The diurnal cycle and propagation of convection in africa
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The diurnal cycle and propagation of convection in Africa. Arlene Laing National Center for Atmospheric Research. Introduction.

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The diurnal cycle and propagation of convection in africa

The diurnal cycle and propagation of convection in Africa

Arlene Laing

National Center for Atmospheric Research


Introduction
Introduction

  • African societies need better precipitation of prediction (economy, health, water resource management, …). Prediction depends on improved understanding of initiation and evolution of organized convection

  • Radar-based studies in US and satellite-based studies in East Asia, Australia, and Europe found that organized convective precipitation displays systematic propagation at regional to continental scales

  • Coherence behavior implies qualitative predictability beyond one or two days.

  • Focus on properties of organized convection in Africa =>Better understanding of water cycle at various scales, improve models & prediction

Laing 2006




Data & Methods

Average Elevation (km)

0 – 20N

20N

May - Aug

  • Reduced Dimension (Hovmoller) techniques. Cold cloud as proxy for precipitating convection

  • Compute phase speeds, duration, span

  • Compare with easterly wave statistics

  • Examine large-scale influences

0

Autumn,

Spring

1.5

1.0

0.5

20S

20W

10W

0

10E

20E

30E

40E

Nov - Feb

40E

20W

0

20E

Average Elevation (km)

35S - 20S

1.2

0.6

  • Meteosat-7 IR, 30min

  • NCEP GFS Analyses

  • May 1999 - March 2004

Laing 2006


US Mainland, Warm Season

Episodes of Precipitation and Cold Clouds exhibit coherence in phase

Laing 2006


Tropical north africa
Tropical North Africa

Episodes of Cold Clouds exhibit coherence in phase,

Average frequency of cold cloud as a function of longitude and time

occur daily

Laing 2006


AFRICA: Transition Season (5S – 15N)

Cold cloud episodes: coherent in phase

Mean Diurnal Cycle

16 - 31 Oct 1999

(Tbb< 233K)

Laing 2006


Sh africa summer 35s 20s
SH AFRICA Summer (35S–20S)

1100 UTC 9 Dec

1100 UTC 9 Dec

8 -11 Dec 2003

1500 UTC 9 Dec

1800 UTC 9 Dec

1800 UTC 9 Dec

2000 UTC 9 Dec

2000 UTC 9 Dec

2330 UTC 9 Dec

10E

20E

30E

40E

1.2

Average Elevation (km)

(35S – 20S)

0.6

0430 UTC 10 Dec

0430 UTC 10 Dec

Cold Cloud Episodes exhibit coherence in phase

Laing 2006


AFRICA: SH Summer

16 – 31 Dec 2003

Tropical Cyclone Cela

Mean Diurnal Cycle

Eastward propagation from high terrain

Laing 2006


Africa sh early summer 35s 15s
AFRICA: SH early summer (35S -15S)

16 - 30 Nov 2003

Mean Diurnal Cycle

Less propagation, more convection with diurnal heating maximum

Laing 2006


Daily mean u wind 500hpa cold cloud episodes
Daily Mean U wind, 500hPa Cold Cloud Episodes

01

1-15 Dec 2003

03

06

09

12

15

0 m/s

Mid-Latitude

S. Africa

  • Propagation influenced by synoptic regime

  • Convection Episodes are less frequent than Sahel and Central Africa

Cela

Laing 2006



Mid latitude southern africa
Mid-latitude Southern Africa

Span vs Duration

Weak correlation

Phase Speed

Laing 2006



Summary mid latitude southern africa
Summary: Mid-latitude Southern Africa

  • Coherent pattern of deep convection

  • Episodes of organized convection are less frequent than other regions. Needs additional forcing from synoptic scale waves

  • Phase speeds less than other continents

  • High terrain aids in convective initiation

  • Regime with propagation exhibit delayed-phase shift in diurnal max

  • Examining westerly wind shear to determine influence on frequency and propagation (propagating vs non-propagating regimes)

Laing 2006


Dynamical similarity of mesoscale convective systems globally
Dynamical similarity of mesoscale convective systems globally

  • Genesis of convective systems in high terrain

  • Systematic propagation on regional-continental scale (coherent behaviour)

  • Diurnal-maximum in precipitation shifted because of propagating cloud-systems

  • Scale-interaction between convective systems and synoptic waves

  • Propagating systems with vertical shear

    • Tropical north Africa (African Easterly Jet, West African Monsoon)

    • Southern Africa, East Asia, US, Europe, Southern Australia (Westerly shear with jet streams undulations)

Laing 2006


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