Vegetation Phenology and the Hydrological Cycle of Monsoons

Vegetation Phenology and the Hydrological Cycle of Monsoons David Lawrence and Julia Slingo CGAM University of Reading, UK

Motivation Early West African monsoon onset in HADAM 3 Evaluate sensitivity of HADAM 3 and MOSES 2 to seasonal cycle of vegetation. Is seasonally varying vegetation important?

Leaf Area Index climatological seasonal cycle 9 -yr ISLSCP II dataset LAI

Seasonal and interannual variability One of the most important outstanding aspects of land-atmosphere interactions is the feedbacks associated with seasonal and interannual vegetation cycles (Hutje, 1998).

Vegetation-atmosphere interaction evaporation surface albedo or sub-surface canopy roughness wind Impacts: hydrological surface energy budget momentum

Vegetation phenology into MOSES 2: Leaf Area Index LAI annual cycle (West Africa) LAI directly or indirectly controls • canopy height • surface roughness • canopy water capacity • surface albedo LAI Maximum: Minimum: MOSES 2 – ISLSCP 2 (UM grid) LAI

Surface albedo Updated soil albedo based on CCRS-ERBE albedo Included linear dependence. CCRS-ERBE of albedo on soil albedo MOSES 2 albedo w/ newwith soil phen + SM depend 2 albedo July - January annual mean moisture albedo with+phen MOSES 2 (w/ 2 soil, SM) –– ERBE CCRS-ERBE MOSES 2 albedo 2 w/ new soil SM depen MOSES albedo clim July -- January annual mean July

Surface albedo verification (West African monsoon region) Surface albedo Annual mean albedo

Model runs LAI-Phen LAI-Mean 25 -yr runs of HADAM 3 with MOSES 2 (first 5 years thrown out for spin-up) • Climatological SSTs • Standard climate resolution (3. 75 o lon x 2. 5 o lat) • LAI prescribed LAI-Phen: LAI varies across season LAI-Mean: LAI set to annual mean value •

Area exhibiting sensitivity to phenology Number of months per year LAI-Phen statistically different from LAI-Mean Daily Max 2 m Air Temperature Latent Heat Flux Precipitation Soil Moisture Content

LAI Surface water balance (West African monsoon) Precipitation 13 mm yr-1 (3%) P = Es + Ev + Ec + Rsurf + Rsub + d. SM/d P = Precipitation Es = Soil evaporation Evaporation 17 mm yr-1 (5%) Ev = Transpiration Ec = Canopy evaporation Rsurf = Runoff surface Rsub = Runoff sub-surface d. SM/dt = Change in soil moisture storage

LAI Surface water balance (West African monsoon) P - E 30 mm yr-1 (45%) Soil moisture content d. SM/dt Runoff 28 mm yr-1 (40%)

LAI Surface water balance (South Asian monsoon) Precipitation 10 mm yr-1 (2%) Soil moisture content Evaporation 4 mm yr-1 (1%) Runoff 13 mm yr-1 (9%)

Surface water balance West African monsoon Evaporation Soil moisture fraction: lay LAI South Asian monsoon Evaporation LAI Soil moisture fraction: lay

Evaporation difference (mm/month) Sensitivity of evaporation to surface soil moisture LAI (LAI-Phen – LAI-Mean) > 1 Significant at 95% LAI (LAI-Phen – LAI-Mean) < -1 Significant at 95%

Rsoil = 100 (SMF 1 / SMFcrit)2 Rsoil = Resistance to evaporation from soil= 0 Lake SMF 1 = Layer 1 soil moisture fraction Rsoil ~ 80 well-water SMFcri = Critical layer 1 soil moisture fraction vegetation West African monsoon Resistance to evaporation from soil Resistance to evaporation South Asian monsoon

Summary MOSES 2 land-surface scheme has been updated so that realistic vegetation phenology, based on satellite data, can be prescribed. Phenology includes seasonal evolution of LAI, surface albedo, and roughness length. Globally, soil moisture, dry season daily maximum temperatures, evaporation, and sub-surface runoff, show significant sensitivity to the incorporation of vegetation phenology. Precipitation, however, does not. In the West African monsoon region, reduced evaporation in spring, when LAI is low and therefore access to sub-surface moisture stores is restricted, leads to an increase in total soil moisture content. The soil moisture increase is sustained until the end of the monsoon season, leading to enhanced sub -surface autumn runoff. Unrealistically low resistances to evaporation from soil may

Wet-dry soil moisture composite Composite West African monsoon evolution based on soil wetness in winter/spring prior to monsoon season Precipitation LAI-Phen Wet SM Dry SM LAI-Mean Wet SM Dry SM

Future Work Evaluate model Sensitivity to resistance to evaporation in bare soil Include canopy heat capacity Interannual variability Run model with observed SSTs over long time period to evaluate whether sensitivity to phenology is related to soil moisture state Analysis of completed runs Examine surface energy balance Look at variability, diurnal cycle, extreme events

MOSES II - Surface tiling Fractional surface cover (IGBP) Broadleaf Trees Needleleaf Trees Need leaf Trees C 3 Grass C 4 Grass Shrub Urban Shrubs Bare Soil Inland Water Bare Soil