Land-Atmosphere Feedback in the Sahel Randal Koster Global Modeling and Assimilation Office NASA/GSFC Greenbelt, MD

Organization of Talk 1.Overview of the processes that control land-atmosphere feedback. (Case study: North America) 2.Application of these ideas to the Sahel: do the observations support the existence of feedback there? 3. Model study of the controls on Sahelian rainfall variability.

Warm season precipitation variance is often high in transition zones between dry and wet areas. Example: North America Observations (Higgins, 50-yr dataset) July Rainfall: Mean [mm/day] July Rainfall: Variance [mm 2 /day 2 ] Koster et al., GRL, 40, 3004

More evidence: tree ring data! (360 years of proxy precipitation data put together by H. Fritts, U. Arizona) Jul/Aug precipitation variances at each tree ring site White dots: Locations of tree ring sites with Jul/Aug precipitation variances in top half of range Shading: Mean annual precipitation (GPCP)

Q: Do we have any reason to suspect that precipitation variances should be amplified in transition zones? A: Yes. Transition zones are more amenable to land- atmosphere feedback. Precipitation wets the surface... …causing soil moisture to increase... …which causes evaporation to increase during subsequent days and weeks... …which affects the overlying atmosphere (the boundary layer structure, humidity, etc.)... …thereby (maybe) inducing additional precipitation

Feedback enhances  2 P through the enhancement of P autocorrelation (on timescales of days to weeks). PnPn P n+2 correlates with means that correlates with PnPn P n+2 wnwn E n+2 w n+2 correlates with correlates with correlates with Observed  2 P

PnPn P n+2 correlates with means that correlates with PnPn P n+2 wnwn E n+2 w n+2 correlates with correlates with correlates with Breaks down in western US: low soil moisture memory Breaks down in western US: low evaporation Observed  2 P Feedback enhances  2 P through the enhancement of P autocorrelation (on timescales of days to weeks).

PnPn P n+2 correlates with means that correlates with PnPn P n+2 wnwn E n+2 w n+2 correlates with correlates with correlates with Breaks down in eastern US: low sensitivity of evaporation to soil moisture Observed  2 P Feedback enhances  2 P through the enhancement of P autocorrelation (on timescales of days to weeks).

PnPn P n+2 correlates with means that correlates with PnPn P n+2 wnwn E n+2 w n+2 correlates with correlates with correlates with Observed  2 P Only in the center of the country (in the wet/dry transition zone) are all conditions ripe for feedback Feedback enhances  2 P through the enhancement of P autocorrelation (on timescales of days to weeks).

We therefore have reason to believe that land- atmosphere feedback can help explain the patterns of observed precipitation variances. Note: up to this slide, we haven’t looked at any model results! What can AGCMs tell us?

AGCM AGCM, no feedback Observations (Higgins, 50-yr dataset) July Rainfall: Mean [mm/day] July Rainfall: Variance [mm 2 /day 2 ] Correlations (pentads, twice removed) [dimensionless] same plots as before

AGCM AGCM, no feedback Observations (Higgins, 50-yr dataset) July Rainfall: Mean [mm/day] July Rainfall: Variance [mm 2 /day 2 ] Correlations (pentads, twice removed) [dimensionless]  The observations show statistics that are similar in location and timing, though not in magnitude, to those produced by the GCM. This is either a coincidence or evidence of feedback in nature. bulls-eye in model is definitely induced by feedback!

Central North America, of course, is just one of the Earth’s wet/dry transitions zones. Another is the Sahel… Annual Precipitation Does nature allow land-atmosphere feedback to affect rainfall statistics in the Sahel?

The comparison between model results and observations isn’t as clear-cut as it is in North America, but it is suggestive… Precipitation Variances (mm 2 /day 2 ) AGCM AGCM with no land feedback Observations

The comparison between model results and observations isn’t as clear-cut as it is in North America, but it is suggestive… Precipitation Variances (mm 2 /day 2 ) AGCM AGCM with no land feedback Observations The dots show where precipitation itself is maximized

Another observational study If land-atmosphere feedback operates in the Sahel, then realistic land initialization there should lead to improved monthly forecasts. Test with comprehensive forecast study: 75 start dates (first days of each month: May to September) 9 ensemble members per forecast In one set of forecasts, utilize realistic land ICs In other set, don’t utilize realistic land ICs Compare

Forecast skill resulting from realistic land surface initialization appears negligible for precipitation… Temperature Precipitation Temperature Precipitation Temperature Precipitation Differences: Added forecast skill from realistic land ICs Skill from knowing SST distribution and realistic land ICs Skill from knowing SST distribution

Precipitation Temperature Added forecast skill from land initialization HOWEVER, locations for which the rain gauge density is adequate enough to properly initialize the model are arguably very limited. Regions w/adequate raingauge density and model predictability

So, for the feedback question, observations are limited. Consider now a pure model study... # of Total Exp. simulations Length years Description A yr 800 AL yr 800 AO yr 720 ALO yr 720 Prescribed, climatological land; climato- logical ocean Interactive land, climato- logical ocean Prescribed, climatological land, interan- nually varying ocean Interactive land, interan- nually varying ocean SSTs set to seasonally-varying climatological means (from obs) SSTs set to interannually-varying values (from obs) LSM in model allowed to run freely Evaporation efficiency (ratio of evaporation to potential evaporation) prescribed at every time step to seasonally-varying climatological means Koster et al., J. Hydromet., 1, 26-46, 2000

Simulated precipitation variability can be described in terms of a simple linear system:   ALO =   AO [ X o + ( 1 - X o ) ]   ALO   AO Total precipitation variance Precipitation variance in the absence of land feedback Fractional contribution of ocean processes to precipitation variance Fractional contribution of chaotic atmospheric dynamics to precipitation variance Land-atmosphere feedback factor The above tautology isolates the relative contributions of SSTs, soil moisture, and chaotic atmospheric dynamics to precipitation variability.

Contributions to Precipitation Variability

Idealized “predictability” (for 1-month forecasts, MJJAS) deduced from aforementioned forecast experiment. (“Ability of model to predict itself.”) Temperature Precipitation Temperature Precipitation Temperature Precipitation Differences: Added predictability from realistic land ICs Predictability from SST distribution and realistic land ICs Predictability from SST distribution

More AGCM results: The GLACE multi-model experiment. In GLACE, land-atmosphere feedback was quantified independently in 12 AGCMs. While the models differ in their feedback strengths, certain features of the coupling patterns are common amongst them. These features are brought out by averaging over all of the model results:

More AGCM results: The GLACE multi-model experiment. In GLACE, land-atmosphere feedback was quantified independently in 12 AGCMs. While the models differ in their feedback strengths, certain features of the coupling patterns are common amongst them. These features are brought out by averaging over all of the model results: The AGCMs tend to agree: land-atmosphere feedback operates in the Sahel.

To summarize: Organization of Talk 1.Overview of the processes that control land-atmosphere feedback. (Case study: North America) 2.Application of these ideas to the Sahel: do the observations support the existence of feedback there? 3. Model study of the controls on Sahelian rainfall variability.

To summarize: Organization of Talk 1. Overview of the processes that control land-atmosphere feedback. (Case study: North America) 2. Application of these ideas to the Sahel: do the observations support the existence of feedback there? 3. Model study of the controls on the West African monsoon. We think we understand the impact of land-atmosphere feedback on the statistics of precipitation in North America. Through feedback, precipitation memory and variance are increased in the transition zones between wet and dry areas. The observations appear to support this.

To summarize: Organization of Talk 1. Overview of the processes that control land-atmosphere feedback. (Case study: North America) 2. Application of these ideas to the Sahel: do the observations support the existence of feedback there? 3. Model study of the controls on the West African monsoon. Observations are too sparse in the Sahel (relative to North America) for an equally clear indication that land atmosphere feedback operates there. Nevertheless, the available observations are not inconsistent with feedback.

To summarize: Organization of Talk 1. Overview of the processes that control land-atmosphere feedback. (Case study: North America) 2. Application of these ideas to the Sahel: do the observations support the existence of feedback there? 3. Model study of the controls on Sahelian rainfall variability. The NSIPP model (and indeed most of the models participating in GLACE) show the Sahel to be a region of strong land- atmosphere feedback.

WAMME W est African Monsoon M odeling and Evaluation The above modeling results may, of course, be model dependent. A new, upcoming experiment may provide a clearer look at the controls on monsoon dynamics… See website: A Spring AGU (Acapulco) session addresses the experiment…

W Simulations: Establish a time series of surface conditions Step forward the coupled AGCM-LSM Write the values of the land surface prognostic variables into file W1_STATES Step forward the coupled AGCM-LSM Write the values of the land surface prognostic variables into file W1_STATES time step ntime step n+1 (Repeat without writing to obtain simulations W2 – W16) Experiment Design All simulations are run from June through August

Experiment Design (cont.) R(S) Simulations: Run a 16-member ensemble, with each member forced to maintain the same time series of surface (deeper) prognostic variables Step forward the coupled AGCM-LSM Throw out updated values of land surface prognostic variables; replace with values for time step n from file W1_STATES Step forward the coupled AGCM-LSM time step n time step n+1 Throw out updated values of land surface prognostic variables; replace with values for time step n+1 from file W1_STATES

Oleson5. NCAR Kanae/Oki2. U. Tokyo w/ MATSIRO Xue12. UCLA with SSiB Koster11. NSIPP with Mosaic Lu/Mitchell10. NCEP/EMC with NOAH Taylor9. Hadley Centre w/ MOSES2 Sud8. GSFC(GLA) with SSiB Gordon7. GFDL with LM2p5 Verseghy6. Env. Canada with CLASS Kowalczyk4. CSIRO w/ 2 land schemes Dirmeyer3. COLA with SSiB McAvaney/Pitman1. BMRC with CHASM ContactModel Participating Groups Country USA UK USA Australia USA Japan Australia Canada

W: GFDL Scale goes from 0 to 1 S: GFDL Scale goes from 0 to 1 Differences: GFDL Scale goes from -0.5 to 0.5

Region considered What controls the timing of the monsoon? Quantify importance of: Another pure model study ( no observations ): monsoon rainfall 1. Average solar cycle. 2. Interannual SST variations 3. Interannual soil moisture variations

All simulations in ensemble respond similarly to boundary forcing  is high Simulations in ensemble have no coherent response to boundary forcing  is low Precipitation time series produced by different ensemble members under the same forcing Illustration of  diagnostic (not for African monsoon region)

solar, SSTs solar, SSTs, soil moisture NSIPP model solar, SSTs (Middle two bars differ because they were derived from different experiments, with different assumptions.)  The contributions of the different boundary forcings to the agreement (between ensemble members) of monsoon structure is established by analyzing the outputs of various experiments…

solar, SSTs solar, SSTs, soil moisture NSIPP model solar, SSTs (Middle two bars differ because they were derived from different experiments, with different assumptions.)  The contributions of the different boundary forcings to the agreement (between ensemble members) of monsoon structure is established by analyzing the outputs of various experiments… In this model, soil moisture variations have a major impact on monsoon evolution