Winds, Surface, and Radiance Products
Jennifer Francis |
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Summary of TOVS Polar Pathfinder Data Set and Applications
- Generated from nearly 20 years of TOVS (TIROS Operational Vertical Sounder) Satellite radiances as a part of the NOAA/NASA Pathfinder Program
- Daily and monthly fields from 1979 to 1998
- 100 km x 100 km resolution, north of 60N, EASE grid, HDF format
- Radiances processed with the Improved Initialization Inversion ("3I") algorithm (Chedin et al, 1985), improved for polar conditions by Francis (1994)
- Extensively validated with data from Russian NP stations, SHEBA, HARA, POLES, and COADS
- Documentation, validation, and data available from NSIDC and http://psc.apl.washington.edu/pathp/pathp.html
Validation of Path-P Products with SHEBA Data
Surface Skin Temperature
Comparisons of Path-P skin temperatures to the IABP-POLES data set of near-surface air temperature reveal large discrepancies. In both January (a)and July (b) the Path-P fields are colder than IABP-POLES over sea ice in a comparison of decadal mean values (1980 to 1989), while over the GIN Seas, Path-P is warmer. We found that some of the difference (up to 1.7 degrees) is caused by intersatellite calibration problems. In summer the detection of low clouds is especially difficult,and we find that Path-P pixels with >95% retrieved cloud cover have large errors (too cold) in surface temperature. Plots (c)and (d) show the decadal differences again, but with these corrections applied. July fields over ice are much improved.
Validation of Surface Skin Temperature
We investigate differences through comparisons to COADS data. Plot (e) compares January-mean IABP/POLES and Path-P values to COADS and NP measurements over the central Arctic Ocean between 1980 and 1991.January differences over sea ice are caused by the IABP-POLES being too warm, probably because data were interpolated over large distances and insulation of buoys by snow. In July (b)shows Path-P values are slightly too cold compared to COADS data, probably caused by cloud effects. In the GIN Seas IABP-POLES is consistently colder than COADS in both January (c)and July (d),which is again likely due to interpolation across large distances. Path-P data compare very well with COADS data in the GIN Seas, suggesting that Path-P data are more realistic than IABP/POLES values in this area. Over sea-ice and open-ocean areas, corrected Path-P temperature retrievals appear to be superior to the IABP-POLES fields.
Examples from Path-P
Sensible Heat Advection
Daily fields of sensible heat convergence are calculated from Path-P temperatures and NCEP-NCAR Reanalysis upper-level winds for 5 tropospheric layers between 1000 and 300 mb, as well as poleward and zonal components. Heating rates [K day-1] resulting from the convergence of laterally advected sensible heat in the 1000-to-700 hPa layer are shown for (a) the annual mean and (b) the winter mean (DJF)over the entire 19-year Path-P record. Values over Greenland are uncertain and should be disregarded.
Winter Advective Heating Poleward and Zonal Components
Winter (DJF) mean heating rates owing to poleward (a) and zonal (b) components of the lateral advection, 1979 to 1998. Near the pole heating results mainly from poleward transport, while over much of northern Europe and Asia heat is transferred primarily from the west.
19-Year Trends in Advective Sensible Heating
Path-P-derived 19-year trends (K day -1 decade -1 )in heating resulting from the poleward sensible heat convergence for (a) winter (DJF), (b) spring (MAM),(c) summer (JJA), and (d) fall (SON). Data are for the deep layer between 1000 and 300 mb. Note increased heating is evident during all seasons in the E. Siberian Sea where observed sea ice extent has decreased, and a slight cooling has occurred near the Canadian Archipelago where ice has increased. Strong warming west of Novaya Zemlya in spring may contribute to observed ice reduction in this area. Basinwide, areas of warming are offset by areas of cooling for no significant net change overall. Are these trends caused by changes in winds or thickness gradients, or both?
Decadal Trends in NCEP v-component Winds
1000-300 mbTrends in NCEP Reanalysis v-component winds (m s -1 dec.-1 ) in the 1000-to-300 mb layer for each season from 1979 to 1998.Patterns are consistent with observed decreased SLP over the pole. Comparison with patterns of trends in poleward advective heating suggests that heating trends are caused primarily by changes in the circulation. In most cases areas of increased heating correspond to increased southerly winds, and areas with cooling coincide with stronger northerly winds. Changes in the thickness gradient also play a role.
Moisture Transport and P-E
Moisture transport is derived from TOVS precipitable water (PW) retrievals and NCEP Reanalysis upper level winds. (a) is the annual-mean PW flux (kg m -1 s -1 ) averaged over 19 years (1980-1998); (b) is the annual mean net precipitation (P-E; cm yr -1 ); and (c) is the seasonal mean, meridional PW flux across 70N.
Trends in Moisture Transport and P-E
Path-P calculated changes in PW flux and P-E: (a)is the difference in PW flux between the 1980s and 1990s; (b) is the decadal difference in P-E; and (c) and (d) show the difference in P-E between positive-AO and negative-AO days during winter and summer. From Groves and Francis [2001a,b ].
Trends in Upper-Level NCEP Winds
Advection of sensible heat and moisture are calculated using upper-level winds from the NCEP Reanalysis. Wind fields are difficult to validate because all available rawinsondes were assimilated into the reanalysis. Rawinsondes from the CEAREX (1988, NE of Svalbard) and LeadEx (1992, Beaufort Sea) field experiments, however, were not assimilated, and therefore constitute independent data. This plot displays summary statistics of the comparison with NCEP winds (ERA winds are very similar).While reanalyses provide the best wind fields available at present, there is a clear need for more accurate data. Layers 1-5 are bounded by (top down): 300, 400, 500, 700, 850, and 1000 mb.
Downwelling Longwave Fluxes from TOVS Path-P
