Hou/JTST NASA GEOS-3/TRMM Re-Analysis: Capturing Observed Rainfall Variability in Global Analysis Arthur Hou NASA Goddard Space Flight Center 2 nd IPWG Workshop, Naval Research Laboratory Monterey, CA, October 2004

Arthur Hou, IPWG Workshop, October  Precipitation products from most operational NWP systems are forecasts rather than analyses of precipitation based on rainfall observations and model information.  NASA has been exploring a different approach to precipitation assimilation that uses rainfall observations to directly estimate and correct errors in the model rain within a 6h assimilation cycle.  A brief description of the variational continuous assimilation (VCA) scheme for precipitation assimilation.  Results from the NASA GEOS-3/TRMM reanalysis (Nov Dec. 2002): - An atmospheric analysis dynamically consistent with a QPE based on TMI and SSM/I rain rates. GEOS-3 = Goddard Earth Observing System – Version 3 Scope of talk

Arthur Hou, IPWG Workshop, October Tropical ENSO rainfall variability: Observation vs analyses TMI Monthly-Mean SST January 1998 January 1999 GPCP January: 1998 Minus 1999 NCEP January: 1998 Minus 1999 ERA40 January 1998 Minus 1999 mm/day 1 mm/day  30 W/m 2 Large discrepancies (for the same SST input) Tropical rainfall analyses are model-dependent and vary with parameterized model physics Present-generation convective schemes are less than perfect - systematic model errors

Arthur Hou, IPWG Workshop, October Sensitivity of tropical analysis to precipitation process Time series of 15-d mean of tropical [v] at 200 hPa Trenberth & Olson, 1988 September 1982: Diabatic Nonlinear Normal Initialization (DNNMI) implemented at ECMWF September 1984: DNNMI introduced at NMC May 1985: Shallow convection (SC) implemented at ECMWF May 1986: SC implemented at NMC ECMWF Reanalysis (80-93) NCEP/NCAR Reanalysis GEOS-1 Reanalysis Variance of Hadley circulation streamfunction

Arthur Hou, IPWG Workshop, October Conventional data assimilation algorithms are based on the assumption that the underlying observation and model error statistics are random, unbiased, stationary, and normally distributed. But model clouds and precipitation are derived from parameterized moist physics, which can have large systematic errors. Unless these (largely unknown) systematic model errors are accounted for in the assimilation procedure, one will always make sub-optimal use of these data. A basic problem is that the observation operator for precipitation is not as accurate as those for conventional data or observables in clear-sky regions. Key issues in precipitation assimilation

Arthur Hou, IPWG Workshop, October What is an observation operator? It relates an observable to model state variables (u,v,T,q, etc.) (u,v,T,q) model grid (u,v,T,q) at observation locations Observation operator (T,q,u,v) grid Precipitation observation operator Cloud, Precipitation random error “Perfect model” Observations in clear-sky regions Systematic error Developing procedures to make online estimation and correction of biases in the observation operator to make more effective use of precipitation data Precipitation observation operator with correction, 

Arthur Hou, IPWG Workshop, October Variational continuous assimilation (VCA) of surface rain A 1+1D observation operator (H) based on a 6h time-integration of a column model of moist physics with large-scale forcing prescribed from “first guess ” Minimizing the cost function: J(x) = (x) T P -1 (x) + ( y o – H(x)) T R -1 ( y o – H(x))  model tendency correction: x  logarithm of observed rain rate: y o  logarithm of model rain estimate: H(x)  error covariance of prior estimate: P  logarithm of relative observation error variance: R Assimilation of 6h surface rain accumulation using 6h-mean moisture tendency correction as the control variable, and applying the correction continuously over a 6h analysis window to ensure dynamical consistency. The scheme estimates and corrects for biases in model’s moisture tendency every 6h to minimize discrepancies in 6h rain between the model and observations. The strategy is to relax the perfect model assumption - i.e., using the forecast model as a weak constraint.

Arthur Hou, IPWG Workshop, October Impact of VCA rainfall assimilation on GEOS-3 analysis MJO in precipitation over tropical oceans (10N-10S) 2001 mm/d GPCP NCEP GDAS ERA-40 Propagation and intensity of tropical rainfall systems are difficult to capture GEOS/TRMM Replicating observed propagation and intensity of tropical rainfall systems and intraseasonal oscillation Rain error reduction (30N-30S, ocean) GEOS = Goddard Earth Observing System

Arthur Hou, IPWG Workshop, October Improved temporal and spatial variability Enhanced frequency-time coherence between GPCP and GEOS-3 analysis Avg. Precipitation ( E, 4S-4N) (Morlet analysis) An atmospheric analysis dynamically consistent with observed rainfall variability

Arthur Hou, IPWG Workshop, October Improved cloud radiative forcing verified against CERES  94% reduction in bias  51% reduction in error standard deviation January 1998 Variational continuous rainfall assimilation improves key climate parameters such as clouds and TOA radiation in the GEOS analysis

Arthur Hou, IPWG Workshop, October Impact on wind and humidity analyses GEOS(TMI+SSM/I PCP+TPW) minus GEOS(CONTROL)Verification: HIRS2 Channel 12 Brightness Temperature Surface rain & Horizontal div. wind at 200 hPa Omega velocity at 500 hPa Specific humidity at 400 hPa Improved latent heating patterns and large-scale motion fields leading to improved upper-tropospheric humidity (verified against TOVS brightness temperature) GEOS control has a moist/cold bias relative to HIRS2 channel 12 (top) Rainfall assimilation leads to a drier upper-troposphere & reduces the err.std.dev by 11% January 1998

Arthur Hou, IPWG Workshop, October Impact on hurricane track and precipitation forecasts Improved initial storm position 5-day track forecast from 12UTC 8/20/98 Blue: No rainfall data in IC Red: With rainfall data in IC Green: NOAA “best track” 5-day rain forecast Hou et al. 2004: MWR, August issue. 5-day track forecast from 00UTC 9/11/99 5-day rain forecast BonnieFloyd

Arthur Hou, IPWG Workshop, October Assimilation of TMI, SSM/I & AMSR-E rain OLR July 2002 W/m 2 OSR July 2002 W/m 2 Precipitation July 2002 mm/d

Arthur Hou, IPWG Workshop, October Optimal use of precipitation information in global data assimilation poses a special challenge because parameterized physics can have large systematic errors, which must be accounted for in the assimilation procedure. –One effective strategy is to assimilate rainfall data using the forecast model as a weak constraint –Exploring advanced techniques such as ensemble DA, which could provide a unified framework for addressing both initial-condition errors and model errors The GEOS-3/TRMM reanalysis provides an atmospheric analysis dynamically consistent with the observed tropical rainfall variability: –Improved climate parameters including TOA radiation, upper- tropospheric humidity, and cloud-radiative forcing –Improved short-range forecasts Summary