# Presentation: 680559

## GEOS-4 Reanalysis: Assessment & Sensitivity study

- Shuhua Li
- Steven Pawson

**February 18, 2004**

**Notes:**
I am going to talk about a reanalysis project using the NASA GEOS-4 data assimilation system.  I will focus on assessment of the assimilation products,  and a sensitivity study with concentration on the impacts of GWD on the analysis and model forecast.

## Motivation for Reanalysis

- Produce a state-of-the-art atmospheric analysis for past periods
- Investigate sensitivities:
- Assimilation methods
- Forecast model
- Input data
- Improve understanding of constituent distributions in the atmosphere

**Notes:**
The motivation for the reanalysis project include:  to produce atmospheric analyses for past periods;  to investigate sensitivity to the assimilation methods, to the forecast model formulation, and to input data types;  also to improve understanding of constituent distributions in the atmosphere on the basis of analysis products and transport study.

## Overview of Assimilation

**Notes:**
A schematic of the GEOS-4 data assimilation system:  The assimilation cycle starts from a GCM including physics to produce a first guess or background forecast;  then the GCM forecast is combined with various observations by an analysis system to generate analysis products used for meteorology, climate diagnostics, and transport studies.  The analysis system also generates statistical quantities for monitoring, such as A-F and O-F.     In addition, the GCM is also used to make multi-day forecast which can be used for assessment of the GCM performance or comparison with analysis products.

## Plan of Talk

- GEOS-4 DAS
- Assessment of 1991-1994 & other years, with some comparison to earlier products
- Sensitivity to
- gravity-wave drag  (submitted to JAS)
- Summary and future work

**Notes:**
Firstly I will briefly talk about the GEOS-4 data assimilation system.  Then I will present some assessment results with comparison to earlier work for GEOS-1 and UKMO analysis.  Then I will talk about a sensitivity study to the representation of gravity wave drag, and finally to summarize.

## Evolution of GEOS Products

- Goddard Earth Observing System (GEOS) data assimilation system (DAS)
- Version 1 began in 1991, a troposphere-stratosphere system supporting the Upper Atmosphere Research Satellite (UARS)
- Numerous improvements in resolution, assimilation methods, model, ...
- GEOS-4 incorporates radiance-based assimilation of nadir sounders (TOVS, AIRS), plus many more data types

**Notes:**
The Goddard Earth Observing System started from Version 1 in 1991, which is a troposphere-stratosphere system supporting the UARS.  Over the last decade, there have been numerous improvements in model formulation, resolution (horizontal & vertical), and assimilation methods (from Optimal Interpolation to PSAS).  The recent version GEOS-4 incorporates radiance-based assimilation of TOVS and AIRS data, plus many more data types.

## Slide 6

**Notes:**
As an example, this diagram is illustrated to emphasize uncertainty between different analysis and independent  datasets.  It shows monthly mean T in July 1992 near the stratopause.  X is latitude from south to north.  The solid line is analyzed T from GEOS-4;  dashed line for NCEP-CPC, and dotted line for analysis from UK Met Office;  the crosses symbol for MLS (Microwave Limb Sounder) and the heavy black dots for HALOE (Halogen Occulation Exp). In general, the GEOS-4 T is about 10K lower than NCEP-CPC.  Analyses from UKMO lie between the two extremes, but are slightly closer to GEOS-4 values.  The MLS estimates are somewhat colder than the NCEP-CPC data at all latitudes, while HALOE data show more variations with latitude, with extreme scatter in the southern mid-latitudes.
The latitude range of HALOE varies as the month progresses, so the north-south structure is under-sampled; this may explain the large deviation from other estimates in the winter hemisphere.

## GEOS-4/Reanalysis

- Produce state-of-the-art analyses for May 1991 until April 1995 (eventually longer)
- Re-examine UARS data in the framework of more accurate meteorological analyses
- Perform a set of sensitivity studies for selected periods, looking at:
- Model formulation (e.g., gravity waves)
- Radiative impacts of Pinatubo aerosol and ozone
- Use of different data types (UARS data)

**Notes:**
The primary objectives of the reanalysis project include: to produce a state-of-the-art analysis for the period 1991-1995, to re-examine the UARS data,  and to perform sensitivity study for selected periods, by looking at GCM formulation, the radiative impacts of aerosol and ozone, the use of different data types, and so on.

## Model and analysis

-  Model:  dynamical core fvGCM based on physics
-                from NCAR CCM3
-                vertical domain: *surface to 0.01 hPa*
-  Analysis: Physical-space Statistical Analysis System
-                 (PSAS) working in observation space
-                 vertically *up to 0.2 hPa (~60 km)*

**Notes:**
The model used in GEOS-4 system is a dynamical core of finite-volume GCM in which physical parameterizations are based on those from the NCAR CCM3.  The model domain extends from the surface to 0.01 hPa (near 80 km).
The analysis system is a global PSAS which works in observation space.  The analysis fields are up to 0.2 hPa (about 60 km high); above this level there is no constraint from observations.

## Meteorological observations: sondes, aircraft, surface stations, ship stations (identical input as the ERA-40 reanalysis)
TOVS (SSU, MSU, HIRS) level-1b radiances (from NESDIS archives): information on thermal structure and tropospheric water
ERS-1 scatterometer: ocean-surface winds
SSM/I total precipitable water 
Cloud-track winds (geostationary platforms)

- TOVS (SSU, MSU, HIRS) level-1b radiances (from NESDIS archives): information on thermal structure and tropospheric water
- ERS-1 scatterometer: ocean-surface winds
- SSM/I total precipitable water
- Cloud-track winds (geostationary platforms)

- ReSTS input data types

**Notes:**
The input data types for the reanalysis include conventional observations from sondes, aircraft, surface stations, ship stations, etc.,  and TOVS data of radiance, ERS-1 scatterometer for ocean surface winds, the SSM/I for total precipitable water, and cloud-track winds.

## GEOS-4 performance

- Tropical tropopause temperature
- Mass-fluxes in tropopause region, compared to earlier work
- Synoptic features in polar structure and water vapor transport

**Notes:**
The performance of GEOS-4 assimilation will be assessed by looking at the tropical tropopause temperature, the mass fluxes across the tropopause with comparison to earlier work for GEOS-1 and UKMO,  as well as the synoptic features in polar vortex structure and water vapor transport.

## ReSTS: 100-hPa temperature 

- show annual progression with inter-seasonal,
- intra-seasonal and interannual variability

- ReSTS: 100-hPa temperature

**Notes:**
This shows the tropopause temperature from GEOS-4 reanalysis for the period of mid-1991 until October 1994.   You can see the signal of annual cycle is clearly produced.  The daily temperatures show progression with seasonal, inter-seasonal and interannual variability.

## Mass flux through 100hPa

**-48**

**-99**

**147**

- Fluxes are weaker in GEOS-4: less cross-tropopause exchange (good for ozone budget)

- Units: 108 kg/s per 4° lat band

**Notes:**
We calculate the vertical mass flux through the tropopause for Dec-Jan-Feb of 1991-92 season.  The flux is over four degree latitude band.  In general, there is upward mass flux in the tropics (30S-30N) and downward flux over the extra-tropical regions.  The numbers indicate the integral mass fluxes for these three regions.  The comparable estimates from earlier work for GEOS-1 and UKMO assimilations are marked as red numbers.  They are similar in magnitude over the three regions.  However, fluxes are weaker in GEOS-4.  This indicates less across-tropopause exchange, which may be good for estimating ozone budget and transport.

## Synoptic features

**-99**

**147**

- North polar region in 1993-1994 winter, showing coherence of features in the modeled water vapor fields, as the polar vortex undergoes a disturbance
- Improvements in PV analyses compared to earlier GEOS systems, building on Coy and Swinbank’s earlier study (1997)

**Notes:**
The synoptic features are examined for North polar region in 1993-1994 winter when a typical disturbance of the polar vortex occurred.  We focus on water vapor fields, T, and the evolution of polar vortex.  Also, comparison of PV analysis with earlier work for GEOS-1 and UKMO analysis indicates improvements in the GEOS-4 analyses, e.g., smoother PV structure.

## Dec. 28, 1993

**Dec. 28, 1993**

- A “wavenumber-1” pattern dominates the flow, with high water vapor concentrations in the polar vortex and low temperature centered over Canada

- Temperature and water vapor

- Specific humidity [mg/kg] (shading)

**Notes:**
This shows water vapor and temperature over the Northern Hemisphere middle and high latitudes, with shading colors for humidity in units of g/kg and contours for T with 2.5K as the interval.  On Dec. 28, 1993, a “wavenumber-1” pattern dominates the flow, with high water vapor concentrations in the polar vortex and low temperature centered over Canada.

## Dec. 30, 1993

**Dec. 30, 1993**

- The distortion intensifies, as the coldest region moves eastward

- Temperature and water vapor

- Specific humidity [mg/kg] (shading)

**Notes:**
Two days later on Dec. 30, the vortex distortion intensifies and the coldest region moves eastward.  Meanwhile, some dry air is transported from middle latitudes to the polar region;  the polar vortex is still well-isolated.

## Jan. 1, 1994

**Jan. 1, 1994**

- The vortex begins to develop a “double” center, as more dry air is transported from the subtropics to the polar region and polar temperatures approach 240K

- Temperature and water vapor

- Specific humidity [mg/kg] (shading)

**Notes:**
After two more days, on Jan. 1 of 1994, the polar vortex begins to develop a “double” center, as more dry air is transported from the subtropics to the polar region.  The polar region temperatures approach 240K, and the coldest region moves equator-ward to as low as 50N.

## Jan. 2, 1994

**Jan. 2, 1994**

- Two “moist” vortex centers emerge. Note that the cold region (T<190K) is displaced to about 60°N, with a maximum temperature in excess of 255K at the same latitude on the eastward extremity of the main vortex

- Temperature and water vapor

- Specific humidity [mg/kg] (shading)

**Notes:**
On Jan. 2, 1994, two “moist” vortex centers emerge.  The cold region below 190K is displaced to about 60N, with a maximum temperature of 255K at the same latitude on the east side of the main polar vortex.  These features are in good agreement with observational disturbance of the polar vortex represented by wind fields and PV.

## Gravity-wave-drag experiments

- GWD scheme (from MACCM3):
- Mountain wave  (phase speed zero)
- Traveling waves (phase speed not zero)
- Configurations
- Control: 0, ±(10, 20, 30, 40) m/s (GW spectrum)
- Two wave: 0, ±(10, 20) m/s
- Mountain wave: 0 m/s
- No wave: no gravity waves
- Experiments (July-August 1999):
- Assimilation and model runs for July 21 – August 31
- Forecasts started every fifth day from control assimilation, using four configurations of model

**Notes:**
Now let’s move on to the 2nd part which is designed to examine the impacts of GWD on the assimilations and model forecast.  The GWD scheme is from NCAR CCM3, which contains a stationary mountain wave (phase speed as zero) and a GW spectrum with none-zero phase speed waves (we also call them “traveling waves”).  We have designed four different configurations: the 1st is control run including the mountain wave and four traveling waves with phase speed as 10, 20, 30, and 40 m/s in each direction; the 2nd excluding the two waves with the largest phase speeds; the 3rd with stationary mountain wave only; and the last for no-wave at all.  The assimilation and model runs are carried out for July-Aug. 1999, and additional free-running model forecasts started every 5th day from control analysis using the four different model configurations, as well as 10-day forecast runs for the control situation.

## Analyzed T & U:  control vs no-GWD

**Notes:**
This diagram shows the cross-section of monthly-mean temperature and zonal wind for August 1999; the control analysis on the left and no-wave analysis on the right. The top level of the analysis is about 60 km.  It may be difficult to read the contours.  However, if we focus on the general distributions between the two extreme cases, they look quite similar: with the stratopause near 1 hPa, and the tropical tropopause near 100 hPa.  For the zonal wind, they clearly show the easterly winds in summer hemisphere and westerly in winter hemisphere, and the polar night jet in the middle stratosphere.  However, there is large localized difference in the upper levels for winter hemisphere high latitudes. as shown in next slides.  

## T difference: control minus no-drag assimilations

**Notes:**
The difference of analyzed temperature between the control analysis and the case without GWD, with positive differences shaded.  What you can see is the largest differences in the winter hemisphere high latitudes near and above the stratopause.  Without GW forcing, the air in this region is too cool.  However, in the troposphere and lower stratosphere there is little impact from GW forcing.

## U difference: control minus no-drag assimilations

**Notes:**
The corresponding difference of zonal wind also shows large difference in the upper levels.  At high latitudes, the zonal wind is too strong without GWD, while in tropical upper levels the zonal wind is too weak or even in opposite direction.
Also, changes in temperature are in thermal wind balance with the wind differences.  The following discussion will focus on the winter hemisphere high latitudes with concentration in the upper stratosphere.

## GWD impacts on assimilation and simulation

- Small model-assimilation bias  with c≠0 waves

- Including (e.g.) convectively forced gravity waves has a substantial impact on model and analyses in winter stratopause region; assimilation can correct much bias

**Notes:**
This diagram gives the time series of  the analyzed and model forecast T at 1 hPa (stratopause), in July-August 1999.  Black lines are for assimilation, and red lines for seasonal  forecast starting on July 20.  The four scenarios are the control run, the two waves plus mountain wave, the stationary wave only, and no-wave at all.  We can learn much information from this diagram.  Firstly, there is very large bias between free-running model and assimilation when traveling waves are omitted;  Secondly, the bias between assimilations with/without traveling waves is less than 10K;  Third, with traveling waves the model-assimilation bias is much smaller than the bias without traveling waves.  Therefore, including non-zero phase speed gravity waves has a substantial impact on model and analysis in winter stratopause region; assmilation can correct much bias.

## GWD impacts on 5-day forecast

**Notes:**
To evaluate the forecast skill of the GCM with different formulations of GWD scheme and their bias from the control analysis (black line), we have run the model forecast only using  the control analysis as initial fields.  This diagram shows 5-day forecast starting on every 5th day, with four configurations of the GWD scheme.  The red curves for multiple waves, orange curves for 2-wave plus mountain wave, green color for mountain wave only, and blue curve for no-wave at all.  Again, for the first two cases with traveling waves, the 5-day forecasts trace the analysis reasonably well; however, the forecast without traveling gravity waves shows rapid going-down drift from the analysis even after a couple of days.

##  T bias (A-F) vs forecast days

**Notes:**
Additional information about the model forecast is given by 10-day control forecast of temperature field.  What I am doing is to run the GCM forecast for 10 days using control analysis on each day as initial fields.  This scatter plot shows T bias (analysis minus forecast) against the forecast length for 32 individual 10-day forecasts.  At 1hPa, there is a tendency for the forecast to be too warm, and the forecast bias intensifies with the forecast length.

## T bias vs forecast days

**Notes:**
At 10hPa, the bias is opposite;  the forecast T tends to be too cool compared to the analysis.  Again, the temperature bias increases with forecast length.  This indicates that even the best model version in this study has a tendency to bias the medium-range forecast.

## Mean O-F (height) at 1hPa: control and no-drag assimilations

**Notes:**
The performance of the analysis system can also be reviewed by the “observation minus forecast”  tendency.  This shows the statistical O-F residual of geopotential height for August 1999 at 1 hPa.  Solid curve for control and dotted curve for the no-wave situation.  The O-F residual shows little difference away from the highest southern latitudes. Over this region, O-F of geopotential height is less than 15gpm for the control situation with a GW spectrum, while the largest values reaching about 55gpm are for the no-wave situation. Therefore, there is substantial reduction in model-data bias when non-zero phase speed gravity waves are included.

## Summary of sensitivity to GWD (upper stratosphere)

- Model “climate” very sensitive to GWD (known result)
- Rapid drift of model forecast to biased climate without convectively forced GWs
- Analyses are also very sensitive (about 7-8K in stratopause temperature) to GWs

- Impacts on analyses are large near the stratopause, and decrease going down to the lower stratosphere (not necessarily true of free-running model)

**Notes:**
A summary of the sensitivity to GWD:
 Model “climate” is very sensitive to GWD; this has been demonstrated  by climate modeling studies.
 There is rapid drift of model forecast to biased climate without non-zero phase speed GWs.
 Analyses are also very sensitive to GWD, with about 7-8K in stratopause temperature.

 The impacts of GWD on analysis are large near the stratopause, and going down to the lower stratosphere.

## Summary and future work

- Reanalysis is justified by improvements in the analysis system
- Analyzed fields are “heading in the right direction”
- Strong sensitivity to model formulation, esp. to GWD near the stratopause

- _Underway:_  Radiative impacts of volcanic aerosol
-                  (e.g., Pinatubo aerosol)