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Intercomparison of ERA-40, ERA-15, and NCEP-NCAR Reanalyses Diagnosed Diabatic Heating
STEVEN C. CHAN, AND SUMANT NIGAM
Department of Atmospheric and Oceanic Science,
University of Maryland,
College Park, Maryland
----------------------------- Corresponding author address: Steven C. Chan, Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland 20742 E-mail: [email protected] Keywords: Reanalysis, Heating, Data Assimilation, Climate
ABSTRACT
Diabatic heating has been diagnosed from the newly available European Centre
for Medium-Range Forecasting (ECMWF) ERA-40 reanalysis. ERA-40 heating is then
compared with ECMWF ERA-15 and U.S. National Center for Environmental
Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalyses heating.
ERA-40 tropical heating is found to be stronger in the tropics (especially in the summer)
than both ERA-15 and NCEP-NCAR reanalyses, which is consistent with the stronger
Hadley Circulation that is found in ERA-40. All three reanalyses have similar Asian
monsoon and winter storm track heating. Both ERA-15 and ERA-40 reanalyses show a
significantly stronger ITCZ than NCEP-NCAR Reanalysis in the East Pacific and the
Atlantic, and this is more consistent with CMAP latent heating estimates. Over Africa,
there is large disagreement between all three reanalyses and observational estimates of
diabatic heating.
1. Introduction
Atmospheric diabatic heating has long been known to play a key role in the large-
scale atmospheric general circulation. A significant fraction of the atmospheric diabatic
heating is caused by water condensation (latent heating) in the tropics, mid-latitude storm
tracks, and regional/seasonal convergence zones like the boreal summer Indian monsoon
and the South Pacific Convergence Zone (SPCZ). The largest latent heating rates are
observed in the tropics, where moisture is abundant in the lower troposphere. Tropical
Rainfall Measurement Mission (TRMM)-estimated mid-tropospheric tropical Pacific
latent heating is up to 10ºC/day (Tao et al. 2003).
In the deep tropics, latent heating is mostly balanced by adiabatic cooling from
large-scale ascent. This is because, the tropics has weak horizontal temperature gradients,
and temperature advection is not sufficient to adjust temperature perturbations (Hoskins
1996). In the extra-tropics, horizontal temperature advection becomes dominant in
adjusting to temperature perturbations. This divides the tropics and extra-tropics into two
different thermodynamic regimes.
Other important sources of diabatic heating include time-mean and transient
horizontal and vertical heat flux convergences. Sensible heat flux in the lower
troposphere and the absorption of radiation (radiative flux convergences) also contribute
to heating, and the Sun is the fundamental source of the heating. Latent heating in clouds
is nothing more than evaporation of wet surfaces into water vapor and subsequent
condensation and latent heat release in the atmosphere. On the planetary scale, mean and
transient heat flux convergences redistribute solar energy from the warmer tropics to the
cooler poles.
Long wave radiation cools down the atmosphere. Globally, on time scales shorter
than a few decades, annual-mean global long wave cooling is in close radiative energy
balance with the annual-mean global incoming solar short wave heating.
The above discussion concentrates in the horizontal distribution of diabatic
heating, but the vertical structure of diabatic heating is just as important. The average
vertical diabatic heating profile is the large-scale manifestation of mesoscale cloud
processes. Nimbostratus and stratus have latent heating that are concentrated in the lower
troposphere, but deep convective clouds often have latent heating maximums in the
middle and upper troposphere. Clouds also have high albedo to visible light, and are
optically thick to infrared red radiation. Mapes and Houze (1993), and Houze (1997)
show convective clouds of different ages have very different diabatic heating vertical
profiles. Schumacher et al. (2003) shows the large-scale responses to tropical latent
diabatic heating are sensitive to the vertical structure of the diabatic heating.
Both divergent flow and diabatic heating are difficult to quantify as there are no
direct ways to measure them. Rotational flow is usually well-constrained in reanalyses
and analyses due to its large horizontal scale and its independence from diabatic heating
(Sardeshmukh 1993). However, divergence is often a small difference between
meridional and zonal wind gradients, and is strongly coupled with the diabatic heating
field. Errors in estimating either diabatic heating or divergence often lead to large errors
in the other. Geostrophic wind balance indicates tropical flow is driven mainly by
divergent circulation due to the low Coriolis parameter. The tropics are also more poorly
observed. Therefore, the quantification of tropical diabatic heating and divergent
circulation is naturally a difficult problem.
Reanalyses and analyses, which are widely used to represent the climatological or
daily/hourly dynamical state of the atmosphere, have questionable divergent circulation,
vertical motion, and diabatic heating fields. As divergent flow itself is hard to separate
from diabatic heating itself, divergent flow and vertical motion fields depend on model
diabatic heating parameterization (like cumulus parameterization) and data assimilation
schemes. Such parameterizations are often simplistic and questionable (Sardeshmukh
1993). Vertical interpolation during post-processing is also known to introduce errors to
diabatic heating fields; this is because the interpolation introduces errors to vertical
temperature advection (Hoerling and Sanford 1993). Obviously, different analyses and
reanalyses use different diabatic heating parameterizations and data assimilation schemes.
There are potentially “large” differences in the 3-D divergent circulations, vertical motion
fields, and diabatic heating in between different reanalyses and analyses. Considering
how widely used these reanalyses and analyses are in current atmospheric science
research and operations, there are strong motivations to quantify these differences.
They are no direct ways to measure diabatic heating, and the validation of
diabatic heating is another difficult problem. Diabatic heating comparisons are often
made among diabatic heating fields from different reanalyses and post-processed in-
situ/remote-sensing data. The latter is sensitive to sampling (e.g. the degree of
representation of the time and location of the in-situ data is taken) and algorithm (e.g. the
inversion of satellite radiances into diabatic heating rates) issues. Precipitation and
outgoing long wave radiation (OLR) offer hope in reasonably estimating latent heating
horizontal distribution and rates in the tropics, because of the close relationship between
vertical motion, cloud development, and diabatic heating.
There are many climate research applications for an “accurate” diabatic heating
dataset. Planetary standing waves, which are often excited by tropical divergences and
diabatic heating in the vortex stretching term of the vorticity equation, play a key role in
determining both the mean state and variability of the atmosphere (Webster 1972). The
El Niño-Southern Oscillation (ENSO) is marked with significant anomalies in tropical
Pacific diabatic heating from the climatological state. With the excitation of
teleconnection patterns from the tropics, ENSO has profound climatological impact
globally. Subtropical highs and deserts have been linked to boreal summer monsoon-
forced diabatic heating and divergences (Hoskins 1996, Hoskins and Rodwell 1996). A
good understanding of both the climatology and variability of the diabatic heating and
divergent circulation is necessary for climate prediction, dynamics, and variability
(Nigam 1983, Ting 1994).
The European Centre for Medium-Range Forecasting (ECMWF) has recently
released the ERA-40 reanalysis (Simmons and Gibbons 2000), and this reanalysis is
made available to US researchers thru the National Center for Atmospheric Research
(NCAR). In this paper, the ERA-40 diabatic heating diagnosis is presented, and is inter-
compared with the ERA-15 (Gibson et al. 1999) and the National Centers for
Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR)
Reanalysis (Kalnay et al. 1996) diabatic heating diagnoses. ERA-40, being released later
than NCEP-NCAR and ERA-15 reanalyses, has more modern data assimilation and NWP
system. It is hoped that ERA-40 represents the state-of-art dynamical representation of
the atmosphere, and the diabatic heating diagnoses and divergent circulations of ERA-40
are hoped to be the most accurate to date.
A discussion of the reanalyses and observations that are used in this study is
presented in Section 2. The heating diagnosis methodology is discussed in Section 3.
The intercomparison between different reanalyses is in Chapter 4. Diabatic heating
validation is discussed in Section 5.
2. Data
a. ERA-40
The ERA-40 is ECMWF’s latest 6-hourly global atmosphere reanalysis. Diabatic
heating diagnoses are completed for all the years that are available from NCAR (1957 -
2002) when this paper is written. The monthly-mean ERA-40 diagnosed diabatic heating
and reanalysis are archived in a 2.5º longitude x 2.5º latitude horizontal grid, and have 23
vertical isobaric levels from 1000-hPa to 1-hPa. Between the surface to the 150-hPa
isobar (the approximate tropical tropopause location), there are a total of 12 isobaric
levels.
b. ERA-15
ECMWF ERA-15 is the forerunner of ERA-40. The archived 5.0º longitude x
2.5º latitude global monthly-mean ERA-15 reanalysis and diagnosed diabatic heating
have 17 isobaric levels from 1000-hPa to 10-hPa. Due to the difference in horizontal
resolution, ERA-40 is re-gridded to a 5.0º longitude x 2.5º latitude grid for comparison.
The archived ERA-15 data share the same isobars with ERA-40 up to 150-hPa isobar.
Only diabatic heating diagnoses below the 150-hPa isobar are discussed in this paper.
The reanalysis and diabatic heating data span 15 years from 1979 to 1993, and these 15
years are chosen to be the base years for the climatology. There are certain known ERA-
15 issues including artificial trends that are caused by assimilation of satellite data, and
these issues are corrected in ERA-40 (Simmons and Gibbons 2000, Trenberth et al. 2001).
c. NCEP-NCAR Reanalysis
The achieved 1949-2002 global 5.0º longitude x 2.5º latitude monthly-mean
NCEP-NCAR Reanalysis and diagnosed diabatic heating have 17 vertical isobaric levels
from 1000-hPa to 10-hPa. Since ERA-15 and ERA-40 reanalyses have the 775-hPa
isobar that the NCEP-NCAR Reanalysis does not have, the 775-hPa isobar is vertically
interpolated for the NCEP-NCAR Reanalysis for inter-comparison purposes. Diabatic
heating fields are available from the original reanalysis. For the sake of consistency,
diabatic heating diagnoses are carried out for the NCEP-NCAR Reanalysis, and the
diagnoses are used instead of the original diabatic heating fields. The original reanalysis
diabatic heating is used to validate the accuracy of the diagnoses.
d. Validation Datasets
The NOAA Climate Prediction Center (CPC) Merged Analysis of Precipitation
Version II (CMAP) (Xie and Arkin 1996) and the NOAA-Cooperative Institute for
Research in Environmental Science (CIRES) Climate Diagnostic Center (CDC)
Interpolated OLR data (NOAA-CIRES CDC 2005) are used to validate the diabatic
heating diagnoses. The archived global monthly-mean CMAP and OLR data have a
horizontal resolution of 5.0º longitude x 2.5º latitude.
3. Methodology of Heating Diagnoses
The ERA-15 and NCEP-NCAR diabatic heating diagnoses are carried out in the
methods that are described in Nigam (1994). There is a slight change to the
methodologies for ERA-40 diabatic heating diagnoses, and the changes are described in
the Appendix. The changes only affect the diabatic heating diagnoses near the poles. A
brief description of the methods of the diabatic heating diagnoses is presented here.
Diabatic heating is diagnosed as a residue quantity, and the governing equation of
the residue diabatic heating diagnosis is:
( ) ( )
∂∂
+⋅∇
+
∂∂
+∇⋅+
∆∆
=•
ppp
pppT
tTtPyxQ ''''vv,,,
00
θωθθωαα
(1)
(a) (b) (c) (d) (e)
( )tPyxQ ,,,•
(on the LHS) is the diagnosed monthly diabatic heating rate (in
temperature per unit time) at a specific spatial grid point on a specific isobar, vector v is
horizontal wind vector, ω is vertical pressure speed, θ is potential temperature, T is
temperature, p is pressure, and 0p is a constant set to 1000-hPa. Over-bars indicate
monthly-means, and primes indicate sub-monthly departures from the monthly-mean. No
time filtering is used in calculated primed variables. Primed variables include both
synoptic transients and “lower-frequency variabilities” like the Madden-Julian Oscillation
or the Pacific-North American Pattern. Beginning from the first term on RHS of the
equation (1), they are: (a) the local time rate of change of temperature that is estimated
from the first and last analysis time (00Z 1st day of the month, and 18Z last day of the
month, respectively) of the month; (b) the horizontal monthly-mean temperature
advection by the monthly-mean horizontal winds; (c) the vertical monthly-mean
temperature advection by the monthly-mean ω; and the eddy (d) horizontal and (e)
vertical heat flux divergences due to sub-monthly variability.
As discussed already, ω and divergences in reanalyses must be treated with
suspicion. The original ERA-40 ω is used to diagnose the diabatic heating. Nigam (1994)
has re-estimated ECMWF un-initialized analysis ω for diabatic heating diagnosis, but the
resultant diagnosed heating fields do not change significantly.
In order to carry out the diabatic heating diagnosis properly, 6-hourly data are
used. The resultant diagnosed heating fields are stored as a monthly mean. Apart from
diabatic heating, monthly eddy heat and momentum covariances (i.e. '','' Tuvu ) are
diagnosed along with wind, temperature, and geopotential variances (i.e. 22 ',' zu ).
4. Intercomparison of Diabatic Heating Diagnoses
a. Global distribution of heating
1) Zonal-mean Heating
Zonal-mean diabatic heating rates not only represent the most basic description of
the global distribution of diabatic heating, it also gives an integrated view of the mean-
meridional circulation and precipitating latitudes. Shown in the two panels in Figure 1
are the January and July 1979-1993 climatological zonal-mean surface-to-125-hPa mass-
weighted vertically-averaged diagnosed diabatic heating rates of the three reanalyses.
For both months, one sees distinct tropical heating maximums between 10ºS-5ºN
(January) and 5ºN-15ºN (July). Those two maximums represent the well-known Inter-
Tropical Convergence Zone (ITCZ) – the ascending branch of the Hadley Circulation.
All three reanalyses capture the broader and double diabatic heating maximum during
January (0.5-1.25 K/day) and the sharper and stronger diabatic heating maximum during
July (1.3-2.5 K/day). All three reanalyses show a region of diabatic cooling in the winter
subtropics (10º-35ºN/S). This is the descending branch of the Hadley Circulation.
Poleward into the winter mid-latitudes, diabatic heating rates begin to increase again.
This is the mid-latitude storm track. Compared to the tropics, the zonal-mean diabatic
heating rates over the mid-latitude stormtracks are much more moderate.
ERA-40 has stronger ITCZ diabatic heating and subtropical diabatic cooling than
the two other reanalyses. For January, ERA-40 ITCZ diabatic heating rates are about
0.25-0.5 K/day more than ERA-15 and NCEP-NCAR reanalyses. Tropical heating
differences between ERA-40 and the two other reanalyses are even larger in July. For
July, ERA-40 ITCZ heating is about 1.2 K/day (about double) more than both ERA-15
and NCEP-NCAR reanalyses. Consistent with stronger ITCZ diabatic heating,
subtropical ERA-40 diabatic cooling is about 0.25 K/day and 0.2K/day stronger in
January and July respectively. It will be shown later that ERA-40 has a stronger
meridional Hadley Circulation than the two other reanalyses, which is consistent with
stronger tropical diabatic heating and subtropical diabatic cooling. The zonal-mean
ERA-15 tropical diabatic heating and subtropical diabatic cooling rates for both January
and July are stronger than the NCEP-NCAR Reanalysis, but the differences between
ERA-15 and NCEP-NCAR are much less than the difference with ERA-40.
In the northern extra-tropics (poleward from 30ºN) during boreal winter, ERA-40
diabatic heating rates are much closer to the two other reanalyses. ERA-15 shows more
diabatic cooling in the midlatitude stormtracks (35º-50º N/S). ERA-40 and NCEP-
NCAR Reanalysis are in good consensus with each other in those latitudes. Due to the
lack of observations, the quality of all three reanalyses in the Southern Hemisphere extra-
tropics is uncertain. During July (austral winter) when the Southern Hemisphere
stormtracks are strongest, ERA-40 has more diabatic heating from 35ºS to 55ºS. The
difference is quite moderate (less than 0.2 K/day) when compared to the differences in
the tropics.
2) Regional Distribution
Clearly, diabatic heating is not distributed in a zonally uniform manner. Due to the
orography and land-sea contrast, standing waves (stormtracks, monsoons, and the Walker
Circulation) organize diabatic heating and cooling to only certain parts of the globe.
Figure 2 and 3 are the January and July 1979-1993 climatological surface-125-hPa global
vertically-integrated diabatic heating rate for the three reanalyses. The diabatic heating
rate differences between the three reanalyses for the same two months are shown in
Figure 4 and 5 respectively.
For January, all three reanalyses show diabatic heating over Indonesia, Amazon-La
Plata Basin, Northwest Pacific and Atlantic (winter storm tracks), equatorial oceans
(ITCZ), Southwest Pacific (SPCZ), and southern Africa. Diabatic cooling can be seen
over continental Eurasia, North Subtropical and Southeast Pacific and Atlantic, and the
Sahara.
For July, all three reanalyses are also in close agreement in the areas with diabatic
heating and cooling. Diabatic heating is observed in equatorial oceans (ITCZ), Pacific
Warm Pool, South and East Asia (Asian Summer Monsoon), Southwest Pacific (SPCZ),
Central America (Central-North American Monsoon), the African Sahel, and Southeast
United States. Diabatic cooling is observed in the extra-tropical East Atlantic and Pacific,
south Indian Ocean, and over Australia.
In January, ERA-40 has stronger diabatic heating over Indonesia than both ERA-15
(up to 1.75 K/day) and NCEP-NCAR (up to 2.25 K/day) reanalyses. The difference
between ERA-15 and NCEP-NCAR reanalyses over Indonesia has no clear spatial
structure. ERA-40 and NCEP-NCAR reanalyses show greater (storm track) diabatic
heating (by about 0.25-0.75 K/day) than ERA-15 over Japan and Eastern US and Canada
Significant differences of the magnitude of July diabatic heating between the two
ERA reanalyses and NCEP-NCAR Reanalysis are seen in the tropical East Pacific and
Atlantic, and the African Sahel. In those regions, both ERA reanalyses show a much
stronger diabatic heating than the NCEP-NCAR Reanalysis. ERA-15 diabatic heating for
those areas is weaker than ERA-40, but is still stronger than NCEP-NCAR Reanalysis.
ERA-40 diabatic heating is up to 2.25K/day and 1.75 K/day stronger than NCEP-NCAR
Reanalysis in the tropical East Pacific and Atlantic respectively. There is also more
diabatic cooling poleward for both ERA reanalyses. The largest cooling differences are
found on the northwest / southwest of the East Pacific/Atlantic diabatic heating centers.
The largest diabatic heating differences between the reanalyses seem to all occur in
poorly observed “wet” regions (Africa and East Pacific). With the lack of observations,
divergent flow becomes more NWP model-dependent. Therefore, it should not be
surprising that diabatic heating differences are largest there. For the same reason, it
should hardly be surprising that there are fewer differences over and downstream of
major North Hemisphere land masses, which are well observed.
b. Divergent circulation and its relationship with heating
1) Meridional Divergent Circulation
It has been noted from other past studies (Nigam et al. 2000) that ERA-15
meridional divergent circulation is stronger than NCEP-NCAR Reanalysis. The analyses
from the past sections imply ERA-40 has an even stronger divergent circulation than
ERA-15, and this is found to be true when the divergent circulations of the three
reanalyses are inter-compared.
The divergent and rotational components of climatological winds are spectrally
decomposed. Zonal-mean meridional wind already represents the divergent meridional
flow. The calculated divergent flow is also used to inter-compare the Walker Circulation
between different reanalyses. Shown in Figure 6 and 7 are the climatological zonal-mean
meridional wind (v), pressure vertical velocity (ω), and the diagnosed diabatic heating for
January and July. The differences between the three reanalyses are shown in Figure 8
and 9 respectively.
The Hadley Cell is strongest in the winter hemisphere in all three reanalyses. The
latitudes of maximum ascent and descent are in good agreement among the three
reanalyses. Maximum ITCZ heating is found between 300- and 600-hPa. Same as
Figure 1, the Hadley ascending branch is wider in January than in July. The narrower
latitude band of tropical diabatic heating during July is consistent with the narrower
latitude vertical ascent latitude band.
ERA-40 has a stronger Hadley Cell for both seasons than the two other reanalyses.
ERA-15 Hadley Circulation is stronger than NCEP-NCAR Reanalysis Hadley
Circulation, but the difference between ERA-15 and NCEP-NCAR reanalyses are less
than with ERA-40. January ERA-40 and ERA-15 middle-troposphere ITCZ ascent are
about -1 and -0.5 Pa/sec stronger than NCEP-NCAR Reanalysis respectively. For July,
ERA-40 and ERA-15 middle-troposphere ITCZ ascent are about -2.5 and -1.2 Pa/sec
stronger.
The Hadley Circulation in both ERA reanalyses also appears to be shallower than
both NCEP-NCAR Reanalysis. In both January and July, both ERA reanalyses show
stronger winter poleward flow between 200- and 300-hPa isobars with opposite-direction
differences further aloft in the 150- and 100-hPa isobars. This indicates both ERA
reanalyses have a shallower upper-troposphere poleward flow than NCEP-NCAR
Reanalysis.
The wind vector differences in the Hadley Circulation are consistent with
diagnosed heating differences. ERA-40, which has stronger ascent, also has stronger
ITCZ heating; while NCEP-NCAR Reanalysis, which has weaker ascent, has also weaker
ITCZ heating. The opposite is true for the subtropical descent and cooling.
The extra-tropics (30º-60º N/S) are marked by the weaker Ferrel Cell. The flow is
close to quasi-geostrophic, and temperature perturbations are balanced mainly by
horizontal temperature advection, and not by vertical motion. During January, the Ferrel
Cell has maximum ascent poleward of 55ºN and 50ºS in the Northern and South
Hemisphere respectively. Below 400-hPa, diabatic heating to close-to-neutral conditions
occur in the Northern Hemisphere Ferrel Cell descending latitudes in the Northern
Hemisphere around 35º-50ºN. During July, the Ferrel Cell is poorly defined in the
Northern Hemisphere, but a well-defined meridional circulation pattern can be seen in the
Southern Hemisphere with weaker ascent poleward of 50ºS.
In the Northern Hemisphere extra-tropics, the diabatic heating differences
between the three reanalyses in the extra-tropics is more decoupled with vertical motion
as stronger ascent/descent is not necessarily linked with stronger diabatic heating/cooling.
In January, stronger ERA-15/40 reanalyses ascent between 50ºS-60ºS does seem to be
linked with stronger middle/lower-troposphere (500- to 700-hPa) warming for those
latitudes. In the Northern Hemisphere, ERA-40 has stronger lower troposphere (under
600-hPa) ascent between 35º-50ºN than the two other reanalyses, but no clear pattern can
be seen with the diabatic heating field except ERA-40 has stronger lower-troposphere
heating poleward of 45ºN. For July, most differences between the three reanalyses
appears in the lower troposphere below 600-hPa. Both ERA reanalyses have stronger
heating in the boundary layer in both hemispheres. For the Southern Hemisphere, there is
more cooling on top of that boundary layer heating from the equator to about 45ºS.
2) The Pacific Walker Circulation
Zonal-asymmetric circulations over the equator are closely related to different
climate regimes that are observed in the deep tropics (the warm West Pacific vs. the cold
East Pacific). In the Pacific, this zonal-asymmetry circulation forms the Walker
Circulation. The inter-annual air-sea coupled variability (ENSO) of the Walker
Circulation is one of the most important global modes of climate variability. Therefore, it
is of scientific interest to quantify the mean state of the Walker Circulation. Shown in
Figure 10 and 11 are the divergent Walker Circulation and diabatic heating cross-section
of the three reanalyses. The differences are shown in Figure 12 and 13. All three
reanalyses show maximum ascent west of the Dateline and west of 160ºW for January
and July respectively. The diabatic heating maximum is found in the middle troposphere
between 600- and 300-hPa isobars. Maximum descent in the East Pacific is between
120ºW-100ºW. In all three reanalyses, January diabatic cooling in the East Pacific show
a west-to-east slant in the lower troposphere and an east-to-west slant in the upper
troposphere. East Indian Ocean and Indonesian (west of 120ºE) diabatic heating and
ascent is weaker (in NCEP-NCAR Reanalysis) or absent (in the two ERA reanalyses) in
July when compared with January. During July, Indonesian diabatic heating and
ascending motion move poleward when the Asian boreal summer monsoon strengthens.
The January differences from the Dateline to 75ºW show little difference between
ERA-15 and ERA-40, but ERA-40 has stronger ascent west of the Dateline. Except west
of 120ºE and east of 75ºW, NCEP-NCAR Reanalysis has generally weaker equatorial
ascent across the Pacific. This is consistent with the weaker zonal-mean Hadley
Circulation in NCEP-NCAR Reanalysis that has been discussed earlier.
For July, NCEP-NCAR Reanalysis shows stronger middle/lower-troposphere
ascent between the Dateline and 120ºW, when compared with the two ERA reanalyses.
This is related to the eastward expanded ascending branch of the Walker Circulation in
NCEP-NCAR Reanalysis. Associated with the eastward expansion, NCEP-NCAR
Reanalysis has a slanted east-to-west lower troposphere diabatic heating structure
between 180º-165ºW. West of 120ºE, NCEP-NCAR Reanalysis also shows ascent and
diabatic heating, which are absent in both ERA reanalyses.
5. Validation of Diagnosis
a) Separation of Radiative and Latent Diabatic Heating
The most important heat source in the tropics is the release of latent heat from
deep convection. Yanai and Tomita (1998) calculated the vertically averaged and mean
vertical profile of Yanai Q1 and Q2 (Yanai et al. 1973), and argue “most” tropical diabatic
heating is driven by condensation in deep convection. With actual field measured
sounding, precipitation rates and radar imagery, Johnson and Ciesielski (2000) use the
residue method to find that radiation has a net cooling effect in deep convection. The
amount of radiative cooling ranges from 0.3-0.5 ºC/day, and daily variability of daily
cooling is highly sensitive to the amount of high clouds.
The residue method that is used in our diabatic heating diagnoses contains no
information about the actual physical process that leads to the residue diabatic heating or
cooling. In numerical models and reanalyses, diabatic heating components (radiative,
sensible, and latent) are separated. It is a more a question whether that information is
achieved during post-processing. However, the numerical model separation of different
diabatic heating components depends on model parameterizations. Although individual
heating components are not constrained under geophysical fluid dynamics, the total
heating is, and that is what the residue method can offer. The physical radiative,
boundary layer, and cloud processes that leads to diabatic heating does go hand-in-hand
with geophysical fluid dynamics.
NCEP-NCAR Reanalysis condensational, radiative and sensible diabatic heating
rates are used to estimate the contribution from radiative and latent heating. Shown in
Figure 14 is the NCEP-NCAR Reanalysis climatological July monthly-mean vertically-
averaged condensational heating and total diabatic heating rates. In the NCEP-NCAR
Reanalysis climate, condensational warming in the tropics sets the upper bound of total
heating. The net diabatic heating is less than the condensational diabatic heating. Total
diabatic heating over Indonesia and Central America is about 1 ºC/day cooler than latent
heating, which is in on the order of magnitude as in Johnson and Ciesielski (2000). This
shows radiation dominates the atmospheric cooling. In the Northeast and Southeast
Pacific and Atlantic where there is little vertically-integrated latent heating, radiation
leads to net cooling. Radiative cooling and the lack of condensational warming in those
regions mark them as Earth’s “cooling window.” It is the safe to argue that diagnosed
diabatic cooling regions are dominated by the radiation effects.
b) CMAP-Estimated Latent Heating Rates
NWP and GCM cloud parameterizations are often questionable. Despite the high
social and economic relevance of precipitation, it is the one of the most difficult variables
to forecast in NWP, and the GCM hydrological cycle is often of suspect. Extreme care
must be taken to interpret reanalysis NWP and GCM latent heating.
CMAP precipitation analyses offer a better way to estimate the tropical latent
diabatic heating. Precipitation is the product of rained-out condensation in the
atmosphere, and non-precipitating clouds do not cause net column diabatic warming to
the atmosphere. In the extra-tropics where horizontal temperature gradients are larger,
mean and transient temperature advection contributes significantly to diabatic heating.
Therefore, the discussion is focused in the lower latitudes (30ºN-30ºS) where latent
heating is the most dominant.
CMAP monthly mean daily precipitation rate R (in mm/day) is converted to
vertically-averaged equivalent latent heat release (in ºC/day) ecipt
TPr∂
∂ by:
DryAirp
onVaporizatiOHlOH
ecip cmmm
lgRtT
,
,)(
Pr *1000
***22
ρ≈
∂∂ (4)
)(2 lOHρ is the density of liquid water (~1000 kg m-3), g is the acceleration due to gravity
near the surface of Earth (~9.81 m s-2), onVaporizatiOHl ,2is the specific latent heat of
vaporization (~2.5X106 J kg-1), DryAirpc , is the specific heat of dry air (~1004 J K-1 kg-1).
The calculated climatological latent heat releases from precipitation for January and July
are shown in Figure 15.
1) January
For January, all three reanalyses are spatially coherent with CMAP-estimated
latent diabatic heating. The ITCZ in the Indian and Pacific Oceans is well captured in the
three reanalyses. In certain parts of the tropical East and Central Pacific, ERA-40
diabatic heating actually exceeds CMAP estimates. This implies ERA-40 January
tropical Pacific diabatic heating may actually to be too strong. There is a disagreement in
diabatic heating distribution between the ERA-15/40 and CMAP estimates over Africa.
CMAP estimates show maximum latent heating in Southeast Africa, while ERA
reanalyses have strongest latent heating in Southwest Africa near Angola and Cameroon.
2) July
July ERA-15/40 diabatic heating rates are more consistent with CMAP in the
tropical East Pacific and Atlantic. CMAP shows East Pacific latent heating extends from
the Latin American coast to around 150ºW, which is in better agreement with the ERA
reanalyses. CMAP July Atlantic heating extends westward off the coast of Sahel Africa,
which again is in better agreement with the ERA reanalyses.
c) Consistency with OLR fields
Outgoing longwave radiation (OLR) is an important signature of deep convection
in the tropics. However, it cannot be compared directly with diabatic heating, as OLR
cannot be casually converted to diabatic heating. OLR is more a proxy for cloud top
height and deep convective activities. However, it is reasonable to argue that areas with
more deep convection are likely to have more latent diabatic heating. Unlike CMAP – a
dataset in which different types of rain gauge and satellite data are assimilated together,
OLR is directly observed with remote sensing. Therefore, OLR is less subject to data
assimilation issues, and is best estimate to deep convection centers.
Only 30ºS-30ºN is used for this comparison, because of the close relationship
between diabatic heating and vertical motion/deep convection. Shown in the two panels
in Figure 16 are the January and July 1979-1999 climatological monthly-mean NOAA-
CPC OLR.
1) January
For January, most active convection is observed over Indonesia (over Java and
Borneo), the Amazon Basin (near the center of the continent), and south central Africa.
All three reanalyses agree well with a maximum of heating over Indonesia. Over South
America, all three reanalyses have heating centered more near the coast of northeast
Brazil, but OLR has deep convection more centered to the west. Over Africa, ERA-15
shows the best spatial correspondence with OLR, with maximum heating/convective
activity in south central Africa.
2) July
For July, OLR data indicate that maximum convection is over the Bay of Bengal,
Pacific Warm Pool, Central America, nearby tropical East Pacific, Equatorial Pacific and
Atlantic, and African Sahel. All three reanalyses have similar spatial structure in diabatic
heating over the Indian monsoon region with a maximum of diabatic heating over
Bangladesh and northern Bay of Bengal. The East Pacific is where considerable
differences are seen among the three reanalyses. Peak convective activity is seen near
Panama and the west coastal waters of Mexico. This is in agreement with the three
reanalyses, but not with CMAP. CMAP has no latent heating maximum over land near
Panama. Lower OLR values can be seen to extend westward from Central America to
about 120ºW. This agrees well with CMAP, ERA-15, and ERA-40, and further supports
that there is not enough East Pacific convective heating in the NCEP-NCAR Reanalysis.
6. Summary and Conclusions
It has been argued that ERA-15 shows stronger divergent circulation (and its
associated diabatic heating) when compared with NCEP-NCAR Reanalysis. Our analysis
indicates ERA-40 shows even stronger heating and divergent circulations. All three
reanalyses are in better consensus in extra-tropical diabatic heating than in the tropics.
The most noticeable differences between ERA-15/40 and NCEP-NCAR Reanalyses are
in the tropics. ERA-40 shows a much stronger Hadley Circulation than both ERA-15 and
NCEP-NCAR reanalyses. Regionally, ERA-40 ITCZ diabatic heating rates may even be
too strong when compared with observational estimates. The largest regional differences
in diabatic heating are seen over the tropical East Pacific, Atlantic, and Africa.
The agreement in diabatic heating between ERA-40 and ERA-15 is better with
NCEP-NCAR Reanalysis. This is hardly surprising as ERA-15/40 data assimilation and
NWP systems are more similar than with each other. Comparing the three reanalyses
with CMAP and OLR data, it does seem ERA-15/40 have better representation in tropical
East Pacific and Atlantic diabatic heating. With the considerable amount of disagreement
among observation- and reanalyses-estimated diabatic heating over Africa, it is hard to
quantify the quality of the diabatic heating diagnoses that area.
This paper only inter-compares and discusses the realism of the heating diagnosis.
The causes for the differences can only be speculated. As discussed in the introduction,
the divergent circulation and heating of the tropics are sensitive to small differences in
wind and the model physics that generates the heating. Wind field differences that are
introduced due to differences in data assimilation and NWP schemes are likely to be
largest in poorly observed areas. It should not be surprising that the largest differences
are observed over tropical East Pacific, Atlantic, and Africa, because it is simply being
poorly observed. Tropical East Pacific, Atlantic, and Africa cover from about 150ºW to
40ºE, and that is more than half of all tropical and equatorial longitudes. Clearly, a more
comprehensive global surface and upper air observation can significantly improve the
understanding of the dynamical and thermodyamical state of the atmosphere.
From the global climate change and dynamics perspective, it is important to know
the differences between different realizations of global climate. Clearly, one does not
want one’s conclusion to be a data artifact. It is hoped that ERA-40 represents the
current best estimate of the state of atmosphere. The word “best” is vague; the
quantification of the accuracy of reanalyses is as hard as constructing the reanalyses.
Acknowledgements. This research was sponsored by the NASA Earth System Science
Fellowship ESSF/04. Many thanks to colleagues Renu Joseph and Alfredo Ruiz-
Barradas for their input.
APPENDIX
Treatment of Vectors in Polar Regions
Vectors near the poles must be done carefully as meaning of the zonal and
meridional wind become questionable there. There are differences in the numerical
algorithms in between ERA-40 and NCEP-NCAR/ERA-15 reanalyses in how vectors
( ''θv ) at the pole are interpolated. In the NCEP-NCAR and ERA-15 reanalyses all
vector quantities are forced to be to zero. In other words, no heat flux is allowed at the
pole. However in ERA-40, only the x component of the vector is forced to be zero. The
y component of the vector is set equal to the wavenumber #1 component at the first off-
polar regular grid.
For all three reanalyses, the polar correction only applies to the heat flux itself.
The original winds of the reanalyses are not modified.
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CAPTIONS
FIG 1. Shown above are the January (upper panel) and July (lower panel) 1979-1993
climatological zonal mean surface-to-125hPa mass-weighted vertical average diabatic
heating rates for the ERA-40, ERA-15, and NCEP-NCAR reanalyses. Units are in
ºC/day.
FIG 2. The above three panels are the January 1979-1993 climatological surface-to-
125hPa mass-weighted vertical-average diabatic heating for the ERA-40 (upper panel),
ERA-15 (middle panel), and NCEP-NCAR (lower panel) reanalyses. Contours are every
1ºC/day, and diabatic heating rates below ± 0.5ºC/day are not shaded. Dark shading
indicates heating for more than 0.5ºC/day, and light shading indicates cooling for more
than 0.5ºC/day.
FIG 3. Same as Figure 3, but it is for July.
FIG 4. Same as in Figure 3, but it is for diabatic heating differences between ERA-40,
ERA-15, and NCEP-NCAR reanalyses. Contours are every 0.5ºC/day, and diabatic
heating rates below ±0.25ºC/day are not shaded. Negative values have light shadings,
while positive values have dark shadings.
FIG 5. Same as Figure 5, but it is for July.
FIG 6. Shown above is the 1979-1993 January climatological zonal-mean diabatic
heating (contours and shadings) and divergent v-ω (vectors) for the ERA-40 (upper
panel), ERA-15 (center panel), and NCEP-NCAR (lower panel) reanalyses. Units for
divergent v and ω are m/s and -Pa/sec respectively. Diabatic heating are contoured every
0.5ºC/day, and heating below ±0.25ºC/day are not shaded. Negative values have light
shadings, while positive values have dark shadings.
FIG 7. Same as Figure 7, but it is for July.
FIG 8. Same as in Figure 7, but for the differences between the three reanalyses.
FIG 9. Same as Figure 9, but it is for July.
FIG 10. Shown above is the 1979-1993 5ºN-5ºS meridional-averaged January
climatological diabatic heating rate (contours and shades) and divergent u-ω (vectors) for
the ERA-40 (upper panel), ERA-15 (center panel), and NCEP-NCAR Reanalysis (lower
panel). Units for divergent u and ω are m/s and -Pa/sec respectively. Diabatic heating
are contoured every 1ºC/day, and heating below ±0.5ºC/day are not shaded. Negative
values have light shadings, while positive values have dark shadings.
FIG 11. Same as Figure 10, but it is for July.
FIG 12. Same as in Figure 10, but for the differences between the three reanalyses.
FIG 13. Same as Figure 13, but it is for July.
FIG 15. Shown above are the NCEP-NCAR Reanalysis 1979-1993 climatological
surface-125hPa mass-weighted vertical-averaged condensational (large-scale +
convective + shallow) (upper panel) and total (condensational + radiative + diffusive)
(lower panel) heating rates. Contours are every 1ºC/day, and diabatic heating rates below
± 0.5ºC/day are not shaded. Negative values have light shadings, while positive values
have dark shadings.
FIG 16. Shown above are the latent heating rates converted from precipitation (in ºC/day)
from the 1979-1993 climatological CMAP precipitation between 30ºS and 30ºN. Upper
panel is for January, and the lower panel is for July. Contours are every 1ºC/day, and
diabatic heating rates below 0.5ºC/day are not shaded. The area between 30ºN/S to 70º
S/N is left blank, so all maps have the same latitude ranges.
FIG 17. Shown above is the 1979-1993 January (upper panel) and July (lower panel)
climatological CPC outgoing long wave radiation. Contours are every 20 W/m2, and
radiative flux above 260 W/m2 not shaded. The area between 30ºN/S to 70º S/N is left
blank, so all maps in this paper have the same latitude ranges.
FIGURES
FIG 1
FIG 2
FIG 3
FIG 4
FIG 5
FIG 6
FIG 7
FIG 8
FIG 9
FIG 10
FIG 11
FIG 12
FIG 13
FIG14
FIG 15
FIG 16