near-term prospects for improving quantitative precipitation estimates at high latitudes
DESCRIPTION
Near-Term Prospects for Improving Quantitative Precipitation Estimates at High Latitudes G.J. Huffman 1,2 , R.F. Adler 1 , D.T. Bolvin 1,2 , E.J. Nelkin 1,2 1: NASA/GSFC Laboratory for Atmospheres 2: Science Systems and Applications, Inc. Outline 1.The Problem 2.Prior Work - PowerPoint PPT PresentationTRANSCRIPT
Near-Term Prospects for Improving Quantitative Precipitation Estimates at High Latitudes
G.J. Huffman1,2, R.F. Adler1, D.T. Bolvin1,2, E.J. Nelkin1,2
1: NASA/GSFC Laboratory for Atmospheres2: Science Systems and Applications, Inc.
Outline
1. The Problem
2. Prior Work
3. Instantaneous Rates
4. Next Steps
5. Summary
1. THE PROBLEM
Retrievals are more challenging at high latitudes
- Different T, RH profiles; sfc. T; tropopause and melting levels
- Generally light precipitation
- Frozen/icy surface knocks out scattering channels
Validation is also more challenging
- Gauges are sparse
- Gauge undercatch more severe
- Radar hasdifficulties with snow and bright band
2. PRIOR WORK
Best solution involves high-frequency microwave channels
- Try to slice atmospheric signal away from difficult surface issues
Some approximate alternatives already exist that can
- Provide answers relatively quickly
- Fill inter-swath gaps in the high-frequency estimates when they arrive
- Stand in for high-frequency estimates where they falter
- Provide a multi-decadal record
One alternative is to work with OLR Precipitation Index (OPI)
- Xie and Arkin (1998) showed that deviations in OLR from local climatology are related to deviations in precip from local climatology
- GPCP uses this OPI in the pre-SSM/I period at high latitudes
- It is available in monthly and pentad files; we have not pursued it at the instantaneous level due to the higher information content used in the next product
2. PRIOR WORK (cont.)
The alternative we chose is working with satellite soundings
- Susskind et al. (1997) developed a calibrated cloud volume proxy from TOVS
Precip = revised cloud depth * cloud fraction * ƒ ( latitude, season )
- The calibration is TOVS swath data vs. daily FGGE station precip data
- Results show low precip rates, very high fractional occurrence
• done as a regression• uses instantaneous data as a proxy for daily data• has only one sample for the day
cloud top ht. – ( scaled RH + scaled cloud fraction )
0 = sat. sfc500 mb9 = dry “
0 = overcast4.5 = clear
2. PRIOR WORK –GPCP Monthly SG
Version 1 deficiencies
- Data voids at high lat.
- Low values in high-lat. ocean
Susskind et al. (1997) TOVS adapted for use in Version 2
- Recalibrated to SSM/I at mid-lat., gauge at high lat.
The accuracy of interannual fluctuations at high lat. is not yet resolved
TOVS algorithm currently applied to AIRS (beginning May 2005)
GPCP V.1 (mm/d) 1988-99
GPCP V.2 (mm/d) 1988-99
2. PRIOR WORK – GPCP One-Degree Daily
SG experience encouraged us to use TOVS at high lat. in 1DD
- By month, at 40°N and 40°S separately, compute rate and occurrence adjustment to daily TOVS to match low-latitude results (from Threshold Matched Precipitation Index), and apply in the appropriate hemisphere 40°-pole
- Very appealing results; minimal data boundaries
TOVS algorithm currently applied to AIRS (beginning May 2005)
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2. PRIOR WORK – GPCP One-Degree Daily (cont.)
Daily averages over the Baltic Sea basin show good skill
- Bias is related to gauge adjustment from monthly product
- Day-to-day events entirely driven by TOVS (in parallel to IR in the band 40°N-S)
Figure courtesy of B. Rudolf, DWD/GPCC
3. INSTANTANEOUSRATES
How best to develop an instantaneous sounding-based scheme?
As we got serious, the A-Train showed up!
- CloudSat provides a “curtain” of cloud/ precip data at all latitudes
- AMSR-E provides 2D maps of precip
- Here, sfc-based CloudSat echo corresponds to AMSR-E rain area
- CloudSat echo based above the sfc shows up in AIRS, but not AMSR-E
A
B
C
A
B
C
C B A
Reflectivity Low High
AMSR-E AIRS
CloudSat
3. INSTANTANEOUS RATES (cont.)
As a first step, we calibrated Susskind et al. (1997) AIRS to AMSR-E for Jan. 2004
- Compare AIRS, AMSR-E, calibrated AIRS for one descending node
- Qualitative agreement
0416-0505 UTC 19 January 2004
AMSR-E AIRS Cal. AIRS
3. INSTANTANEOUS RATES (cont.)
Example of AIRS filling in a feature over snow where AMSR cannot reliably estimate
AMSR-E
CalibratedAIRS
16 January 2004 mm/d
16 January 2004 mm/d
Land precip feature
3. INSTANTANEOUS RATES (cont.)
Month-average of Susskind et al. (1997) AIRS calibrated to AMSR-E for July 2006
- calibration by lat. bands:
Ocean: 90-30°N, 30°N-S, 30-90°SLand: 90-40°N, 40·°N-S, (40-90°S)Coast: global Cal.AIRS (mm/d) July 2006
AMSR (mm/d) July 2006
Diff. (mm/d) July 2006
- Note opposing within-band (east-west) differences
- Implies regime dependence – same scaled cloud volume maps to different AMSR-E rain rates in different places
4. NEXT STEPS
Design and implement a new AIRS cloud volume scheme based on comparison with AMSR-E and CloudSat
Develop a merged AMSR-E / AIRS swath dataset
- How can we gracefully transition from AMSR-E to AIRS at high latitudes and in cold/frozen land?
Apply the revised cloud volume scheme to ATOVS and TOVS to develop an improved long-term record at high latitudes
Throughout, particularly with the operational ATOVS, sounding retrievals work best in clear cases and worst (or fail) for precipitating cases
Explore model data- Include model precip in high-lat. comparisons- Consider similar profile-based estimates for models (T and RH profiles better
than precip?)- Look toward combinations of observation- and model-based estimates
5. SUMMARY
Historically, we lack the physically direct sensors for high-latitude and cold-region precip that are available for tropical rain
The Susskind et al. (1997) scaled cloud volume algorithm for TOVS (and AIRS) has seen successful use in GPCP Version 2 monthly and 1DD
Early development work with AMSR-E and CloudSat data seems promising for an instantaneous version
Once high-frequency microwave sensors/algorithms are in place, scaled cloud volume could serve at high latitudes as IR serves at low, by providing
- Lower instantaneous skill, but availability to fill holes
- A long record