergence in stratiform over Niamey than in other tropical regions.

Contrasts in the Vertical Profiles of Reflectivity from TRMM PR [Frequency] Fuentes (2008)

Convective and Stratiform Areas Schumacher and Houze (2006)

Climatological Rainrate in CFSR vs TRMM TRMM 3B42 Rainrate (mm day -1, shaded) CFSR Rainrate Bias (mm day -1, shaded) Relative to 3B42 CFSR Rainrate Bias (mm day -1, shaded) Relative to 3B42 [mm day -1 ] dry wet

Heating over Land: Comparison with Radar Observations Pressure (hPa) Regression between rainrate (derived from ZR relationship, Russell et al., 2010) and divergence estimated from the radial wind (similar to Mapes and Lin, 2005). Divergence (x10 -5 s -1 per mm hr -1 ) JAS 2006  MIT C-Band radar operated in Niamey, Niger during JAS  Radar observations suggest a peak heating rate ~ hPa consistent with CFSR. Approx. Peak Heating CFSR explicit latent heating (K day -1, shaded) and ω (hPa day -1, contours) 5-13°N JAS 98-09

Heating over East Atlantic: Comparison with GATE Apparent heat source (Q 1 ) derived from Global Atmospheric Research Program Atlantic Tropical Experiment (GATE). During Aug Sep. 18, Apparent heat source (Q 1 ) derived from Global Atmospheric Research Program Atlantic Tropical Experiment (GATE). During Aug Sep. 18,  The level of peak heating over the East Atlantic in the CFSR is also qualitatively similar to the results from GATE. Thompson et al., (1979) CFSR explicit latent heating (K day -1, shaded) and ω (hPa day -1, contours) 5-13°N JAS 98-09

How do we Explain the Atlantic- Continent Heating Profile Differences? Houze (1997)  The Atlantic region has more stratiform area but the heating profiles look more convective?  Increased shallow rain over the Atlantic.  Increased low-level cooling in the continental stratiform.  More top-heavy convective profiles over land than the ocean.  Convective/stratiform area ≠ rain amount.

PV Tendency in CFSR PV Tendency due to explicit latent heating (PVU day -1, shaded) Diabatic + friction PV tendency (PVU day -1, shaded) Diabatic + friction PV tendency (PVU day -1, shaded) [ PVU day -1 ]  The largest positive contribution to the PV tendency came from the explicit latent heating. 5-13°N JAS 98-09

Vorticity (contours, x10 -6 s -1 ) and (Vorticity΄) 2 (x s -2, shaded) Vorticity (contours, x10 -6 s -1 ) and (Vorticity΄) 2 (x s -2, shaded) Omega(x10 -2 Pa s -1 ) and RH (%, shaded) Omega(x10 -2 Pa s -1 ) and RH (%, shaded)

Characteristics of AEW Vortices

700 hPa Relative Vorticity

925 hPa Relative Vorticity

925 hPa Potential Temperature

850 hPa Vertical Velocity

Perturbation Temperature and Vorticity

Vertical Velocity

Vorticity Flux Convergence

Future Work Improved quantification of the divergence estimates, analysis of select events, and comparison with data from Bamako, Mali. Cloud resolving simulation of AEWs and diagnosis of the profiles of vorticity generation. – Importance of interactions between the transient and mean terms.

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