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GEOS–5 vs. GEOS–4 precipitation fields

GEOS–5 vs. GEOS–4 precipitation fields. In GEOS–Chem, we have been using the following formula to compute the precipitation field (QQ) that is used in the wet scavenging algorithm: ! If there is total precipitation in the (I,J) column, then: !

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GEOS–5 vs. GEOS–4 precipitation fields

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  1. GEOS–5 vs. GEOS–4precipitation fields

  2. In GEOS–Chem, we have been using the following formula to compute the precipitation field (QQ) that is used in the wet scavenging algorithm: ! If there is total precipitation in the (I,J) column, then: ! ! (1) Compute FRAC, the large scale fraction (if LS = .TRUE.) ! or convective fraction (if LS = .FALSE.) total ! precipitation. FRAC is computed from PREACC and PRECON. ! ! (2) Compute QQ, the rate of formation of precipitation ! [cm3 H2O/cm3 air/s]. From MOISTQ [kg H2O/kg air/s], ! the unit conversion is: ! ! kg H2O | m^3 H2O | AIRDEN kg air m^3 H2O ! ------------+-------------+--------------- ==> ------------- ! kg air * s | 1000 kg H2O | m^3 air m^3 air * s ! ! and ! ! m^3 H2O cm^3 H2O ! ------------- is equivalent to -------------- ! m^3 air * s cm^3 air * s! ! ! since the same conversion factor (10^6 cm^3/m^3) is in both ! the numerator and the denominator. ! ! Therefore, the equation for QQ is: ! QQ(L,I,J) = FRAC * MOISTQ(L,I,J) * AIRDEN(L,I,J) / 1000.0

  3. Thus the precip field QQ consists of 3 terms: • FRAC • MOISTQ • AIRDEN • AIRDEN is the air density from GEOS–Chem (straightforward) • MOISTQ is the tendency (i.e. 1st deriv.) of the specific humidity. • In GEOS–4 the MOISTQ field only considered the tendency of vapor, due to moist processes. • In GEOS–5, we don't have a MOISTQ field, but we computed it from these GEOS–5 met fields: MOISTQ = DQVDTMST + DQLDTMST + DQIDTMST.  • Thus the GEOS–5 "MOISTQ" field now also contains contributions from liquid, ice, and vapor tendencies, whereas GEOS–4 did not.

  4. MOISTQ: sum of liquid, ice, vapor tendencies GEOS–5 DQIDTMST (ice tendency) GEOS–5 DQLDTMST (liquid tendency) Zonal Means GEOS–5 DQVDTMST (vapor tendency) GEOS–5 equivalent MOISTQ= sum of all three tendencies Units have been converted to g/kg/day for comparison to the GEOS-4 fields. Sample plot of how GEOS-5 MOISTQ is created from liq, vap, ice tendencies

  5. FRAC is the precipitating fraction of the grid box. We must compute FRAC separately for large-scale and convective precipitation. • FRAC is currently computed in GEOS–Chem for two conditions: • 1) If there is precip reaching the surface, then compute FRAC as: • IF ( PREACC(I,J) > 0d0 ) THEN • ! Large scale or convective fraction of precipitation • IF ( LS ) THEN • FRAC = ( PREACC(I,J) - PRECON(I,J) ) / PREACC(I,J) • ELSE • FRAC = PRECON(I,J) / PREACC(I,J) • ENDIF • PREACC is the total 2-D precipitation at the surface and • PRECON is the total 2-D convective precipitation at the surface. • ( PREACC – PRECON ) / PREACC is the fraction of LS precip in the box • PRECON / PREACC is the fraction of convective precip in the box

  6. 2) If there is no precipitation reaching the surface, then consider any precipitation at altitude to be large-scale precipitation. • ! Assume large-scale precipitation! • IF ( LS ) THEN • FRAC = 1d0 • ELSE • FRAC = 0d0 • ENDIF • Fraction of large-scale precip in the box • Fraction of convective precip in the box • . . . . . . . • So what does QQ (and its related terms) look like in GEOS–Chem? Here is some output from simulations: • GEOS-Chem v8–01–01 • GEOS–4 and GEOS–5 met • 1-week Rn–Pb–Be simulations (Jan 1–8, 2005)

  7. QQ: GEOS–Chem Precipitation Fields Also note GEOS– 5 has more conv precip over tropics! NOTE: The GEOS–5 precipitation is much higher at about 1km than in GEOS–4!

  8. So what could be causing this discrepancy in the QQ precip field?Let’s look at each of the terms in the equation. First let’s look at MOISTQ. At first glance, the MOISTQ fields don’t look terribly different.

  9. Here are the large-scale FRAC terms for GEOS–4 and GEOS–5 Note that in GEOS–5, we have large scale precip in a much wider area of the globe than we do in GEOS–4. The GEOS–5 FRAC value is close to 1 pretty much everywhere except in the ITCZ, where convective precipitation dominates:

  10. Here are the large-scale and convective FRAC terms for GEOS–4 & GEOS–5 On the other hand, the FRAC value for convective precipitation follows the ITCZ, whereas in GEOS-4 it occupies a much wider latitudinal distribution. The GEOS–5 FRAC is almost zero over the N midlatitudes, over continents.

  11. FRAC is based on PREACC and PRECON. Here are the PREACC fields. PREACC is total 2-D accumulated precipitation at the ground. GEOS–5. PREACC is qualitatively similar to GEOS–5. There seems to be stronger precip along the equator over Indonesia & Micronesia in GEOS–5.

  12. And here are the PRECON fields. The PRECON field (2-D accumulated convective precipitationat the surface) also appears much weaker in GEOS–5 than in GEOS–4, especially in the equatorial region near Indonesia & Micronesia. Note that in GEOS–4,that there is nonzero PRECON extending up to 60 N near Greenland. This does not happen in GEOS–5.

  13. GEOS–4 vs. GEOS–5 wetdep of Pb210 by large scale precipitation This plot (and the plots on the next 3 pages) contain the tracer removed from the entire column during 1-week GEOS–4 and GEOS–5 simulations. For the most part, less Pb is removed via LS precip in the tropics in GEOS–5.

  14. GEOS–4 vs. GEOS–5 wetdep of Pb210 by convective precipitation More Pb is removed in the tropics in GEOS–5…stonger convective precipitation at altitude?

  15. GEOS–4 vs. GEOS–5 wetdep of Be7 by large-scale precipitation Note smaller magnitude of differences between GEOS–5 and GEOS–4. In general, there is slightly more Be removed in LS precip in GEOS– 5

  16. GEOS–4 vs. GEOS–5 wetdep of Be7 by convective precipitation Ditto for Be7 in convective precip

  17. Rn222 zonal mean profiles in GEOS–4 and GEOS–5 Seems to be more Rn222 in the PBL at N midlats in GEOS–5

  18. Pb210 zonal mean profiles in GEOS–4 and GEOS–5 Also seems to be more Pb210 in the PBL at N midlats in GEOS–5

  19. Be7 zonal mean profiles in GEOS–4 and GEOS–5 Be7 looks very similar in both GEOS-5 and GEOS-4

  20. Summary • The GEOS– Chem precip field QQ is a product of 3 terms: MOISTQ, FRAC, and AIRDEN. (We’ll ignore AIRDEN, that is computed straightforwardly from the pressure fields.) • Of these terms, FRAC seems to be the most different between GEOS–4 and GEOS–5. The differences seem to be due to the differences in the PREACC & PRECON fields between GEOS–4 and GEOS–5. There could also be a mismatch between the 2-D PREACC & PRECON fields with the 3-D MOISTQ field. • FRAC as currently computed (for both LS and CONV precip) depends on the 2–D fields PREACC & PRECON. This might be the heart of the discrepancy. • We can investigate computing the QQ field from the GEOS– 5 fields DQRCON and DQRLSC (the conv & LS rain production rates in the met fields). Stay tuned …

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