NCEP reanalysis model levels
Grid Resolution of the NCEP/NCAR reanalysis data

The data is available on different grids depending on the variables of interest:


The original grid is a T62 Gaussian grid, which grid cells do not have a constant lon-lat spacing over the whole globe (see e.g. Gaussian grid at Wikipedia), with 28 sigma levels. The sigma levels are listed on this page, where also the surface pressure is available for download. According to the FAQ, the sigma coordinate equals "pressure in current level"/"surface pressure". Thus, the pressure on each sigma level can be obtained via the available data.
Secondly, some variables, such as air temperature, are available interpolated onto an equally spaced lon-lat grid of 2.5° x 2.5° resolution with 17 pressure levels. These data are available here including data files with the geopotential height. The pressure levels are listed on the same page.


Comparability with A and B coefficients of ECMWF

The ECMWF uses the coefficients A and B as well as the reference pressure $p_0$ to describe the levels. NCEP uses just one coefficient ($\sigma$) for this purpose:

$$
\begin{align}
&amp; \text{ECMWF:} &amp; p\left(\eta,p_s\right) &amp;= A\left(\eta\right) \times p_0 + B\left(\eta\right) \times p_s \\
&amp; \text{NCEP:} &amp; p\left(\eta,p_s\right) &amp;= \sigma\left(\eta\right) \times p_0 \\
\end{align}
$$

When we had $p_0 = 0\ \text{bar}$, both representations were the same. In the given situation, there is no unambiguous mapping from a known $\sigma\left(\eta\right)$ to a set of $A\left(\eta\right)$ and $B\left(\eta\right)$. At least if there are no further limiting conditions.

Normally, I do not deal with vertical coordinate systems. Therefore, I am not sure how data on different vertical layers is compared with each other (except of the obvious: interpolating the one data set onto the vertical coordinates of the other data set).

Summary

1) Yes, here and here.


The variables Air Temperature, Geopotential Height, Relative Humidity, Specific Humidity, Omega (Vertical Velocity), U-Wind, and V-Wind are available on the second grid (2.5° x 2.5° horizontal resolution; 17 pressure levels).
The variables Divergence, Specific Humidity, Virtual Air Temperature, and Relative Vorticity are available on the first grid (T62 Gaussian grid; 28 sigma layers). In order to calculate the model pressure at each sigma level, the surface pressure is additionally provided (T62 Gaussian grid but no vertical resolution). Also the topographic data (Orography) is provided on the same resolution as the surface pressure.


2) The vertical layer representations by $\sigma$ coordinates, on the on side, and by a set of $A$, $B$ and $p_0$, on the other side, are similar to each other but not directly comparable to each other. However, for most practical applications the data on the second grid, which is easier to handle, might be sufficient.

I am not sure if these answers to the question are satisfactory.