Database Grid Structure

Variables in the database are written on an equally spaced horizontal grid of 96 longitude points numbered from 0 $^\circ$ to 356.25 $^\circ$ East in steps of $3.75^\circ$ and 48 latitude points numbered from -88.125 $^\circ$ to 88.125 $^\circ$ in steps of 3.75 $^\circ$ (latitudes South of the equator are indicated by negative numbers). See Figure 1.

**Figure 1:** The database horizontal grid of 90 longitude and 48 latitude points.
$\begin{figure} \centerline{ \psfig {file=hgrid.ps,width=150mm} }\end{figure}$

Vertical Structure

The database vertical coordinate is defined as

$\begin{displaymath} \sigma~=~\frac{p}{p_s}\end{displaymath}$

(1)

where p is the atmospheric pressure and p_s is the surface pressure. Thus $\sigma$ is 1 at the surface, 0 at infinite height and the $\sigma$ levels follow the model orography. The sigma levels are arranged as in Table 1 and illustrated in Figure 2.

**Table 1:** Database $\sigma$ levels and approximate heights based on $-10 \ln(\sigma)$ km.
$\sigma$	Approximate Height	$\sigma$	Approximate Height
0.9995000	5m	0.2039437	15.9km
0.9980984	19m	0.1235681	20.9km
0.9955622	44m	0.0716660	26.4km
0.9909766	91m	0.0403225	32.1km
0.9827373	174m	0.0221810	38.1km
0.9680972	324m	0.0119545	44.3km
0.9425909	591m	0.0062904	50.7km
0.8996294	1.1km	0.0031992	57.4km
0.8311734	1.8km	0.0015417	64.7km
0.7310061	3.2km	0.0006778	73.0km
0.6009113	5.1km	0.0002497	83.0km
0.4552628	7.9km	0.0000563	97.8km
0.3166897	11.5km

**Figure 2:** The database $\sigma$ levels in a region with large topographic features; a section through Olympus Mons is the main peak visible. An isothermal atmosphere with a scale height of 10km has been assumed for this simple illustration.
$\begin{figure} \psfig {file=sigma.ps,width=150mm}\end{figure}$

Temporal Structure

Data are stored in the database 12 times per Martian day using prime meridian time, not local time. At universal time level 1 the local time at 0 $^\circ$ longitude is 12pm (given that the Martian day is 24 ``hours'' long). Hence at 90 $^\circ$ longitude it is 6am, at 180 $^\circ$ it is 12am etc. At time level 2 the local time at 0 $^\circ$ is 2am, etc.. All times are True Solar Times, not Mean Solar Time. Prime Meridian Time can be computed from local time and longitude using the following FORTRAN commands (see routine atime.f in the appended code listing).

iut=mod(nint(-longitude/30.+localtime/2.+12.),12)+1 pmtime=float(iut) if (iut.eq.13) pmtime=1.0