Vectorizing in Python
I was the one who recommended vectorizing.  I think the first approach would be to convert your code to using 2D indexes.  I think this will make the vectorizing clearer:

for j in range(rows):
    for i in range(cols):
        i1 = i+1 if i&lt;cols-1 else i-1
        i2 = i-1 if i&gt;0 else i+1
        j1 = j+1 if j&lt;rows-1 else j-1
        j2 = j-1 if j&gt;0 else j+1

        w1 = U[j, i]
        w2 = U[j, i-1] if i &gt; 0 else U[j, i]
        w3 = V[j, i]
        w4 = V[j-1, i] if j &gt; 0 else V[j, i]

        p = parray[j, i]
        p1 = parray[j, i1]
        p2 = parray[j, i2]
        p3 = parray[j1, i]
        p4 = parray[j2, i]

        zarray[j, i] = (w1 + w2 + w3 + w4)*p - (w1*p1 + w2*p2 + w3*p3 + w4*p4)


So, if I am reading this code right, you are shifting some rows and columns around.  So the next step is to re-implement this by making copies of the arrays that follow the same patterns:

U1 = np.empty_like(U)
V1 = np.empty_like(V)
parray1 = np.empty_like(parray)
parray2 = np.empty_like(parray)
parray3 = np.empty_like(parray)
parray4 = np.empty_like(parray)

U1[:, 1:] = U[:, :-1]  # this is w2
V1[1:, :] = V[:-1, :]  # this is w4
U1[:, 0] = U[:, 0]
V1[0, :] = V[0, :]

parray1[:, :-1] = parray[:, 1:]  # this is the result of k1
parray2[:, 1:] = parray[:, :-1]  # this is the result of k2
parray3[:-1, :] = parray[1:, :]  # this is the result of k3
parray4[1:, :] = parray[:-1, :]  # this is the result of k4

parray1[:, -1] = parray[:, -2]
parray2[:, 0] = parray[:, 1]
parray3[-1, :] = parray[-2, :]
parray4[0, :] = parray[1, :]

zarray = (U+U1+V+V1)*parray - (U*parray1 + U1*parray2 + V*parray3 + V1*parray4)


You could also use np.hstack and np.vstack instead of slices:

U1 = np.hstack([U[:, :1], U[:, :-1]])
V1 = np.vstack([V[:1, :], V[:-1, :]])

parray1 = np.hstack([parray[:, 1:], parray[:, -2:-1]])
parray2 = np.hstack([parray[:, 1:2], parray[:, :-1]])
parray3 = np.vstack([parray[1:, :], parray[-2:-1, :]])
parray4 = np.vstack([parray[1:2, :], parray[:-1, :]])

zarray = (U+U1+V+V1)*parray - (U*parray1 + U1*parray2 + V*parray3 + V1*parray4)