here's a start:

float[] explX = new float[1000];
float[] explY = new float[1000];
float[] explXVel = new float[1000];
float[] explYVel = new float[1000];


void setup(){
size(400,400);
randomExplosionSet();
}
void draw(){
background(255);
explosionDraw();
}



void randomExplosionSet(){
     for (int i=0; i<=999; i++){
 explX[i] = random(-5,5);
 explY[i] = random(-5,0);
 explXVel[i] = random(-5,5);
 explYVel[i] = random(-5,0);
}
}
void explosionDraw(){
 for (int i=0; i<=999; i++){
 explYVel[i] += .1;
 explX[i] += explXVel[i];
 explY[i] += explYVel[i];
   noStroke();
 fill(100,100,255);
 ellipse(width/2+explX[i],height/2+explY[i],random(3),random(3));
 }
}


I took this from an explosion thing I made for a lunar lander game.

mess around with it and see if it can help you work out how to do what you want to do.

sorry if a side view isn't really what you wanted. It's also set up so that the effect makes a rectangle-sh shape. I'll let you mess with it to make it more realistic.

anyway hope this helps.