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, 2007.

• 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 10000 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

• 7/22 1030 UTC a stratiform case.• Divergence and its standard error is

calculated at each pressure and range

446994119

How Does Continental Africa Compare to the Rest of the Tropics?

Comparison to EPICNIAMEY

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 10-5 s-1 per mm/hr

Divergence 10-5 s-1 per mm/hr

Convective and Stratiform Regressions JAS (2006)

Convective StratiformConvective

• 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

[mm day-1]

[mm day-1]

dry

wetwet

Heating over Land: Comparison with Radar Observations

Pre

ssur

e (h

Pa)

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 2006MIT C-Band radar operated in Niamey, Niger during JAS 2006-2007.

Radar observations suggest a peak heating rate ~300-500 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 (Q1) derived from Global Atmospheric Research

Program Atlantic Tropical Experiment (GATE).

During Aug. 30 - Sep. 18, 1974.

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)

[ PVU day-1]

The largest positive contribution to the PV tendency came from the explicit latent heating.

5-13°N JAS 98-09 5-13°N JAS 98-09

Vorticity (contours, x10-6 s-1) and (Vorticity΄)2

(x10-10 s-2, 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.

Sahelian Monsoon Convection Among Most Intense in the World