2.6.4. Schumacher and Houze (2006) This Lecture: Review of Schumacher and Houze, 2006: Stratiform precipitation over sub-Saharan Africa and the Tropical East Atlantic as observed by TRMM, QJ, 2235-2255
2.6.4. Schumacher and Houze (2006) • INTRODUCTION • Comparison of MCSs over land and over ocean (East Atlantic) • Compared to Ocean MCSs, Land MCSs are: • Faster and short-lived • Have stronger 85GHz ice-scattering signatures • Have more lightning • Have more rain that peaks late afternoon-midnight (ocean peaks in day) • Have less stratiform rain
2.6.4. Schumacher and Houze (2006) • INTRODUCTION • Comparison of MCSs over land and over ocean (East Atlantic) • Compared to Ocean MCSs, Land MCSs are: • Faster and short-lived • Have stronger 85GHz ice-scattering signatures • Have more lightning • Have more rain that peaks late afternoon-midnight (ocean peaks in day) • Have less stratiform rain • WHY?
2.6.4. Schumacher and Houze (2006) • INTRODUCTION • Two types of MCS exist over West Africa: • rapidly moving squall lines and slower moving non-squall MCSs • Conditions conducive for genesis and longevity: • potentially (?) unstable low-level moisture source overlain by dry desert air • vertical wind shear beneath the AEJ • Most of the rainfall over West Africa comes from MCSs, so it is important to understand these systems including the stratiform component. • Comparision with MCSs in East Atlantic is important in general but it is also relevent to studies on tropical cyclogenesis.
2.6.4. Schumacher and Houze (2006) 2. TRMM PRECIPITATION DATA DATA: TRMM 1998-2003, from the Precipitation Radar (PR) Radar echo is subdivided into convective and stratiform elements in TRMM product 2A23. Determined by examining the vertical profile of reflectivity and the horizontal variability of the echo. Radar echo is further subdivided into 18 subtypes in the TRMM product Version 5 2A23 algorithm. This paper uses this to define four general echo types.
2.6.4. Schumacher and Houze (2006) ATLANTIC OCEAN Shallow isolated echos: few km in width and height, often seen over ocean in low rain regions Convective echos: exhibits a spectrum of sizes; Figure shows weak to moderate convective cells. Stratiform echos: ppn reaches surface and has a brightband (maximum reflectivity in melting layer) Can form due to late stages of convective life-cycle or in association with a “robust” MCS
2.6.4. Schumacher and Houze (2006) AFRICA Convective echos: More intense than Atlantic case shown Anvil echos: Echos that do not reach the surface – maybe attached to convective or stratiform echos Squall line with leading convective leading edge and trailing stratiform
2.6.4. Schumacher and Houze (2006) 3. TROPICS-WIDE STATISTICS OF REFLECTIVITY PROFILES Vertical Distribution of tropics-wide (20N-20S) reflectivity from the four echo types Stratiform: bright band (4-5km), narrow distribution above 0C – median sfc value of ~1mm/hr Convective: broader distribution above 0C, higher values, increases towards surface, mean sfc value of ~6mm/hr Anvil: weak reflectivity with little or no sfc rain Shallow isolated:does not extend above the freezing level
2.6.4. Schumacher and Houze (2006) 3. TROPICS-WIDE STATISTICS OF REFLECTIVITY PROFILES Using Z-R relationships that relate reflectivity (Z) to rainfall (R) rainfall statistics are constructed for each echo type. Separate Z-R relationships are used for convective and stratiform rain.
2.6.4. Schumacher and Houze (2006) 4. REGIONAL PRECIPITATIONCLIMATOLOGY ASSOICATED WITH RADAR ECHO TYPES MAM: PPN just north of Equator; Average Stratiform Rain Fraction – 40% over E Atlantic, 23% over W. Africa (Tropics Average is 40%); Convective rainrates 14mm/hr over West Africa, 7mm/hr over E Atlantic; Lightning Maximum over Congo Basin (rare over ocean) JJA: PPN shifts north, over ocean PPN total same but stratiform rain fraction increases to 50%. PPN overland more than doubles to 170mm/mo; average stratiform fraction increases to 35% Atlantic: Conv. And Strat. rain rate similar in MAM and JJA Africa: Conv. - Slight decrease? Strat. RR increases! MAM JJA
2.6.4. Schumacher and Houze (2006) 4. REGIONAL PRECIPITATIONCLIMATOLOGY ASSOICATED WITH RADAR ECHO TYPES Note higher convective rainfall amounts west of high topography – related to monsoon westerlies impinging the high terrain. MAM JJA
2.6.4. Schumacher and Houze (2006) 5. REFLECTIVITY PROFILES BY GEOGRAPHICAL REGION AND SEASON JJA Major difference in convective profiles – weak spread above 0C in E Atlantic; and weaker above 5km than over West Africa – suggests stronger and more varied vertical velocities above 5km over West Africa Stratiform profiles more similar between regions; slightly stronger over land – suggests that there is no fundamental difference in the nature of stratiform ppn
2.6.4. Schumacher and Houze (2006) 5. REFLECTIVITY PROFILES BY GEOGRAPHICAL REGION AND SEASON MAM-JJA Stratiform profiles do not change much above 0C Stratifom at low levels are more intense in JJA Convective varies more at upper-levels above 0C level
2.6.4. Schumacher and Houze (2006) 6. SHALLOW RAIN AND ANVIL MAM: Shallow rain covers up to 30% of total rain area in rain areas over ocean Very little shallow rain over land MAM: Anvil has opposite pattern; Anvil covers up to 30% of total reflectivity coverage (at 9km) over land (10-15% over ocean). JJA: main areas of shallow and anvil move polewards with peak ppn. More shallow seen over land now – less impact of anvil on ocean. JJA: Fractional rain area from shallow increases especially over land – also reduced anvil over land
2.6.4. Schumacher and Houze (2006) 6. SHALLOW RAIN AND ANVIL “A predominance of shallow rain and a relative lack of anvil observable by the PR appear to be indicators of regions with robust stratiform rainfall production”
2.6.4. Schumacher and Houze (2006) “Yuter and Houze (1998) and Houze (2004) suggest that large sizes attained by oceanic MCSs over the Western Pacific are a result of MCSs being in regions of high convective sustainability, i.e. located in regions where the environment, particularly the boundary layer, favours continual formation of new convective cells within or near the existing MCS.” Ocean , sustainability associated with warm, moist boundary layer and weak diurnal cycle. Favour continual production of new convective cells. Land, sustainability is weaker linked to strong heating during the day that produces short-lived intense convection with much less occurring during the night The larger population of shallow cells over the ocean is likely an indicator of convective sustainability – an environment that can consistently produce moderate convection necessary for stratiform rain areas to form and mature
2.6.4. Schumacher and Houze (2006) 6. SHALLOW RAIN AND ANVIL The anti-correlation between shallow and anvil amounts is consistent with this – intense short-lived convection over land produces large ice particles aloft (see figure below) that fall out rapidly. Some small particles are left aloft but because of less continuous production of convective cells, less material is available to form a robust stratiform area and anvil is preferentially created.
2.6.4. Schumacher and Houze (2006) 7. RELATIVE HUMIDITY AND WIND SHEAR RH: Greater in JJA for both regions, esp. mid troposphere. Higher stratiform rain fractions are seen in JJA (cause or effect?) Convective sustainability does not vary between seasons over ocean – but increases over land (RH 60% to 70%). Moister mid-levels over land in JJA is good for sustaining convection for the growth of larger MCSs and associated stratiform convection (what about downdrafts??)
2.6.4. Schumacher and Houze (2006) 7. RELATIVE HUMIDITY AND WIND SHEAR SHEAR: AEJ affects both regions in MAM and JJA Strong mid-level winds may increase entrainment in stratiform clouds which would increase sublimation and evaporation, directly reducing stratiform rain in mesoscale regions AEJ is less pronounced over E Atlantic in JJA – the season of higher stratiform rain fraction.
2.6.4. Schumacher and Houze (2006) 7. RELATIVE HUMIDITY AND WIND SHEAR SHEAR: Strong shear beneath the AEJ in both seasons Strong shear above the AEJ in MAM over Atlantic when stratiform rain fractions are lowest Propose that upper-level shear (Above 500mb) has the potential to cause hydrometeors originating in deep convective cells to spread beyond the stratiform rain area, resulting in less stratiform rain Good shear bad shear?
2.6.4. Schumacher and Houze (2006) 7. RELATIVE HUMIDITY AND WIND SHEAR SHEAR: TEJ is present in JJA – reduces upper-level shear in both regions – consistent with higher stratiform rain fractions? Reduced upper-level shear may explain why weak TEJs are assd with dry years! “An increase in stratiform rain fraction is generally linked to high rain amounts, and upper-level shear, if strong enough appears to be inimical to stratiform rain production in at least some types of MCSs” Here: stronger TEJ – weaker shear – more stratiform- more rain Later: more rain –stronger “Hadley Cell” – stronger TEJ
2.6.4. Schumacher and Houze (2006) 7. RELATIVE HUMIDITY AND WIND SHEAR Saharan Air Layer (SAL): SAL may increase sublimation and evaporation within MCSs, and less stratiform rain Note northerlies at mid-levels and kink in humidity profile.
2.6.4. Schumacher and Houze (2006) 8. CONCLUSIONS • Most rain occurs in JJA over West Africa • The fraction of rain that is stratiform increases from MAM to JJA in both regions • Stratiform rain fraction is always higher over the ocean • Stratiform rain rates similar in both regions; convective rain rates higher over land • Despite weaker convection over ocean over East Atlantic, stratiform rainfall associated with MCSs is a large contributor to area’s rainfall. • Stability and upper-level shear help explain these variations – especially regarding stratiform rain production.
2.6.4. Schumacher and Houze (2006) 8. CONCLUSIONS • Stability variations over land versus ocean likely play an important role in convective system characteristics – “convective sustainability” • Continuous production of moderate convective cells important for build up of stratiform rain. • Less continuous production of convective cells results in non-raining anvils. • Upper-level wind shear can also be important • A decrease in upper-level shear linked to a weaker AEJ and/or stronger TEJ occurs during periods of higher stratiform rain fraction • If upper-level shear is too strong, a stratiform area is less likely to form. Since the hydrometeors essential for continued growth will be advected too far away and anvil will preferentially form. • How strong is strong? Does it depend on the MCS?
2.6.4. Schumacher and Houze (2006) 8. CONCLUSIONS TRMM PR : similar stratiform reflectivity profiles at upper-levels over Ocean and land and at the same location in seasons suggests that ice particles aloft are similar in size TRMM PR and Lightning: indicates that convection over West Africa produces large ice particles (graupel) that facilitates electrification (large ice particles appear to fall out quickly and do not contribute to the stratiform cloud mass) Final Word: Sublimation and evaporation from higher winds and the SAL can also affect the stratiform rain production by decreasing the stratiform cloud mass but upper-level winds and stability appear to be the main contributors to explain the observed variations in convective, stratiform and anvil echos through the production and supply of hydrometeors.