Referee 1 ****************************************************************

This is a very good paper on the technical aspects of
carrying out large-scale thermal convection.  It should
be published after some improvements , which i will point out.
These points are raised in the hope that the authors can 
broadcast their work better.

(1.) In the abstract , more important technical details
such as the number of elements and GFlops attained
for how many processors. These are important information
which should hit the reader's eye at first reading.

(2.) some reference to previous work on finite-elements on
the sphere by Baumgardner and Frederikson , S.I.A.M., 1984 ,
Stuhne and Peltier, J. Computational Physics, 1999.
and issues involving spectral-transform over the sphere
( Lesur and Gubbins, Geophys. J. International, 1999).
Also finite-differences over spherical shell
Fornberg and Merrill, Comparison of finite-difference- and
pseudospectral   methods  for  convective  flow  over  a  sphere,
Geophys. Res. Lett., 24, 3245-3248, 1997.
The authors should discuss the relative merits of
finite-element methods in treating the singular polar
problem. This may be a distinct advantage of finite-elements
in using integration by parts to get rid of the singularity.
They should look definitely into the icosehedral method
by Stuhne and Peltier ( 1996, 1999, both in J. Computational
Physics ) in this connection.

An additional paper by Schmalzl and Hansen ,
Schmalzl, J. and U. Hansen, A fully implicit model for simulating
dynamo  action in a Cartesian domain, Phys. Earth Planet. Inter.,
120, 339-349, 2000.
should be rferened in regard to their usage of the finite-volume
method for discretization as opposed to finite-elements.
An advantage of the finite-volume method is a lower
overhead in the memory requirements unlike finite-elements
which need more memory.

(3) some more discussions about future prospects of this method
in relationship to the growth of computational power in the near
future ( next 2 years ).

Referee 2 ****************************************************************

This is an interesting paper that needs to complement the excellent application
discussion with a "computer science" describing parallelism issues including
performance.

Referee 3 ****************************************************************

This paper presents a numerical simulation of thermally driven convection in
a rotating spherical shell modeled on the Earth ’s outer core using the
Thermal-Hydraulic subsystem of GeoFEM. These simulations have been typically
done using spherical harmonic expansions. The authors motivation is using
FEM is to make it suitable for massively parallel computations. In this
paper, the authors concentrate on comparing the results generated by the
parallel FEM simulations to those produced by the spectral harmonics
expansion. In the results, the authors demonstrate that the results differ
by about 10% that the authors attribute to differences in the spatial
resolution, radial resolution, and initial temperatures in the two
simulations. Furthermore, the authors mention that the radial resolution in
the FEM formulation can cause "serious problems" and require a smaller time
step in this case -- making it computationally more expensive.

While the paper is interesting and appears technically sound, I have some concerns
about the paper, which does not address the parallel formulation of the problem, its
parallel FEM implementation or the issues/requirements/challenges that arise when
moving this class of problems to massively parallel platforms. It is only mentioned
that the implementation uses the GeoFEM package and relies on this package to support
parallelism. The parallel runtimes listed are for 32 processors (not "massively
parallel") and show a scalability of about 2.7 when moving from 8 to 32 processor.
Furthermore, the results indicate that the FEM simulation is at least an order of
magnitude more expensive than the spectral method simulation. This is attributed to
the smaller time step that is required for the FEM case, and also to the inefficiency
of the parallel FEM code (attaining only 5% of peak).  The authors do mention that
the FEM formulation is new.

The overall paper requires some effort to read and understand and has many
grammatical and spelling errors. A few of these are listed below.

I would suggest that the paper be enhanced with a discussion of the
requirements/challenges for a parallel formulation of the problem and possibly an
investigation of the overheads and lack of scalability of the current GeoFEM based
implementation.

Presentation Changes:
The paper has many grammatical errors that make it quite difficult to read.
A few typos are listed below
1.	Page 1, section 1, line6 -- should be " Furthermore, the dynamo process is
not only a complicated nonlinear system....."
2.	Page 1, section 1, last line -- should be "Most of these simulations,
however, have applied the......"
3.	Page 2, section 1, line 4 -- should be "...... the finite difference method is
applied, however they have considered....."
4.	Page 2, section 1, line 12 -- should be "...... rotating spherical shell on
massively parallel computers."
5.	Page 2, section 1, line 21 -- should be "..... convection column which are
parallel......"
6.	Page 2, section 1, line 22 -- should be "..... the columns propagate westward
in both cases....."
7.	Page 2, section 2, line 4 -- should be "...... and rotates with a uniform
angular......"
8.	Page 5, section 2, line 9 -- should be "..... each triangle is divided into
three....."
9.	Page 6, section 2, line 11 -- should be "..... a large truncation error....."
10.	Page 6, section 3, line 12 -- should be "..... which are parallel ....."
11.	Page 7, section 3, line 3 -- should be "..... Z-component....."
12.	Page 7, section 3, line 6 -- should be "..... position of each convection
column....."
13.	Page 9, section 3, line 12 -- should be "..... is estimated by the phase....."
14.	Page 9, section 3, line 15 -- should be "..... the convection pattern
propagates rapidly in the case of GeoFEM....."
15.	Page 10, section 4, line 2 -- Table number missing.
16.	Page 10, section 4, line 3 -- should be ".....a 10 % difference is seen in
several values in the results....."
17.	Page 10, section 4, line 5 -- remove "In these differences,"
18.	Page 10, section 4, line 6 -- should be "..... around both the boundaries....."
19.	Page 12, units for column 3 missing in table 2.