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West Africa Meteorology Lecture notes1


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This is an introduction to West Africa Meteorology

This is an introduction to West Africa Meteorology

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  • 2. COURSE OUTLINE1. INTRODUCTION 1.1 Definitions – Weather, Climate, Climate Variability and Change 1.2 Climate, Climate Variability and Change Issues2. CLIMATOLOGICAL FEATURES OF WEST AFRICA 2.1 Surface Pressure and Temperature Distributions 2.2 Vertical Temperature Distribution (Assigns. 1 & 2) 2.3 Equivalent Potential Temperature and Moist Static Energy (Assign. 3) 2.4 Humidity (Moisture) Fields 2.5 Wind Fields and Tropospheric Jets 2.6 Horizontal Divergence and Vertical Motion Fields 2.7 Inter-Tropical Discontinuity (ITD) (Assigns. 4, 5, 6 & 7) 2.8 Rainfall Distributions3. SCALE INTERACTIONS IN WEST AFRICA 3.1 Summer Situation 3.2 Winter Situation
  • 3. 4. INSTABILITY PROCESSES IN THE ATMOSPHERE 4.1 Static Instability 4.2 Conditional / Convective Instability (Insitu Development) 4.3 Conditional Instability of the Second Kind (CISK) 4.4 Baroclinic Instability 4.5 Barotropic Instability5. WEST AFRICAN WEATHER SYSTEMS 5.1 African Easterly Waves 5.2 Meso-scale Convective Systems (MCS) and Squall lines 5.3 Thunderstorms and the Monsoon 5.4 ENSO and Drought 5.5 Dust Haze and Fog6. EARLY WARNING SYSTEMS AND FORECASTING METHODS 6.1 Early Warning System 6.2 Forecasting MethodsGrading System• Continuous Assessment {Attendance (65% Attendance minimum)– 10% + Assignments – 30%} – 40%• Examination – 60%• Examination Malpractice is not spared but the victim should face disciplinary panel of the University and be rusticated. Be warned!
  • 4. 1. INTRODUCTION1.1 Definitions• There is a difference between weather and climate and likewise between climate variability and climate change.• Weather is the daily conditions of the atmosphere of a particular place expressed by elements such as temperature, pressure, precipitation, humidity, winds, cloudiness, etc.• Climate is the synthesis of atmospheric conditions characteristic of a particular place in the long-term expressed by means of averages and including extremes values of the various weather elements.• Climate may include four seasons a year - spring, summer, autumn and winter - or a wet and a dry season. Our climate depends on our position on the earth and our distance from the sun.• The minimum average of climate element/parameter being considered is 30 years as proposed by the World Meteorological Organization (WMO).• Climate Variability occurs if over three thirty-year periods the climate element/parameter varies and then returns to what it used to be or its original value.• Climate Change occurs if over three thirty-year periods the climate element/parameter varies and never returns to what it used to be or its original value.
  • 5. 1.2 Climate, Climate Variability and Change Issues• Climate has become key issues nationally and internationally. Climate variability and change have become the talk of the day.• Climate change and the need for environmental protection are global problems and call for a knowledgeable response from all countries in order to be effectively addressed.• In Nigeria, there has been much discussion on desertification in the North and deforestation in the South owing to human activity leading to global warming, climate variability and climate change.2. CLIMATOLOGICAL FEATURES OF WEST AFRICA2.1 Surface Pressure and Temperature Distributions• All year round, low pressure occurs where high temperature prevails – heat low, while high pressure occurs where low temperature prevails.• Isobars and isotherms are zonal. The areas where there is strong temperature gradient is known as baroclinic zone.• The baroclinic zone is north of the Inter-Tropical Discontinuity (ITD) in winter (∼ 10 – 20°N) and south of the ITD in summer. This leads to the formation of the mid-tropospheric jet, African Easterly Jet (AEJ) which contributes to the occurrence of thunderstorms over West Africa during summer.
  • 6. • Fig 1a shows surface pressure (solid lines) and temperature (dotted lines) distributions over West Africa during winter (January/February). Low pressure center is at about 6.5°N and the Saharan anticyclone is centered at 27°N.• Fig 1b gives surface pressure and temperature distributions over West Africa during summer (July/August). Low pressure center advances to a maximum position of 21-22°N.• Temperature distribution depicts:  Near zonal isotherms, decreasing pole wards with strong gradient (baroclinic zone), particularly between 14 and 24°N in winter.  In summer this baroclinic zone is between 19 and 16°N south of the low pressure center.  The center of low pressure coincides with the maximum temperature both in winter and summer – hence the name thermal low.• Fig. 1c, d and e show the surface temperature fields in January, April and August respectively for 1960-1990 (WMO).• In January, the baroclinic zone is between 15° and 25°N, by April the zone splits into two – one between 5° and 12°N and the other between 18° and
  • 7. (1c) (1d)Fig. 1c and d: Surface temperature fields in January and April respectively(1960-1990, WMO)
  • 8. Fig. 1e: Surface temperature fields in August(1960-1990, WMO)
  • 9. • By August, the southern baroclinic zone has shifted to 10° – 16°N with the heat low now at about 21° – 24°N over most parts of West Africa but oriented NE/SW 24°/19°N over western coast of Mauritania and Senegal.• All these lead to different wind profiles during the course of the seasons as discussed later in section 2.5.
  • 10. 2.2 Vertical Temperature Distributions• Figs. 2A and B – the mean meridional vertical cross-section of temperature (A) December – February (B) June – August for the 0 – 30km (~1000 – 10hPa) altitude region.• Meteorologists divide this region of the atmosphere into two layers based on the vertical gradient of temperature.• In the lower layer, the troposphere temperature generally decreases with height with lapse rate γ = – ∂T/∂z ~ 6.5K/km, while in the upper layer, the stratosphere, the temperature generally gradually increases with height.• The troposphere and the stratosphere are separated by the tropopause, a level of temperature minimum which varies from about 18km near the equator to about 9km near the poles.• Lowest temperature occurs over the tropics (West Africa in particular) at the tropopause while highest temperature occurs at the surface.• The pole to equator temperature gradient in the troposphere is much larger in the winter than in the summer.• Within the tropics, the temperature gradient is very weak unlike within the mid- latitudes and polar regions.• The minimum temperature of about -830C occurs around the equatorial region (which includes part of West Africa) and at about 100hPa level for the winter situation while that of summer of about -770C.
  • 11. • Fig. 3a shows the monthly mean temperature (in 0C) during February (northern winter) at 850, 700, 500, 300 and 200hPa levels over Africa.• The mean meridional temperature gradient from equator to around 15 0N over West Africa is positive at 850 and 700hPa though that of 700hPa is weaker, while for 500 and 300hPa, the gradient is negative. That of 200hPa is back again to positive value.• This characteristic change in temperature distribution gives rise to two wind maxima at about 700 – 600hPa and 300 – 200hPa – the African Easterly Jet (AEJ) and the Tropical Easterly Jet (TEJ) respectively in the tropics.• The characteristic change in temperature distribution contributes much to the existence of the AEJ but weakly to the existence of TEJ. The TEJ results mainly from the mass flux from the Himalayan highlands during summer.• Fig. 3b – the monthly mean temperature ( 0C) during August (Northern Summer) at 850, 700, 500, 300 and 200hPa.• Here, the high temperature around 30 0C over the Sahara Desert and the baroclinic zone becomes extremely weak at 15 0N as one proceeds upward to about 500hPa, however, one can trace an equator ward tilt of the major thermal system and the associated thermal gradients.• The mean meridional temperature gradient from equator to around 15 0N over West Africa is positive at all the levels.
  • 12. Assignment: 1 Given that the surface wind over Akure on a day in May is 5m/s, surface pressure is 975hpa and given that the mean temperature gradient across Akure is 1K/300km. Find the thermal wind between surface and 850hPa and using the same gradient calculate also the thermal wind between 850-800 and 800-700hPa. Assume further the same gradient (but negative) across Akure between 700-500hPa, find also the following: (a) Winds at 850, 800, 700 and 500hPa levels. (b) Plot the vertical profile for zonal (u-component) of the wind. (c) comment on your profileAssignment: 2 Using Figs. 1c and 3a, the monthly mean temperature (in °C ) over Africa during January/February (northern winter) from 850 - 200hPa, and given that the wind at surface is 5m/s and from northeast at latitude 10°N and 5°E and surface pressure is 980hPa. Calculate and plot the zonal wind profile from surface to 200hPa and comment on the profile.Assignment: 3 Using Figs. 1e and 3b, the monthly mean temperature (in °C ) over Africa during July/August (northern summer) and using the same data as above calculate and plot the zonal wind profile from surface-200hpa
  • 13. 2.3 Equivalent Potential Temperature and Moist Static Energy• Fig. 4 gives the latitude-pressure cross-section of mean equivalent potential temperature θ e along longitude 20 30’E and from 50S to 200N covering West Africa.• This is obtained from the time averaged maps of equivalent potential temperature at 1000, 850, 700, 500, 300, 200 and 100hPa levels.• The figure shows a mid-tropospheric θ e minimum at about 700hPa and this is typical of tropical atmosphere.• Within 1000 – 850hPa from ∼130N – 200N is a zone where ∂θ e /∂z >0 i.e change of θ e with height is positive and indicates a stable zone while from 5°S – 12°N is the zone where ∂θ e /∂z <0 i.e. change of θ e with height is negative and it indicates an unstable zone in the lower and middle troposphere.• In the layer between 850 and 500hPa levels are the absorbers of convective instability for tall (cumulonimbus) towers to shoot to higher levels.• The moist static energy is a thermodynamic variable that describes the state of an air parcel, and is similar to the equivalent potential temperature.• The moist static energy is a combination of a parcels kinetic energy due to an air parcels temperature, its potential energy due to its height above the surface, and the latent energy due to water vapor present in the air parcel.
  • 14. Fig. 4:
  • 15. • It is a useful variable for researching the atmosphere because, like several other similar variables, it is conserved during adiabatic ascent and descent.• The moist static energy Hm is approximately equal to the product of specific heat capacity at constant pressure and the equivalent potential temperature given by Hm ≈ Cpθ e ≈ CpT + gz + Lq• Where first, second and third terms are internal kinetic energy due to an air parcel’s temperature, its potential energy due to its height above the surface and the latent energy due to water vapour present in the air parcel respectively.• Moist static energy is maximum in the boundary layer and minimum at the AEJ level.Assignment: 3 Prove that the moist static energy Hm ≈ Cpθ e ≈ CpT + gz + Lq. (Hint: Begin from the left hand side of the equation and substitute the expression for θ e and noting that (1000/p)R/Cp is unity at low levels where q is large. Also use the first law of thermodynamics and expression for geopotential)
  • 16. 2.3.1 Profiles of Potential, Equivalent Potential and Saturated Equivalent Potential Temperatures• Fig. 5 shows the vertical profiles of potential temperature θ, equivalent potential temperature θe, and equivalent potential temperature of a hypothetically saturated atmosphere θes in a tropical region.• This is obtained by averaging the θ, θe, and θes values along tropical latitudes at the given pressure levels in the atmosphere.• The formulae for the three parameters are given below: θ = T(1000/p)R/Cp , θe = θexp(Lq/CpTv), θes = θexp(Lqs/CpT) where T=temperature in Kelvin, Tv= T(1 + 0.61q) = virtual temperature in Kelvin, L = latent heat of vapourization, q = specific humidity, q s = saturation specific humidity, Cp = specific heat capacity at constant pressure.
  • 17. Fig. 5:
  • 18. 2.3.2 Stability Perspective• Vertical motions, cloud formation and associated rainfall depend on vertical stability (from temperature and moisture analysis).• θe and θes are realized by condensing out all the moisture in the air before compressing it through adiabatic descent to the 1000hPa level.• Moisture is a crucial factor for tropical atmosphere.• Potential temperature profile, Fig. 5, implies that dry tropical atmosphere is statically and unconditionally stable but is conditionally and convectively unstable when moisture is considered (i.e. for θe and θes).• West African atmosphere, like everywhere in the tropics, is inherently convectively and conditionally unstable.• Convective instability is usually investigated through the use of equivalent potential temperature θe whereas its saturation value θes is used to study conditional instability.• Below about 300hPa, all the profiles coincide because there is little or no moisture.• The instability still needs to be released through large scale convergence/divergence, strong insolation (leading to super-adiabatic lapse rates in the lower layers) and forced (orographic) ascent.
  • 19. • Regions between latitude 5°S and about 13°N are conditionally unstable from the surface and 700hPa. The atmosphere is statically stable north of this latitude.• There is a minimum in equivalent potential and saturated equivalent potential temperatures at about 700hPa.• It is obvious from the figure that mean tropical atmosphere is conditionally unstable in the lower and middle troposphere,• However, this observed profile does not imply that convective overturning will spontaneously occur in the tropics.• The release of conditional instability requires not only ∂θes/∂z < 0, but also a saturated atmosphere, and the mean relative humidity in the tropics is well below 100%.• Thus, low level convergence with its resultant forced ascent or vigorous vertical turbulent mixing in the boundary layer is required to produce saturation.• The amount of ascent necessary to produce a positively buoyant parcel can be estimated simply from Fig. 5.
  • 20. Fig. 6: (After Grist and Nicholson, 2001)
  • 21. 2.4 Humidity (Moisture) Field• Fig. 6 gives the vertical structure of mean relative humidity distribution in August averaged for the wet (abundant rainfall) and dry (deficient rainfall) composites in the sector 10°W to 20°E over tropical Africa.• Over West Africa (0 - 15°N), the atmosphere below 800hPa is very moist with relative humidity higher than 70% at all times, particularly in the boundary layer (BL). Deep layer of humidity >80% exits in June – August.• Fig. 7 shows the moisture (specific humidity) anomalies situation for typical dry and wet years over Kano, in Nigeria (after Omotosho, 2007). For abundant precipitation:  The boundary layer must be very moist.  Mid-troposphere should be moister than normal, but drier than the boundary layer.  Positive moisture anomalies during wet years are about twice those in the dry (drought) years.
  • 22. 2.5 (Fig. 8, January) Fig. 8: (Fig. 8)
  • 23. Fig. 9Fig.9
  • 24. • Fig. 10 shows the wind component profiles over (A) Venezuela, (B) West Africa and (C) West Pacific.• The profiles highlights the uniqueness of the West African wind regime:  A rather shallow low-level monsoon layer overlain by two jets all in the troposphere.  No other tropical region with such wind distribution.• Fig. 11 gives the meridional distributions of maxima in temperature and relative humidity in relation to the African Easterly Jet (AEJ) over West Africa.• It shows an important inter-relationship between temperature and humidity distributions and the African Easterly Jet located at 700hPa.• The arrangement is crucial for the maintenance of the jet.• From the top figure, the maximum moisture is not at the coast but around 10°N.• The reason is that the wind from the Atlantic Ocean carries moisture across the coast with speed of about 20m/s and on reaching the land it is retarded inland so that the maximum moisture is about 10°N where the retardation is maximum and then the moisture reduces northwards.• The temperature distribution increases from coast to a maximum at around 22°N and reduces further north. Temperature gradient is +ve from 10-22°N.
  • 25. Fig.10:
  • 26. • The latitudinal position of about 15°N where the specific humidity and temperature lines intersect is the same position of the core of the AEJ.• Active weather occurs to the south of the AEJ core. Drought years are associated with the southward position of the AEJ while good rain years are associated with the northward position of the AEJ.2.6 Horizontal Divergence and Vertical Motion Field• Vertical motions in the atmosphere are indicative of instability and are the potential for convective cloud formation depending on the level of instability and available moisture.• Vertical motions are small over the tropics (order of 10 -8mb/s) and related to horizontal divergence through the continuity equation: ∂ω/∂p = - ▽·V gives, on integration ωp2 = ωp1 - ∫(▽·V)dlnp• Thus, vigour of the ascending motion depends on strength of the convergence/divergence in the layer.• Hadley circulation over the tropics (Fig. 11b below) shows convergence at about 8°-10°N and descending branch around 20°-26°N.• Warm air rises, with consequent precipitation, on the equator-ward side of the circulation while cold air descends on the pole-ward side.
  • 27. Fig. 11b: Hadley circulation over the tropics.
  • 28. Fig. 11c:
  • 29. • African easterly and tropical easterly jets (AEJ and TEJ respectively) are denoted by the letter “J” in small circles. Comparison of Fig. 11b and 1b indicates the AEJ positioned above the baroclinic zone around 15°N but about 5° north of the TEJ.• Fig. 11c (above) shows ascending motion between the equator and about 16°N with maximum ascent of 5 x 10-8mb/s over the Guinea coast. Areas of ascending motion generally coincide with region of upper tropospheric outflow (see Fig. 11b).
  • 30. 2.7 ITD AND ITCZ• The area of lowest pressure over West Africa (meteorological equator at surface) is the Inter-Tropical Discontinuity (ITD) and it separates two air masses – the moist southwest monsoon and the dry northeast trades.• The extension of the ITD over the ocean is the Inter-Tropical Convergence Zone (ITCZ). The ITD and ITCZ are semi-global phenomenon.2.7.1 ITD• The ITD over West Africa could be located with observation of wind direction change and the dew point temperature of 15 0C. To the south, the dew point temperature is >150C and to the north it is < 150C.• Clouds are generally found to the south of the ITD and not to the north except at high levels (altocumulus and cirrus). South of the ITD could be foggy while north of the ITD is dusty. Maximum rainfall occurs 500-700km south of the ITD.• The ITD undergoes three distinct and superimposed movements, differing both in amplitudes and duration: the diurnal movements, consisting of shifts southwards during the morning and northwards during afternoon within average amplitude of 200km. annual migrations which approximately follow the displacement of the sun during the year, but with a delay of six to eight weeks and
  • 31.  intermediate oscillations, with an amplitude around an average seasonal position of several hundred kilometers and a duration of several days, this movement is often observed during northern winter months.• In West Africa the ITD at the ground reaches it most northerly position in July/August between 210N and 220N and its most southerly position in January/February around 70N on the average.2.7.2 ITCZ• The Inter-tropical Convergence Zone (ITCZ) occurs over ocean where northeast trades and southwest or southeast trades converge.• There seems to be no difference in value of temperature and humidity to the north or to the south of the ITCZ.• The only difference between north and south of the ITCZ is the wind direction. ITCZ depends on the position of the sun and movement of the anticyclones.• It is a semi-global phenomenon. ITCZ resembles fronts in terms of zone of horizontal wind shear but unlike fronts the air masses are not different.
  • 32. 2.7.3 VERTICAL STRUCTURE OF THE ITD/ITCZ• Fig. 12a shows the mean position of the ITD/ITCZ at the surface, 850, 700, and 500hPa levels in January.• The ITD/ITCZ tilts southwards with height west of about 15 0E and northward with height east of about 250E.• The surface mean southernmost position of the ITD over West Africa is around 70N while over east Africa it is around 150S. So from West Africa to East Africa, the ITD/ITCZ tends to “bend” southwards.• Fig. 12b gives the average position of the ITD/ITCZ at surface, 850, 700 and 500hPa levels in July.• In this situation, the ITD/ITCZ tilts southwards with height everywhere unlike that of January.• By making a complete surface and upper air analysis of the ITD in conjunction with the synoptic situation, it has been possible to define quasi-permanent structure which can be related to the types of weather observed in West Africa during northern spring and autumn.• Fig. 13 shows schematic longitude (meridional) cross-section representation of the ITD showing zones of weather types relative to the surface ITD and the predominant cloud types for each zone (Dhonneur, 1970).
  • 33. 12a:
  • 34. 12b:
  • 35. • By considering the depth of the monsoonal flow relative to the surface ITD it is possible to distinguish four zones at the surface, each corresponding to well-defined weather types (Hamilton and Achibold, 1945): Zone A which lies to the north of the line of the ITD at the ground corresponding dry air and usually a cloudless sky. Zone B which lies to the south of the afore-mentioned line, is characterized by a shallow monsoonal flow; its meridional extent is variable, depending on the season and the values of criteria differentiating the air masses which are present.o The weather in zone B is characterized by clouds formations showing a large diurnal variation and isolated afternoon thunderstorm may develop in this zone. Zone C which lies to the south of zone B, is the zone in which the influence of monsoonal flow is at a maximum.o There is usually strong convergence in this zone (Morth and Johnson, 1963). Dhonneur (1970) has further sub-divided zone C into two zones, C 1 and C2. C1 is the zone which is traversed by the moving thundery disturbances of Africa, whether they be easterly waves or line-squalls.
  • 36. 13:
  • 37. o C2 is the zone in which the depth of the monsoon is at the maximum, the disturbances are usually zonal, showing little diurnal variation and causing heavy rainfall. Zone D which lies to the south of zone C, corresponds to an air mass from the south which is almost homogeneous from the surface up to the medium levels.o The trans-equatorial south-easterly flow has not yet acquired a marked westerly component and shows divergence (Morth and Johnson, 1963).o This zone gives rise to a particular kind of dry season in the regions lying to the south of 90N in July/August.• In Fig. 13, the cloud masses associated with each zone have been shown and also the boundaries separating equatorial air at upper levels from the air masses directly associated with the sub-tropical highs over the Azores-Sahara region and the Island of St. Helena.• In southern meteorological hemisphere an upper-air discontinuity corresponds to the boundary between air from the south which has evolved in the lower layers and air from south at medium levels.
  • 38. Assignment 4: Using Figs. 12a & b, plot the position of the ITD at surface, 850, 700, and 500hPa at 100W, 00, and 100E for January and July cases and describe the level of moisture available at those longitudes.Assignment 5: Show that the southward movement of ITD is twice as fast as the northward movement given that the ITD reaches its southernmost position in January at 70N and the northernmost position in July at 21 0 or 220N.Assignment 6: Using Figs. 14a, b & c, plot the latitudinal positions of ITD with respect to months from January to January at 5°E. Join the points with two straight lines (lines of best fit) – one representing the northward movement and the second the southward movement. Determine the gradient of these lines. Compare the two graphs and make comments.
  • 39. 14a:
  • 40. 14b:
  • 41. 14c:
  • 42. • Figs. 14a, b and c show the main discontinuities at the surface for each month of the year (ASECNA,1973).• The heavy line separates different flow regimes. It is interest to note that southerly flows cross the Equator northward into West Africa during all months of the year.• Although this separation line stays north of the equator over West Africa, its behaviour over East Africa is much influenced by the Asian monsoons.• Fig. 15 a and b show the long-term average climatology for the advance and retreat respectively of ITD latitude (1974-2003) after Lele and Lamb (2008).• The distributions are oriented WNW – ESE. The northward advance is gradual and reaches its maximum position near 21°N in early August (Fig. 15a) while Fig. 15b shows an abrupt southward retreat.
  • 43. Fig. 15: Long-term average climatology of ITD latitude (1974-2003) – (a) forNorthward Advance (b) for Southward Retreat (After Lele and Lamb, 2008)
  • 44. THE INTER-TROPICAL DISCONTINUITY (ITD) – SUMMARY• An important factor for West Africa weather and climate.• Determines at any point in time and space, the level of moisture available.• Terminology (ITD) applies over land areas only and used to differentiate it from ITCZ (Inter-Tropical Convergence Zone) over the oceans.• Associated with weather zones (Fig. 13).• Mean Southernmost position of about 70N in during winter in January/February, northernmost position about 21 – 22 0N at the peak of summer in July/August.• Remain north of the equator at surface throughout the year.• Southward retreat about twice as fast as northward advance.• Meso-scale convective systems (MCSs) occur around the ITCZ but are found about 1.50latitude (or 170km) south of the ITD.• Located by the 150C dew point isotherm limit (which essentially coincides with the centre of the thermal low in Figs. 3a and b. and wind convergence.• Slopes equator ward (Figs. 7a & b) from its mean surface position of 7 0N to about 50S over West Africa at 500hPa in Jan/Feb and from its surface position of about 21-220N to about 20N over W. Africa at 500hPa level in July/August.