Regional Differences in Convection over During the

West African Monsoon

November 9, 2012 Tropical Lunch

Matthew A. Janiga and Chris D. ThorncroftUniversity at Albany, SUNY

What is still not understood (or well documented)?

Relationship between TCWV and Heavy RainfallQuestion 2: Obviously rainfall increases with TCWV, but is this

because of convective or stratiform rain and intensity or coverage?

Importance: Understand the morphology of convection responsible for heavy rainfall.

Regional Differences in Convection / EnvironmentQuestion 1: What is the land/ocean contrast in updraft depth

and intensity? Importance: Cloud parameterization is a challenge for

numerical models…what do the observations tell us?

TRMM’s View of Convection

Conv.

Stra.

Climatological Rain Rate from TRMM PR

This plot shows the rainrate from averaging all the TRMM PR passes. The unconditional rainrate includes zeros. The PR

samples each point about every 3 days. About 400 passes used.

Climatological Rain Rate from TRMM PR

The plot shows the rainrate (intensity) and stratiform fraction (color) from the TRMM PR.

Conditional Rain Rate• The total conditional rain rate is slightly higher over the Sahel and Congo than the East Atlantic.

• The convective conditional rain rate is much higher.

• The stratiform conditional rain rate is higher over the East Atlantic.

• Precipitation features (PFs) were identified as contiguous regions of rain rate > 0 from the TRMM PR swaths.

(TOP) The cumulative distribution function (CDF) of PF frequency.

(BOTTOM) The contribution of each size to the total rainfall in the three domains.

Systems Accounting for > 50% of Rainfall

This plot shows the PF characteristic (size, intensity, etc…) that accounts for 50% of the rainfall. Gray regions indicate that 50% of

the rainfall comes from PFs with no 40 dBZ echoes.

• To examine the influence of the diurnal cycle on the properties of convection certain PF classes were created.

• These correspond to isolated cells, small MCSs, and large MCSs.

Size Illustration

very small (<23 km) small (23-96 km) medium (96-192 km) large (>192 km)

No 40 dBZ low (0.5-5.5 km) medium (5.75-7 km) high (>7.25 km)

The Land/Ocean Contrast in Convection

From Liu et al. (2007)

Deep and Intense Convection

Analogous differences between the West Pacific and East Atlantic

Deep but Weak Convection

42% 24%

Echo Tops as a Function of TCWV

Many studies have found a shallow to deep transition over the ocean. This transition is not clear over the Sahel…rain rate due to

reduced evaporation or collision coalescence?

Convective Properties as a Function of AEW Phase

Key Results1.) There are strong land/ocean contrasts in 40 dBZ echo tops but less so

in the 18 dBZ echo tops. This is consistent with contrasts in CAPE and the level of neutral buoyancy.

2.) There is a strong relationship between rain rate and TCWV: a.) Stratiform rain rate is more sensitive than the convective rain rate.b.) Increases in rain coverage are more important than increases in rain intensity.

3.) Caveats associated with the land/ocean contrast in conditional convective rain rates…a.) The Sahel has a lack of weak convective rain not more intense convective rain.b.) At unusually high TCWV values (> 90th percentile) rainfall is much more intense in the East Atlantic despite not being as deep.