Semi-direct effect of biomass burning on cloud and rainfall over Amazon Yan Zhang, Hongbin Yu, Rong Fu & Robert E. Dickinson School of Earth & Atmospheric Sciences, Georgia Institute of Technology ??? III LBA Scientific Conference, July 27-29, 2004, Brasilia, Brazil

Direct effect of biomass burning aerosols Smoke Atmospheric Heating Reduce surface solar flux, cooling surface Absorb solar radiation in atmosphere, warming in the smoke layer Net effect: stabilize atmosphere

Indirect effect ( through cloud microphysics) More aerosols  more but smaller cloud droplets in clouds for a fixed water content  More reflective clouds Suppress or delay warm rain in shallow clouds  prolong clouds and promote deep/stronger convection.

Semi-direct effect: Surface solar flux  : –Surface SH   shallower ABL –LH   RH ABL  Previously believe: suppress convection. However, q + W E   RH ABL  in early afternoon  may not suppress convection The sign of semi-direct effect is uncertain morningnoon SHLH q + W E,Dry air entrainment height Atmospheric Boundary layer Without aerosols With aerosols

What controls the semi- direct? How does semi- direct effect affect rainfall? –Much of the previous studies have focused on indirect effect (e.g., Kaufman and Fraser, 1997; Rosenfeld, 1999; Feingold et al., 2001). –However, the semi-direct effect can be more important (e.g., Koren et al., 2004) Questions: MODIS, Sept. 2002, warm clouds, southerly V-index Warm cloud fraction decreases and cloud top become lower with increase of aerosol optical depth.

Factors that could influence the semi-direct effect: Soil moisture, Yu et al. 2002; Surface cooling (scatter) vs. atmospheric warming (absorption); Vertical distribution of smoke layer, Yu et al., 2002; Structure, diurnal development of atmospheric boundary layer.

NCAR Regional Climate Model Aerosol forcing Why Regional Climate Model? –Resolution: high enough to resolve Andes –Domain: large enough to include moisture transport from ocean to Amazon. RegCM consists of: Atmosphere : MM4 Land : BATS Radiation : CCM3 Domain: 20W~~100W, 35S~~25N Time simulated : Jun ~~ Oct, Data used: Landuse and Topography USGS GTOPO30 _10 MIN data Initial and Boundary Conditions Reanalysis Data: NCEP Data Derived from an integration of MODIS retrievals and GOCART simulations (Yu et al., 2003)

Experiment design: CONTROL run: without smoke aerosol forcing AER runs: Using MODIS + GOGART (Aug-Oct. 2001) Assumptions on the thickness of the smoke layer: –AER1: 1.5 km from the surface –AER2: 3.5 km from the surface (Reid et al. 1998). Same aerosol loading for AER1 & AER2. For Aug-Oct & 2002, respectively. Observation from SMOCC field experiment, Sept-Nov. 2002, Andreae et al., 2004, Science.

Table 1 Comparisons ABRACOS observations and RegCM3 simulations with root depths (d) of 1.5 m and 3 m, respectively. The observations were conducted from 29 Jun to 5 July, The RegCM3 model simulation is July average for Rn (Wm -2 ) SH (Wm -2 ) LH (Wm -2 ) ABRACOS Observation RegCM3 (d = 1.5 m) RegCM3 (d = 3 m)

The influence of aerosols peaks in late morning (10 am LT), instead of noon when incoming solar radiation peaks —change of clouds Diurnal cycles of the net downward surface solar flux: Without change in cloud Aerosol forcing Change in surface solar flux Sept 2002

Influences on surface net radiation, latent and sensible fluxes: Biomass burning aerosols result in stronger reduction in surface SH.

Cloud burning Pressure (hPa) Top of ABL Direct effect: Aerosols  SH   Z ABL  Cloud Fraction (%) Cloud burning can largely compensate the direct effect of aerosols on depth of ABL in early afternoon.

Observations Model results Biomass burning aerosols  higher RH ABL (model&observation). Presumably, weaker diurnal growth of ABL  weaker entrainment of dry air into ABL  higher RH in ABL.

The direct and semi-direct effects of aerosols on total rainfall aerosols, 1.5 km Without aerosols aerosols, 3.5 km

Summary: morningnoon SHLH q + W E,Dry air entrainment RH ABL can increase, instead of decrease. Without cloud burning Without aerosols Cloud burning   downward solar flux  compensates the direct aerosol effect

What have we learned from this study? The diurnal growth (esp. the entrainment) of the ABL plays a key role in determining semi-direct effect of biomass burning aerosols. Cloud burning can reduce the surface cooling in early afternoon, thus compensate the negative direct aerosol effect on rainfall. The semi-direct effect of biomass burning aerosols is highly sensitive to the vertical distribution of aerosols.

Leticia: specific & relative humidity

Manaus: specific & relative humidity

NCAR Regional Climate Model Model domain and topography (m) Why Regional Climate Model? –Resolution is high enough to resolve Andes and domain large enough to include all the key processes, especially moisture transport from ocean to Amazon. About the model runs: RegCM consists of: Atmosphere : MM4 Land : BATS Radiation : CCM3 Domain selected: 20W~~100W, 35S~~25N Time simulated : Jun ~~ Oct, 1993 Data used: Landuse and Topography USGS GTOPO30 _10 MIN data Rotated Mercator: Suitable for most latitudes Initial and Boundary Conditions Reanalysis Data: NCEP Data Map Projection :

Distribution of aerosol optical depth at 550nm for different months in It was derived from an integration of MODIS retrievals and GOCART simulations (Yu et al., 2003) used in our RegCM simulations: