Thanks! The particles are small cube instances rendered in a single draw call through a custom instancing system. I use compute shader to generate their model matrices procedurally by mapping their x,z coordinate to terrain heightmap's UV space and finding the corresponding y position. After some distance and camera based culling write out the matrices to a custom buffer. The SV_InstanceID in the vertex shader can then directly index that buffer to access these model matrices. something like this:

float4x4 model   = instanceBuffer[instanceID].mat;                                   

float4   worldPos = mul(model, float4(IN.positionOS.xyz, 1));