// Spread random velocity mainly along x-z axis

particleEmitter.rndVelocity = Vector3(2, 0.1, 2);

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        particleEmitter.rndVelocity = new Vector3(2, 0.1F, 2);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        particleEmitter.rndVelocity = Vector3(2, 0.1F, 2)